1. What physical order defines quantum computational power

Presenting Author: Arnab Adhikary, Leibniz University of Hannover
Contributing Author(s): Wang Yang, Robert Raussendorf

In measurement-based quantum computation (MBQC), the computational power of the scheme depends primarily on the initial resource state. Typically, the states used for MBQC are symmetry-protected topologically (SPT) ordered and thus short-range entangled. By utilizing classical MBQC relations and the symmetry of the state involved, we demonstrate that MBQC can, in fact, be performed in states where the correlation length diverges. This opens the possibility of using genuinely topologically ordered states. Additionally, we show that our methods are equally applicable to the Toric code state, which serves as an example of symmetry-enriched topological (SET) order. Thus, both SPT and SET ordered phases appear to possess non-trivial MBQC power. This raises a crucial conceptual question: what precisely is the physical order counterpart to MBQC power?

Read this article online: https://arxiv.org/abs/2307.08903

2. Designing Robust, Decorrelated Quantum Sensors for Multiparameter Estimation

Presenting Author: Shah Saad Alam, University of Colorado JILA
Contributing Author(s): Victor Colussi, John Wilson, Jarrod Reilly, Michael Perlin, Murray Holland

Quantum sensors have emerged as a rapidly growing application of quantum devices for precision measurement, offering multiple advantages over classical sensors. Various protocols for designing highly sensitive quantum sensors exist for both single and vector parameter sensing (for example, [1,2, 3]). In practice, a quantum sensor's performance in measuring "target" parameters is diminished due to the presence of "nuisance" parameters that are difficult to control, calibrate or subject to noise. This is especially true if the quantum sensor's design leads to noise in the nuisance parameters being correlated with the sensor's readout. However, the effect of the uncertain nuisance parameters on the sensor's sensitivity can be characterized through quantum information and multiparameter estimation theory [4,5]. We develop a general protocol, applicable to any hardware platform, for designing robust quantum sensing designs whose performance is decorrelated from undesirable fluctuations in nuisance parameters. Our approach relies on defining variational information-theoretic goals using quantum and classical Fisher information matrices that allow a search algorithm to find globally optimal sensing designs with a large space of potential sensing protocols. Our method allows for end-to-end optimization of the sensor's design, rather than optimizing component by component. Using reinforcement learning as our variational algorithm [1,6], we demonstrate an application of our method for a programmable optical lattice to design an accelerometer whose performance is decorrelated from fluctuations in the optical lattice depth. We also show the effect of successful decorrelation on the sensor's measurement outcomes and Bayesian inferencing through statistical analysis in target and nuisance parameter space. Finally, we discuss the implications of our method for quantum metrology and computing.

[1] Liang-Ying Chih and Murray Holland, Phys. Rev. Research 3, 033279 - Published 27 September 2021
[2] Raphael Kaubruegger, Athreya Shankar, Denis V. Vasilyev, and Peter Zoller, PRX Quantum 4, 020333 - Published 1 June 2023
[3]  Catie Le Desma, Kendall Mehling, Murray Holland, arXiv:2407.04874
[4] Jing Liu et al 2020 J. Phys. A: Math. Theor. 53 023001
[5] Jarrod T. Reilly, John Drew Wilson, Simon B. Jäger, Christopher Wilson, and Murray J. Holland, Phys. Rev. Lett. 131, 150802 - Published 11 October 2023
[6] Le Desma et al.  ,arXiv:2305.17603 - Funding Acknowledgements: NSF OMA 2016244 Q-Sense, NSF PHY-2317149, NASA Quantum Pathways Institute, NSF Grant 2231377

Read this article online: https://arxiv.org/abs/2405.07907

3. Efficiency in Measurement-Based Nonlinear Dynamics

Presenting Author: Joseph Andress, University of Colorado
Contributing Author(s): Yuan Shi, Scott Parker

We present a quantum algorithm based on repeated measurement to solve initial-value nonlinear ordinary differential equations (ODEs). The algorithm relies on classical evaluation of a summation over sub-Hamiltonians, weighted by the expectation values of paired observables. Standard quantum Hamiltonian simulation bridges the short times between evaluation of new Hamiltonian matrices. This algorithm requires an ensemble of quantum states, where each step consumes a subset of quantum states, which are used for measurements and are discarded from further time advance. Having demonstrated that our algorithm is capable of solving nontrivial problems, in this work, we explore the question of efficiency: For what class of problems is the algorithm efficient? For a range of problem classes, we use classical simulations to estimate how algorithmic errors scale with simulation time. For problems where the algorithm is efficient, the scaling is polynomial, rather than exponential, indicating that more accurate results is attainable with a small increase of computational resources. We present potential physical systems for the problem classes, providing further analytical analysis.

4. Fault-tolerant Clifford circuits using flag qubits

Presenting Author: Benjamin Anker, University of New Mexico CQuIC
Contributing Author(s): Milad Marvian

Current strategies for implementing fault-tolerant Clifford gates often depend upon transversality, which preclude the use of transversal non-Clifford gates due to the Eastin-Knill theorem. Therefore, costly methods (e.g. magic state distillation, code switching) are necessary for implementing non-Clifford gates to achieve universality. Recently [1], we developed a framework for designing flag gadgets using classical codes. Using this framework we showed how to perform fault-tolerant syndrome extraction for any stabilizer code using exponentially fewer qubits than conventional methods when qubit measurement/reset is slow. In particular, our method requires only O(t^2 log(w)) flag qubits to fault-tolerantly measure a weight w stabilizer of a distance 2t + 1 code. In this work we propose an extension to our construction to apply not only to stabilizer measurements, but also to general Clifford circuits. We suggest a method to make a Clifford circuit on n qubits, consisting of Θ(n^2) two-qubit gates, fault-tolerant to distance 2t + 1 using O(t^4 (log n)^2 + n) ancilla qubits (linear in n for a fixed code). As suggested, this could see applications in alternative schemes enabling fault-tolerant universality for codes with transversal T gates, as well as in fault-tolerant state preparation. For state preparation, this scaling is similar to that of Shor's method (O(t^2 n)), but our construction applies to general Clifford circuits.

[1] B. Anker and M. Marvian (2022) arxiv:2212.10738

5. Decay of fully excited two-level atoms in delayed non-Markovian waveguide QED: Spontaneous dark state generation

Presenting Author: Pablo Barberis Blostein, National Autonomous University of Mexico
Contributing Author(s): William Alvarez-Giron, Pablo Solano, Kanupriya Sinha

The decay of fully excited interacting atoms gives rise to the well-known superradiance effect. In theoretical calculations, interacting atoms are typically assumed to be in close proximity, and delays in interactions are often neglected. We demonstrate that when the atoms are separated but continue to interact, for example through a waveguide, qualitatively new behaviors emerge. These include decay rates exceeding those predicted by superradiance, entangled atomic states, a small probability of creating a NOON state, and a two-atom cavity with two photons inside.

Read this article online: https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.6.023213

6. Optimizing Unitary Compilations for Noise Resilience

Presenting Author: Joseph Barreto, University of Southern California
Contributing Author(s): Luis Pedro García-Pintos

Until the advent of fault-tolerant quantum computers, it remains an important task to mitigate or avoid the effects of noise in near-term quantum algorithms. We consider a setting where certain circuit patches of limited width and depth are known a priori to suffer from increased noise rates. Relying on the notion of "fragility" introduced in [1], we consider to what extent variational quantum compiling (VQC [2]) can discover sets of angles which recompile these patches into ansatze which are more resilient to certain coherent noise models. We aim to bias VQC towards those solutions which minimize the amount of curvature induced by the coherent noise as quantified by the quantum Fisher information, and we study to what extent we can optimize objectives which incorporate functions of this curvature. We find in some cases that overparameterized ansatze yield a wider "curvature disparity" amongst good-enough solutions. Those solutions of lower curvature can have average fidelities on par with shallower versions of the ansatz, potentially allowing for the benefit of increased solution quality that comes with deeper ansatze while sidestepping increased levels of noise.

[1] Garcia-Pintos et al. "Resilience-Runtime Tradeoff Relations for Quantum Algorithms"
[2] Kunal Sharma et al. "Noise resilience of variational quantum compiling" 2020 New J. Phys. 22 043006

7. Impulse Measurements Beyond the Standard Quantum Limit

Presenting Author: Jacob Beckey, Lawrence Berkeley National Laboratory
Contributing Author(s): Daniel Carney, Tsai-Chen Lee, Giacomo Marocco

The detection of very weak impulses using levitated dielectric particles has received considerable attention in recent years. As these experiments quickly approach the quantum-limited regime, it becomes essential to understand how experiments can surpass the standard quantum limit (SQL). In this work, we provide a number of protocols that enable sub-SQL impulse sensing. We start with the canonical example of cavity opto-mechanics before treating a harmonically suspended dielectric slab which captures much of the relevant physics involved in levitated dielectric nano-particles in one dimension.

8. Quantum metrology and simulation with matter-waves in a high-finesse cavity

Presenting Author: Eliot Bohr, University of Colorado JILA
Contributing Author(s): Chengyi Luo, Chitose Maruko, John Wilson, Haoqing Zhang, Anjun Chu, Ana Maria Rey, James K., Thompson

In conventional quantum simulations and condensed matter systems, interactions are typically limited to pairwise (2-body) interactions. However, exploring n-body interactions (n > 2) opens new avenues for efficient quantum gates and the realization of exotic many-body states. In this study, we experimentally demonstrate a 3-body Hamiltonian interaction using an ensemble of laser-cooled rubidium atoms in a high-finesse optical cavity. By encoding pseudo-spin 1/2 in two atomic momentum states and applying two dressing laser tones, we induce a virtual six-photon process, where lower-order interactions destructively interfere, yielding a pure 3-body interaction. This collective momentum-exchange interaction, mediated by the cavity photons, is of particular interest for entanglement generation and high-order correlation studies. Additionally, we observe signatures of 4-body interactions mediated by virtual eight-photon processes, showcasing the scalability of our approach. Our results demonstrate the feasibility of engineering higher-order interactions in quantum systems, paving the way for advanced quantum simulations and entanglement-enhanced sensing applications. This work also recaps previous studies on exchange interactions and Doppler dephasing suppression.

9. A quantum algorithm to simulate Lindblad master equations

Presenting Author: Evan Borras, University of New Mexico CQuIC
Contributing Author(s): Milad Marvian

We present a quantum algorithm for simulating a family of Markovian master equations that can be realized through a probabilistic application of unitary channels and state preparation. Our approach employs a second-order product formula for the Lindblad master equation, achieved by decomposing the dynamics into dissipative and Hamiltonian components and replacing the dissipative segments with randomly compiled, easily implementable elements. The sampling approach eliminates the need for ancillary qubits to simulate the dissipation process and reduces the gate complexity in terms of the number of jump operators. We provide a rigorous performance analysis of the algorithm. We also extend the algorithm to time-dependent Lindblad equations, generalize the noise model when there is access to limited ancillary systems, and explore applications beyond the Markovian noise model. A new error bound, in terms of the diamond norm, for second-order product formulas for time-dependent Liouvillians is provided that might be of independent interest.

Read this article online https://doi.org/10.48550/arXiv.2406.12748

10. FOCQS: Feedback Optimally Controlled Quantum States

Presenting Author: Lucas Brady, NASA - Ames Research Center
Contributing Author(s): Stuart Hadfield

Quantum optimization, both for classical and quantum functions, is one of the most well-studied applications of quantum computing, but recent trends have relied on hybrid methods that push much of the fine-tuning off onto costly classical algorithms. Feedback-based quantum algorithms, such as FALQON, avoid these fine-tuning problems but at the cost of additional circuit depth and a lack of convergence guarantees. In this work, we take the local greedy information collected by Lyapunov feedback control and develop an analytic framework to use it to perturbatively update previous control layers, similar to the global optimal control achievable using Pontryagin optimal control. This perturbative methodology, which we call Feedback Optimally Controlled Quantum States (FOCQS), can be used to improve the results of feedback-based algorithms, like FALQON. Furthermore, this perturbative method can be used to push smooth annealing-like control protocol closer to the control optimum, even providing and iterative approach, albeit with diminishing returns. In numerical testing, we show improvements in convergence and required depth due to these methods over existing quantum feedback control methods.

11. Generalized geometric speed limits for observables

Presenting Author: Jacob Bringewatt, Harvard University
Contributing Author(s):

Energy-time uncertainty relations and the associated quantum speed limits were first formalized by Mandelstam and Tamm. They provided a lower bound for the time for a quantum system in a pure state to reach an orthogonal state, given unitary evolution under some Hamiltonian. Since then, a wide variety of other bounds, all under the general heading of "quantum speed limits," have been derived, providing bounds on the rate of change of quantum states and observables with applications to metrology, quantum thermodynamics, quantum control theory and the analysis of quantum algorithms. Leveraging quantum information geometry, we derive a new set of generalized quantum speed limits on the rate of change of the expectation values of observables undergoing arbitrary probability-conserving dynamics. These bounds subsume and, for Hilbert space dimension greater than three, tighten existing bounds---in some cases by an arbitrarily large multiplicative constant. The generalized bounds can be used to design "fast" Hamiltonians that enable the rapid driving of the expectation values of observables. Our theoretical results are supported by illustrative examples and an experimental demonstration using a superconducting qutrit. Possibly of independent interest, along the way to one of our bounds we derive a novel upper bound on the generalized quantum Fisher information for unitary dynamics in terms of the variance of the associated Hamiltonian and the condition number of the density matrix.

12. Dynamical Sweet Spot Manifolds of Bichromatically Driven Floquet Qubits

Presenting Author: D. Dominic Briseno-Colunga, Chapman University
Contributing Author(s): Bibek Bhandari, Debmalya Das, Yosep Kim, Long B Nguyen, David I Santiago, Irfan Siddiqi, Justin Dressel, Andrew N Jordan

Flux-tunable superconducting circuits are vulnerable to noise induced dephasing except at sparse flux bias sweet-spots, limiting the utility of the flux control. Replacing the DC flux bias with an AC signal reveals continuous manifolds of noise protected dynamical sweet spots, allowing for flux modulation while limiting dephasing. Here, we utilize multi-mode Floquet theory (MMFT) to analyze the dynamical sweet spots induced by weak and strong bichromatic flux drives. Specifically, we characterize the Floquet quasienergy spectrum, multiphoton resonance (AC stark shift and power broadening), and dephasing lifetime for a two level system as functions of the driving frequencies and AC amplitudes. We find that, unlike in the DC driving case, the dephasing lifetime of the Floquet qubits achieve either their minimum or maximum at the avoided crossings, but always achieve their maximum when the contribution from the regularized pole in the 1/f spectrum is minimum. We obtain a maximal dephasing lifetime when both driving strengths are non-negligible, suggesting a better lifetime with bichromatic driving compared to the monochromatic case. Finally, we simulate a 10ns single-qubit bichromatic Floquet gate on fluxonium with > 99.99% fidelity.

13. Efficient Chromatic-Number-Based Multi-Qubit Decoherence and Crosstalk Suppression

Presenting Author: Amy Brown, University of Southern California
Contributing Author(s): Daniel Lidar

The performance of quantum computers is hindered by decoherence and crosstalk, which cause errors and limit the ability to perform long computations. Dynamical decoupling is a technique that alleviates these issues by applying carefully timed pulses to individual qubits, effectively suppressing unwanted interactions. However, as quantum devices grow in size, it becomes increasingly important to minimize the time required to implement dynamical decoupling across the entire system. Here, we present "Chromatic-Hadamard Dynamical Decoupling" (CHaDD), an approach that efficiently schedules dynamical decoupling pulses for quantum devices with arbitrary qubit connectivity. By leveraging Hadamard matrices, CHaDD achieves a circuit depth that scales linearly with the chromatic number of the connectivity graph for general two-qubit interactions, assuming instantaneous pulses. This includes ZZ crosstalk, which is prevalent in superconducting qubit devices. CHaDD's scaling represents an exponential improvement over all previous multi-qubit decoupling schemes for devices with connectivity graphs whose chromatic number grows at most polylogarithmically with the number of qubits. For graphs with a constant chromatic number, CHaDD's scaling is independent of the number of qubits. Our results suggest that CHaDD can become a useful tool for enhancing the performance and scalability of quantum computers by efficiently suppressing decoherence and crosstalk across large qubit arrays.

Read this article online: https://arxiv.org/abs/2406.13901

14. Scalable electronic control of trapped-ion qubits

Presenting Author: William Cody Burton, Oxford Ionics
Contributing Author(s): C. M. Löschnauer, J. Mosca Toba, A. C. Hughes, S. A. King, M. A. Weber, R. Srinivas, R. Matt, R. Nourshargh, D. T. C. Allcock, C. J. Ballance, C. Matthiesen, M. Malinowski, T. P. Harty

A central challenge in scaling quantum computers is classical control signal delivery to the quantum processing unit. Current trapped-ion quantum computers use several independent signals per qubit for trapping and separate control signals for each gating zone. We present a plan to drastically reduce control signal requirements in the quantum charge-coupled device (QCCD) architecture. Qubit trapping and sorting will follow the WISE architecture ¹, which takes advantage of in-vacuum switch networks and symmetries in surface trap designs to reduce the number of analog signal sources to ~ 200 for a 1000-qubit system. Independently controllable electronic single- and two-qubit gates will be implemented with a combination of a common current-carrying trace and local tuning electrodes in the surface trap ². We demonstrate single-qubit gates with 99.99916(7)% fidelity and find consistent performance with low crosstalk across a seven-zone ion trap that can control up to ten qubits. We additionally electronically generate two-qubit maximally entangled states with 99.97(1)% fidelity and long-term stable performance over continuous system operation.

¹ Malinowski, M., et al., PRX Quantum 4, 040313 (2023).
² Löschnauer, C. M., et al., arXiv:2407.07694 (2024).

15. Quantum sensing and parametric amplification in a Penning ion trap

Presenting Author: Allison Carter, National Institute of Standards and Technology, Boulder
Contributing Author(s): Bryce Bullock, Jennifer Lilieholm, John Bollinger

Penning ion traps allow for the well-controlled formation of two-dimensional trapped ion crystals with hundreds of ions using static electric and magnetic fields to provide confinement. The control of trapped ions, including the generation of spin-motion entanglement, enables their use as a platform for quantum sensing and simulation. One application is sensing weak displacements and electric fields resonant with a collective motional mode of the ions. These weak electric fields can theoretically arise, for example, from the coupling of axion dark matter candidates to strong magnetic fields, such as those used in Penning traps. By applying a small spin-independent displacement while the spin states of the ions are coupled to their collective motional state, we demonstrate a sensitivity of 8.8 dB below the standard quantum limit.

These results can be improved with the use of parametric amplification. Modulating the DC trapping potential at approximately twice the frequency of the center-of-mass mode of the ion crystal squeezes the motional mode. With a measured 5.4 dB of motional squeezing below the ground state, theory predicts a comparable improvement in the sensitivity of our displacement sensing. Parametric amplification can also be used for enhancing the strength of the spin-motion coupling without sacrificing coherence of the spin states. We highlight an additional parametric amplification protocol that could be used to improve quantum simulation experiments.

Read this article online: https://link.aps.org/doi/10.1103/PhysRevA.107.032425

16. High-Fidelity Two-Qubit Gates Between Fluxonium Qubits Using a Tunable Coupler

Presenting Author: Abhishek Chakraborty, Chapman University
Contributing Author(s): D. Dominic Briseño-Colunga, Noah Stevenson, Noah Goss, Lucas Burns, Bibek Bhandari, Chuan-Hong Liu, Zahra Padramrazi, Justin Dressel, Andrew Jordan, David Santiago, Irfan Siddiqi

Tunable couplers provide a promising platform to realize efficient two-qubit gates, capitalizing on their potential for achieving a substantial on/off coupling ratio and eliminating residual interactions within a single design. In this work, we develop fast, high-fidelity two-qubit gates between fluxonium qubits using a floating SQUID as a tunable coupler. We investigate the performance of both fast-flux and parametric gate designs by modulating the flux through the coupler and consider the impact of junction asymmetry.

17. Trotter error time scaling separation via commutant decomposition

Presenting Author: Yi-Hsiang Chen, Quantinuum
Contributing Author(s):

Suppressing the trotter error in dynamical quantum simulation typically requires running deeper circuits, posing a great challenge for noisy near-term quantum devices. Studies have shown that the empirical error is usually much smaller than the one suggested by existing bounds, implying the actual circuit cost required is much less than the ones based on those bounds. Here, we improve the estimate of the trotter error by introducing a general framework of commutant decomposition that separates different error components that scale differently with time. In particular we identify two error components that scale as (dt^p)*t and dt^p for an order-p product formula with step size dt. Under a fixed step size dt, that means one would scale linearly with time t and the other would be constant of t. We show that this formalism not only straightforwardly reproduces previous results but also provide a better error estimate for higher-order product formulas. We demonstrate the improvement both analytically and numerically. We also apply the analysis to observable error and find behaviors that may provide insights for other research areas like Floquet dynamics and thermalization.

18. Costs of accuracy enhancements for qubit phase-shift parameter estimation with very noisy states

Presenting Author: David Collins, Colorado Mesa University
Contributing Author(s): Andrew Griffenberg

We consider estimating the phase shift parameter in a single-qubit phase shift operation when the available physical resources are suboptimal; specifically, the available probe initial states are highly mixed. We quantify the estimation accuracy via the quantum Fisher information, which can be evaluated for various protocols, each starting at the same initial states, following this with a parameter-independent unitary on the qubits and then invoking the phase shift once on one of the qubits.

It is known that, for very highly mixed probe states, one particular correlated-state protocol, using n qubits and a specific preparatory unitary prior to the phase-shift invocation, provides an n-fold quantum Fisher information gain over a protocol which invokes the phase shift once on a single qubit. We now ask how the gain is related to the cost of implementing the preparatory unitary.

We show how to construct the preparatory unitary from single qubit unitaries and one particular two-qubit unitary. We quantify the cost of the protocol in terms of the two-qubit unitaries that appear and provide an expression that shows a trade-off in quantum Fisher information gains versus the preparatory unitary cost. We use this to evaluate when the particular correlated-state protocol is worth its cost as a tool for enhancing parameter-estimation accuracy.

19. Protocol for photonic graph state generation from T-centers in silicon

Presenting Author: Arshag Danageozian, Virginia Tech
Contributing Author(s): Yujun Choi, Sophia E. Economou

The T center is a point defect in silicon with a characteristic transition frequency in the telecommunication O-band, which makes it an attractive candidate as a building block of quantum repeaters connected with fiber optical cables. We consider cavity-assisted photon emission from T centers to design protocols for the generation of useful all-photonic graph states from silicon. Namely, we consider two complementary roles for the T center: a quantum emitter and a mediator of photon-photon CZ gates. Using a master equation formalism for quantum pulses to investigate the effects of noise on the photon generation protocols for both time-bin and polarization encodings, we show how the optimal cavity parameters are somewhat different for each role, hence pointing to the necessity of careful engineering of the defect-cavity parameters. For the atom-photon CZ gates, we study further physical noise sources from the reflected pulse that are not present for the emitter application. Our analysis shows how the cavity parameter regimes for the emitter and control system roles can be compatible for implementation on a single silicon chip.

20. Approaching optimal unambiguous state discrimination using the optimal nonprojective measurement for two coherent states

Presenting Author: Morteza Darvishi, University of New Mexico CQuIC
Contributing Author(s): Francisco Elohim Becerra

Generalized quantum measurements allow for realizing tasks beyond conventional measurement paradigms. Among different generalized quantum measurements, the optimal inconclusive measurement (OIM) is a non-projective quantum measurement that generalizes the minimum error discrimination (MED) and the unambiguous state discrimination (USD) of nonorthogonal states, achieving the minimum error probability for a given probability of a conclusive result [1]. As a particular instance of the IOM, a USD measurement discriminates between two states with zero-error probability, and with minimal probability of inconclusive results, called the Ivanovic-Dieks- Peres (IDP) bound [2]. However, any real-world demonstration of a USD measurement, experimental imperfections make ideal (error-free) USD impossible, always yielding discrimination errors. In particular, for coherent state discrimination, the impossibility of implementing an ideal optimal USD comes from the impossibility of realizing perfect displacement operations to the vacuum state in the presence of experimental imperfections, which aim to realize orthogonal projections onto particular subspaces containing the different input states. This problem is particularly acute for USD implementations that exclusively rely on displacement operations to vacuum for simultaneous, or sequential, state hypothesis testing [3, 4]. Here, we investigate the use of the OIM to implement the optimal USD measurement for two coherent states under realistic imperfections. The OIM in this zero-error regime, does not fully rely on displacement operations to the vacuum state, like the simple USD measurement without feedback [3], and we expect that it is more robust to experimental imperfections. This robust measurement can be useful in realistic implementations of quantum communication protocols based on USD of coherent states.

[1] K. Nakahira and T. S. Usuda, Physical Review A 86, 052323 (2012).
[2] S. M. Barnett, S. Croke, arXiv:0810.1970 (2009).
[3] K. Banaszek, Physics Letters A 253, 12 (1999).
[4] S. J. van Enk, Physical Review A 66, 042313 (2002). Work supported by NSF Grant #PHY- 2210447.

21. Quantum Bayesian Games

Presenting Author: John B. DeBrota, University of New Mexico CQuIC
Contributing Author(s): Peter J. Love

We apply a Bayesian agent-based framework inspired by QBism to iterations of two quantum games, the CHSH game and the quantum prisoners' dilemma. In each two-player game, players hold beliefs about an amount of shared entanglement and about the actions or beliefs of the other player. Each takes actions which maximize their expected utility and revises their beliefs with the classical Bayes rule between rounds. We simulate iterated play to see if and how players can learn about the presence of shared entanglement and to explore how their performance, their beliefs, and the game's structure interrelate. In the CHSH game, we find that players can learn that entanglement is present and use this to achieve quantum advantage. We find that they can only do so if they also believe the other player will act correctly to exploit the entanglement. In the case of low or zero entanglement in the CHSH game, the players cannot achieve quantum advantage, even in the case where they believe the entanglement is higher than it is. For the prisoners dilemma, we show that assuming 1-fold rational players (rational players who believe the other player is also rational) reduces the quantum extension [Eisert, Wilkens, and Lewenstein, Phys. Rev. Lett. 83, 3077 (1999)] of the prisoners dilemma to a game with only two strategies, one of which (defect) is dominant for low entanglement, and the other (the quantum strategy Q) is dominant for high entanglement. For intermediate entanglement, neither strategy is dominant. We again show that players can learn entanglement in iterated play. We also show that strong belief in entanglement causes optimal play even in the absence of entanglement -- showing that belief in entanglement is acting as a proxy for the players trusting each other. Our work points to possible future applications in resource detection and quantum algorithm design.

Read this article online: https://arxiv.org/abs/2408.02058

22. Exact spectral gaps of locally random 1-D quantum circuits

Presenting Author: Andrew Deneris, Los Alamos National Laboratory
Contributing Author(s): Pablo Bermejo, Paolo Braccia, Marco Cerezo, Lukasz Cincio

Random quantum circuits play an important role in quantum computation and quantum information. Their study has allowed researchers to find experiments capable of achieving a quantum advantage by offering classically-hard to simulate sampling experiments, providing insights into quantum chaos and entanglement transitions, as well as improving the study of the trainability of variational quantum algorithms. In particular, random quantum circuits composed of local unitary gates sampled independently and identically distributed according to the Haar measure over the standard representation of U(2d) and acting on neighboring sets of "m" qudits in a one-dimensional lattice, have received considerable attention. While several works have provided bounds for their spectral gap-the absolute value of the largest non-one eigenvalue of the t-th moment operator of the random circuit's unitary distribution- its exact value has not been reported. In this work we contribute to the body of knowledge of random quantum circuits by exactly computing the t = 2 spectral gap for unitary circuits with open and closed boundary conditions, showcasing that the latter is exactly the square root of the former for all system sizes. Our exact formulas provide tighter bounds on the number of layers needed to converge to a 2-design. Moreover, we also numerically compare such spectral gaps to those obtained from random circuits with local gates sampled from the orthogonal and symplectic groups.

23. Learning Arbitrary Gaussian Optical States with Quantum Computers

Presenting Author: Spencer Dimitroff, University of New Mexico CQuIC
Contributing Author(s): Ashe Miller, John Kallaugher, Mohan Sarovar

Concerning the task of learning a quantum state, there has recently been much theoretical work indicating that nonclassical measurement techniques are often necessary to achieve optimal accuracy. In some cases, the quantum advantage over classical methods can scale with the complexity of the system. A notable example of this is for systems of qubits, where one can learn properties of the multimode state with exponentially fewer copies if given access to a quantum computer than could be achieved without one. Additional works have looked at tomography of continuous variable states where due to infinite-dimensional Hilbert spaces, learning can be very inefficient. However, certain subsets, such as Gaussian states, can be learned efficiently but require joint (interferometric) measurements to approach the optimal bound. Our theory work somewhat connects these two problems. Here, we consider the transduction of optical Gaussian states into qubits followed by quantum computation to learn properties of the initial Gaussian states in hopes of finding a scaling advantage in terms of sample size. This is an alternative scheme for joint measurements of multimode light fields that does not require optical interferometry, but instead demands efficient transducers. We believe this scheme may be more experimentally feasible in the future with advancements of transducer and quantum computer technologies.

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

25. Learning with SASQuaTCh: Quantizing Transformer Neural Networks with Kernel-based Self-Attention

Presenting Author: Ethan Evans, US Department of Defense
Contributing Author(s): Matthew Cook, Zachary P. Bradshaw, and Margarite L. Laborde

The recent exploding growth in size of state-of-the-art artificial intelligence and machine learning models highlights a well-known issue where exponential parameter growth, which has grown to trillions as in the case of the Generative Pre-trained Transformer (GPT), leads to training time and memory requirements which limit their advancement in the near-term. The predominant models use the transformer neural network, which are based on the so-called self-attention mechanism and have a large field of applicability including text and images, classification, and even predicting solutions to the dynamics of physical systems. In the latter context, the continuous analog of the self-attention mechanism reveals a convolution kernel nature that can be exploited by the Fourier transform. Our work explores quantum circuits that can efficiently express the self-attention mechanism through the perspective of kernel-based operator learning, where we are able to represent deep layers of a vision transformer network using simple gate operations and a set of multi-dimensional quantum Fourier transforms. Contrasted against the classical exploding parameter complexity, quantum circuits of this kind offer a logarithmic qubit complexity due to dense embedding schema and a logarithmic parameter complexity with respect to the size of the classical data. We demonstrate our approach with simulated classification experiments and validate with quantum computer hardware experiments.

Read this article online: https://arxiv.org/abs/2403.14753

26. Moving Goalposts: Optimizing Warm-Start QAOA to Outperfom its Warm-Start

Presenting Author: Sean Feeney, Los Alamos National Laboratory
Contributing Author(s): Reuben Tate, Stephen Eidenbenz

We study a variant of warm-start QAOA, in which the Quantum Approximation Optimization Algorithm (QAOA) for the Maximum Cut problem on 3-regular graphs is initialized with a superposition of computational basis states that is dominated - to an extent that we can control with a parameter θ - by a classically found cut of the problem graph. This warm-start QAOA version stands out from others as lower bounds on the worst-case guaranteed approximation ratio achieved by single-round versions are known for all values of the control parameter θ. We show through numerical experiments for input graphs up to size 28 that, while warm-start QAOA does not surpass the initially provided classical cuts at depth p=1, it outperforms theoretical lower bounds in many scenarios. In order to make warm-start QAOA more competitive against classical methods, we introduce a novel metric for optimizing QAOA parameters to maximize the probability of sampling a larger cut, rather than the expected approximation ratio. Numerical evaluation shows that this new optimization metric leads to persistent non-zero probabilities (in the lower single-digit percentage points) of sampling better than the initial classical cut obtained for instance through the Goemans-Williamson algorithm or other optimized cut, for certain value of the θ parameter. Thus, we show that repeated sampling of singe-round warm-started QAOA will lead to better cuts.

27. Automation of frequency tuning of magneto-optical trap laser for neutral atom quantum computing

Presenting Author: Jonathan Flores, California Polytechnic State University
Contributing Author(s): Tomas Ludin, Mason Stobbe, Jay Erker, Katharina Gillen

The quickly advancing field of neutral atom quantum computing and other applications of light traps and cold atoms require carefully tuned lasers to pre-cool the atoms in a magneto-optical trap (MOT). While integrated tuned laser systems are commercially available, some laboratories require a more cost-effective solution. We use home-built tunable external-cavity diode lasers and a dichroic atomic vapor laser lock (DAVLL) to create the laser light for our MOT. Our original process of tuning the lasers requires human vision and decision-making at several points and is time-consuming. Here we present our work towards automating it and potentially turning hours of work into a few keystrokes. We have developed, and are continuing to improve, an initial control software framework from which we can create further connections to various parts of our laser trapping setup. A suite of python code instructs an Arduino microcontroller with custom electronics to generate a ramp signal for scanning the laser frequency using a piezo-electric transducer. The scanning range is large enough to capture all Doppler-broadened D2 transition peaks in rubidium. The next step to automate is using saturated absorption spectra to identify the correct transitions in 87Rb to DAVLL lock our lasers to. We believe that this acceleration of the atom-trapping process using readily accessible components will provide benefits to a broader group of neutral atom quantum computing and atom trapping labs.

28. Hybrid and Optimal Measurements of Atomic Ensembles with nonGaussian States for Metrology

Presenting Author: Andrew Forbes, University of New Mexico CQuIC
Contributing Author(s): Ivan Deutsch

We present a protocol for generating nonGaussian states of atomic ensembles through measurement backaction, in a hybrid of continuous-variable and discrete-variable protocol. By squeezing the atomic ensemble's projection noise via homodyne detection, one can effectively increase the coupling strength between the ensemble and a probe beam for a subsequent single photon detection. This enhancement could prove useful in the absence of an atomic cavity. The result is a squeezed first excited Dicke state of the ensemble. We benchmark the protocol's utility using the Fisher information in metrology as a measure of quantum advantage, and we study the decay of this quantity under the effects of decoherence. We employ the Holstein-Primakoff approximation to study the atomic ensemble using a bosonic mode, and apply formalism previously developed by our group to analytically study the evolution of the state under the combined effects of simultaneous measurement and optical pumping. Finally, we explore optimal POVMs to saturate the quantum Cramer-Rao bound. We demonstrate that certain POVMs are naturally more robust than others to noise, and we study how one might make use of this in our protocol.

29. Continuous quantum correction on Markovian and Non-Markovian models

Presenting Author: Juan Garcia Nila, University of Southern California
Contributing Author(s): Todd A. Brun

We investigate quantum error correction through a continuous quantum-jump process, comparing performance under a Markovian error model to two distinct non-Markovian models. The first non-Markovian model involves an interaction Hamiltonian coupling the system and an environmental qubits through $X-X$ bit flip simulating crosstalk, and adding a "cooling" bath to simulate Markovianity. This model is known to exhibit abrupt transitions between Markovian and non-Markovian behavior \cite{Pang2017}. The second non-Markovian model is based on the post-Markovian master equation (PMME), which incorporates the bath correlation through a memory kernel, which we assume is exponentially decaying in time or with both underdamped and overdamped dynamics. We systematically compare these non-Markovian error models against the Markovian case and against each other.

Our analysis begins applying the three models to a single-qubit system coupled to a bath qubit, both initially in its ground states, allowing for analytical solutions. We extend the study to include the three-qubit repetition code and the five-qubit "perfect" code. Generally, we find that fidelity in the Markovian case decays more abruptly than in non-Markovian models, suggesting that non-Markovian models can enhance error correction performance. For instance, in the single-qubit model, fidelity exhibits a linear decay at short times in the Markovian case, whereas non-Markovian models show a quadratic decay. Similarly, for the three-qubit and five-qubit codes, fidelity decays quadratically in the Markovian case and cubically in the non-Markovian case. This difference is attributed to the presence of a quantum Zeno regime in both non-Markovian models.

30. The Weak Generalized Bunching Conjecture

Presenting Author: Shawn Geller, National Institute of Standards and Technology, Boulder
Contributing Author(s): Scott Glancy, Emanuel Knill

There has been interest in the dynamics of noninteracting bosons because of the boson sampling problem. These dynamics can be difficult to predict because of the complicated interference patterns that arise due to their indistinguishability. However, if there are unobserved, hidden degrees of freedom, the indistinguishability can be disrupted in the observations. The generalized bunching probability is defined to be the probability that $n$ noninteracting bosons all arrive in a subset of sites $S$. The strong generalized bunching conjecture states that among models where the hidden state of the bosons is separable, the generalized bunching probability is maximized when the bosons are perfectly indistinguishable. This conjecture was shown to be false [Seron et. al. Nat. Phot. 17, 702-709 (2023)]. We show that among models where the hidden state is permutation invariant, the generalized bunching probability is maximized by perfectly indistinguishable bosons if and only if Lieb's permanental dominance conjecture for immanants is true. We discuss applications of this conjecture to measuring the temperature of bosons trapped in an optical lattice.

31. A trapped-ion platform for quantum rotations

Presenting Author: Neil Glikin, University of California Berkeley
Contributing Author(s): Ryan Tollefsen, Yu-Lung Tang, Neha Yadav, Hartmut Haeffner

I will present an overview of the scientific progress and prospects of a geometrically unique surface-electrode ion trap, where ions are trapped with a high degree of cylindrical symmetry. In stark contrast to the purely oscillator-like motion of other trapped-ion systems, this symmetry allows the motion of a trapped-ion crystal to be rotational. Our system is thus a highly controllable platform for exploring the Hilbert space of a free planar rotor. Such Hilbert spaces exhibit unique properties such as inherent periodicity, leading to desirable properties for quantum information, simulation, and sensing applications. I will discuss tools that we have developed for engineering the state of our rotor in its rotational Hilbert space, and how we have utilized these tools to study decoherence laws for rotors and to make progress towards realizing a two-atom rotational interferometer. I will also discuss other prospects which exhibit the potential of the trapped-ion rotor as a platform for quantum science, such as more sophisticated state engineering techniques, interaction with structured light, and quantum simulations with perfect periodic boundary conditions.

32. Finding Goldilocks models by fighting the gauge problem

Presenting Author: Juan Gonzalez De Mendoza, University of New Mexico CQuIC
Contributing Author(s): Corey Ostrove, Robin Blume-Kohout, Stefan Seritan

Tomography is one of the most powerful tools available for the characterization of quantum computers. However, this power comes with great cost, in part because the space of all possible errors that a quantum processor may experience is incredibly vast. In practice we observe that in experiments only a small fraction of possible errors are relevant for any given device, making the rest unnecessary for the description of that quantum computer's operations. The inclusion of such unneeded errors in our models makes the task of performing and interpreting tomography results harder. Automated Model Selection is an algorithm that reduces this problem by finding a model with the least number of parameters that still can effectively describe the data collected from a device. An obstacle for this algorithm, is that due to a property called gauge freedom, many different models give the same physical predictions and thus are equally effective in representing empirical data. As a consequence, traversing the space of models with different a set of parameters is not trivial. In this project, we implement the automated model selection algorithm using first-order gauge invariant (FOGI) parameters, which alleviates the gauge freedom problem and simplifies the landscape. We have implemented these two ideas together, and observed successful results for a variety of error models. SNL is managed and operated by NTESS under DOE NNSA contract DE- NA0003525.

33. Measurement choice back-action effects on energy and chaos

Presenting Author: Alex Gran, Carleton College
Contributing Author(s): Yuelin Kuang, Maya Khesin, Yusuf Ismail, Sacha Greenfield, Justin Dressel, Arjendu Pattanayak

Quantum backaction from weak measurement affects quantum dynamics. We consider quantum state evolution for a nonlinear driven quantum oscillator under continuous measurement. In a system monitored through a homodyne measurement system the post-processing choice of measurement angle of the local oscillator (LO) laser, phi, changes the form of the energy dissipation via the nonclassical spread variables. This phi dependence is an effect of the noise introduced by the measurement system and is inherent to any open quantum system. This phi dependence arises from the measurement backaction; by taking an ensemble of trajectories, we can model a system with inefficient measurement. We discuss examples, including where this phi dependence is seen in the energy spread variables of a finite ensemble. We sketch applications and implementation ideas.

34. Building a Stylus Trap and Deep Parabolic Mirror to Study and Control Quantum Jumps

Presenting Author: Jane Gunnell, University of Washington
Contributing Author(s): J. Gunnell, C. Thomas, H. Parker, B. Pashinsky, J. Liteanu, D. Tchaikovski, B. B. Blinov

In this poster we present a method for studying quantum jumps in trapped barium ions using a stylus trap and deep parabolic mirror. This novel design minimizes the solid angle blocked by the trap electrodes allowing about 95% of the photons from the ion to hit the surrounding mirror. Combined with the 90% reflectivity of the bare aluminum mirror, this experimental design results in a total single photon collection efficiency of over 85%. High quantum efficiency single photon detectors, such as avalanche photodiodes or transition-edge detectors, will be used to maximize the overall photon detection efficiency. Part of the motivation for this research comes from work of Minev et al. [1], who found they could predict, and even reverse, a quantum jump in a superconducting artificial three-level atom. Once our trap is complete, we plan on attempting to replicate these results.

[1] Minev, Z K et al., To catch and reverse a quantum jump mid-flight. Nature 570, 200-204 (2019)

35. Perturbation theory of long-range Ising model using irreducible representations of SU(2)

Presenting Author: Ivy Gunther, University of New Mexico CQuIC
Contributing Author(s): Andrew Kolmer Forbes, Pablo M. Poggi, Ivan H. Deutsch

Ising models dictate a broad class of natural spin-dynamics bridging integrability and chaos, of interest to quantum simulation and metrology. When they exhibit permutation symmetry by infinite-range interactions, they collapse to a single irreducible representation (irrep) of SU(2) of the collective spin (the symmetric subspace). Finite but long-range interactions tend to preserve much of the collective character, avoiding chaos when initialized in the largest SU(2)-irrep thanks to quantum many-body scars (QMBS) overlapping with the symmetric subspace. Meanwhile irreducible spherical tensors form physically motivated orthonormal operator bases within and between irreps. We truncate a transverse Ising model with slow-decaying interactions, keeping first-order accessible subspaces from this symmetric irrep by optimizing over the space of irrep degeneracies, and analytically decomposing the truncated Hamiltonian into generalized spherical tensors. This truncated Hamiltonian includes the minimal matrix elements necessary for a first-order perturbation theory of the QMBS from the symmetric subspace. We study the dynamics as the system leaks into lower-spin irreps via observable phenomena like dynamical quantum phase transitions and entanglement growth. We probe the quality and duration of approximation using Loschmidt echos and time-dependent variational principles (TDVP) at system sizes too large for direct simulation.

36. Exploring Band Theory in Fractional Quantum Media

Presenting Author: Brenden Guyette, Colorado School of Mines
Contributing Author(s): Joshua Lewis, Lincoln Carr

This study explores the adaptation of solid-state physics principles to the fractional Schrödinger equation, focusing on the resulting modifications to band structure. We investigate how varying the fractional order influences the energy gap between the ground state and the first excited band, which increases monotonically with the fractional order. Additionally, we observe the unique phenomenon of spontaneous band inversion when the fractional order is increased while holding the well parameters (such as height and width) constant. Further analysis reveals that band inversion occurs above a fractional order of 2.0 when the well parameters are varied, and fractional order is held constant. The transition to inversion follows a power-law relationship with a power equivalent to the corresponding fractional order, indicating a predictable and systematic effect. These findings suggest that the fractional Schrödinger equation can drive significant changes in electronic properties, potentially leading to new applications in materials science where control over band structure is crucial.

37. Error tomography in many-body quantum simulation

Presenting Author: Bharath Hebbe Madhusudhana, Los Alamos National Laboratory
Contributing Author(s): Aditya Prakash

Quantum simulation of many-body Hamiltonians constitutes early advances in multi-qubit quantum control. This has been demonstrated in linear ion traps and in ultracold atoms trapped in optical lattices and tweezer arrays. An important next step is to characterize the accuracy of this quantum control in terms of a figure-of-merit. This problem is challenging due to the large Hilbert space dimension and well-known techniques such as randomized benchmarking are not effective for a specific multi-qubit gate. Here, we develop an experimental protocol to characterize the errors in many-body Hamiltonians in trapped ultracold atom experiments.

We consider two forms of errors:

1. Unitary errors arising out of systematic errors in the applied Hamiltonian and
2. canonical non-Markovian errors arising out of random shot-to-shot fluctuations in the applied Hamiltonian.

38. Error propagation in 'ping-pong' quantum teleportation circuits on Ion-Trap platforms

Presenting Author: Hector Hernandez, University of New Mexico CQuIC
Contributing Author(s): Kenneth M. Rudinger, Antonio E. Russo, Andrew J. Landahl, and Brandon P. Ruzic

Mid-circuit measurements are essential for quantum computing as they play an integral role in quantum error correction. However, these measurements can introduce non-Markovian errors that may limit performance of syndrome extraction protocols, especially for many-qubit systems. To investigate the fidelity impact of mid-circuit measurements on scalable trapped-ion quantum processors, we have developed a simple "ping-pong" quantum teleportation circuit, in which a quantum state is teleported back and forth between different qubits several times. Like many-qubit syndrome extraction, the ping-pong circuit involves a mid-circuit measurement for each round of teleportation, and the number of these measurements grows linearly with both the number of qubits involved and the rounds of teleportation.

We have performed numerical simulations of the ping-pong circuit and studied how the fidelity impact scales with the number of mid-circuit measurements. By constructing low-level, physical models of the QSCOUT quantum testbed at Sandia, we have characterized the physical noise sources in the device that limit performance, and we have found that excitation of ion motion during mid-circuit measurements leads to non-Markovian errors that play a key role in circuit performance. We have also implemented a novel circuit compilation for the ping-pong circuit that takes advantage of virtual (error-free) native gates, compensates for electronic crosstalk errors in the device, and uses the minimum number of single-qubit gates. This compilation resulted in a significant increase in performance compared to standard circuit compilations. In addition, we have derived analytic expressions for many physical gate errors, from which we gain physical insight into their effects and greatly increase the numerical efficiency of our simulations.

39. Scalable benchmarking of mid-circuit measurements using Pauli noise learning

Presenting Author: Jordan Hines, University of California Berkeley
Contributing Author(s): Timothy Proctor

Current benchmarks for mid-circuit measurements (MCMs) are limited in scalability or the types of error they can quantify, necessitating new techniques for quantifying MCM performance. Here, I will show how to efficiently learn Pauli noise in MCMs and introduce scalable benchmarks for MCMs that use this noise learning technique. I will first introduce MCM cycle benchmarking, a benchmark for individual layers of MCMs and gates. This method extracts detailed information about individual Pauli error rates from a single layer of gates and MCMs, and I will demonstrate how its results can be used to quantify crosstalk errors during MCMs on current quantum hardware with experiments on IBM Q processors. MCM Pauli noise learning can be integrated with existing Pauli noise learning techniques to scalably characterize the errors in complete syndrome extraction circuits. As an example of this, I will introduce a method for learning a detailed Pauli error model for each layer of a syndrome extraction circuit using few circuits by adapting Averaged Circuit Eigenvalue Sampling to MCMs. Finally, I will discuss using MCM cycle benchmarking to benchmark sets of layers, including parity checks, and how this approach enables learning more parameters of the error than single-layer benchmarking. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Read this article online: https://arxiv.org/abs/2406.09299

40. Tunable open microcavity: towards quantum computing and networking with solid state emitters

Presenting Author: Thi Hoang, University of Colorado JILA
Contributing Author(s): Yuan Zhan, Frankie Ngan, Shuo Sun

In quantum computing and networking, one ideal setup involves using material spins as qubits and photons as information carriers. Material spins can interact to create quantum gates and memory, while photons can transmit information over long distances. At the heart of this configuration is an efficient spin-photon interaction platform. Microcavities with small mode volume and high quality factor have emerged as a prominent method of enhancing spin-photon interaction and improving photon collection efficiency. We report our progress in building a tunable Fabry-Perot microcavity with mode volume of a few λ^3 with moderate finesse and quality factor to be operated in a closed-cycle cryostat. Our ongoing work targets realizing efficient single photon sources and coherent spin control with quantum dots and defect centers, towards applications in building quantum gates and generating photonic cluster states.

41. Performing optimal frequency measurements with a universal quantum processor

Presenting Author: Ross Hutson, Quantinuum
Contributing Author(s):

The use of qubits to perform frequency measurements of classical fields arises naturally in a variety of contexts, from operating atomic clocks to calibrating quantum processors. In this context, it has long been known that the generation of entanglement with ensembles of qubits can provide a metrological advantage over the "standard quantum limit" achieved by unentangled qubits. However, practical demonstrations saturating the optimal "Heisenberg limit" remain elusive due to requirements on fidelities of the required single- and two-qubit manipulations. We report on the use of an H-series universal quantum processor to perform frequency measurements on ensembles of up to 15 qubits using optimal input states and measurements.

42. Universal framework for simultaneous tomography of quantum states and SPAM noise

Presenting Author: Abhijith Jayakumar, Los Alamos National Laboratory
Contributing Author(s): Stefano Chessa, Carleton Coffrin, Andrey Y. Lokhov, Marc Vuffray, Sidhant Misra

We present a general denoising algorithm for performing simultaneous tomography of quantum states and measurement noise. This algorithm allows us to fully characterize state preparation and measurement (SPAM) errors present in any quantum system. Our method is based on the analysis of the properties of the linear operator space induced by unitary operations. Given any quantum system with a noisy measurement apparatus, our method can output the quantum state and the noise matrix of the detector up to a single gauge degree of freedom. We show that this gauge freedom is unavoidable in the general case, but this degeneracy can be generally broken using prior knowledge on the state or noise properties, thus fixing the gauge for several types of state-noise combinations with no assumptions about noise strength. Such combinations include pure quantum states with arbitrarily correlated errors, and arbitrary states with block independent errors. This framework can further use available prior information about the setting to systematically reduce the number of observations and measurements required for state and noise detection. Our method effectively generalizes existing approaches to the problem, and includes as special cases common settings considered in the literature requiring an uncorrelated or invertible noise matrix, or specific probe states.

Read this article online: https://quantum-journal.org/papers/q-2024-07-30-1426/

43. Barycentric bounds on the error exponents of quantum hypothesis exclusion

Presenting Author: Kaiyuan Ji, Cornell University
Contributing Author(s): Hemant K. Mishra, Milán Mosonyi, Mark M. Wilde

Quantum state exclusion is an operational task that has significance in studying foundational questions related to interpreting quantum theory. In such a task, one is given a system whose state is randomly selected from a finite set, and the goal is to identify a state from the set that is not the true state of the system. An error, i.e., an unsuccessful exclusion, occurs if and only if the state identified is the true state. In this paper, we study the optimal error probability of quantum state exclusion and its error exponent --- the rate at which the error probability decays asymptotically --- from an information-theoretic perspective. Our main finding is a single-letter upper bound on the error exponent of state exclusion given by the multivariate log-Euclidean Chernoff divergence, and we prove that this improves upon the best previously known upper bound from [Mishra et al., Letters in Mathematical Physics 114, 76 (2024)]. We also extend our analysis to the more complicated task of quantum channel exclusion, and we establish a single-letter and efficiently computable upper bound on its error exponent, even assuming the use of adaptive strategies. We derive both upper bounds, for state and channel exclusion, based on one-shot analysis and formulate them as a type of multivariate divergence measure called a barycentric Chernoff divergence. Moreover, our result on channel exclusion has implications in two important special cases. First, for the special case of two hypotheses, our upper bound provides the first known efficiently computable upper bound on the error exponent of symmetric binary channel discrimination. Second, for the special case of classical channels, we show that our upper bound is achievable by a nonadaptive strategy, thus solving the exact error exponent of classical channel exclusion and generalising a similar result on symmetric binary classical channel discrimination from [Hayashi, IEEE Transactions on Information Theory 55, 3807 (2009)].

Read this article online: https://arxiv.org/abs/2407.13728

44. Lumped-Element Filters for Protecting Qubits from Control and Readout Ports

Presenting Author: Kaixuan Ji, National Institute of Standards and Technology, Boulder
Contributing Author(s): José A Estrada, Katarina Cicak, Zachary L Parrott, Kristen L Genter, Raymond W Simmonds

It is essential to isolate qubits from their environment, which includes all control and readout ports. Each port can open up the qubit system to noise and dissipation. "Purcell filters" have typically described filters that isolate the qubit from the cavity readout port, decoupling qubit relaxation from cavity decay. In this talk, we describe our efforts to decouple the qubits from both the readout and the parametric control ports. We have designed and tested custom band-stop and band-pass filters using microstrip, coplanar waveguide, and lumped-element technologies. These modular designs provide flexibility to implement the filtering in an external package or directly on the qubit chip. In this talk, we will discuss the design methodology, fabrication, and cryogenic characterization of these filters.

45. Transport Enabled Gates with integrated photonics on a surface electrode ion trap

Presenting Author: Evan Johnson, Sandia National Laboratories
Contributing Author(s): Craig Hogle, Jonathan Sterk, Daniel Stick

In trapped ion quantum computing, shuttling is an integrable part of a scalable architecture. In order to reduce the need for optical modulators, it would be advantageous to perform gate operations while the ion is being shuttled using "transport enabled gates" [1]. Using integrated photonics allows for enhanced scalability through individual addressing with on-device waveguides. Here, I will discuss our progress towards demonstrating transport enabled gate operations on Calcium-40 using a Sandia fabricated surface-electrode Paul trap with integrated waveguides.

[1] H.N. Tinkey, et.al. Phys. Rev. Lett. 5, 128 (2022) Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.

46. Investigating Nonlinear Motional Dynamics and Their Impact on Gate Fidelities in Trapped Ion Quantum Processors

Presenting Author: Wes Johnson, Sandia National Laboratories
Contributing Author(s): Scott E Parker, Brandon Ruzic

Trapped ion crystals are a well-established platform for quantum information science. However, nonlinear resonances between the collective motional modes used to couple the qubits become more prevalent in scalable trapped ion quantum computers. Here, we discuss modeling these nonlinear resonances and their impacts on entangling gate fidelities. Our work introduces a straightforward yet effective model for examining these nonlinear dynamics, shedding light on a critical error mechanism that emerges as the system scales. This understanding is crucial for the development of high-fidelity operations on scalable quantum processors.

This work was supported by the U.S. Department of Energy under Grant No. DE-SC0020393. This material was funded by the U.S. DOE, Office of Science, ASCR Quantum Testbed Program. SNL is managed and operated by NTESS LLC, a subsidiary of Honeywell International, Inc., for the U.S. DOE's NNSA under contract DE-NA0003525. The views expressed here do not necessarily represent the views of the DOE or the U.S. Government.

47. Dynamic, symmetry-preserving, and hardware-adaptable circuits for quantum computing many-body states and correlators of the Anderson impurity model

Presenting Author: Eric Jones, Infleqtion
Contributing Author(s): Cody James Winkleblack, Colin Campbell, Caleb Rotello, Edward D. Dahl, Matthew Reynolds, Peter Graf, Wesley Jones

We present a hardware-reconfigurable ansatz for the variational preparation of many-body states of the Anderson impurity model (AIM), which conserves total charge and spin z-component within each variational search subspace. The ground state of the AIM is determined as the minimum over all minima of distinct charge-spin sectors. Hamiltonian expectation values are shown to require between a linear and quadratic number of symmetry-preserving, parallelizable measurement circuits, each amenable to post-selection. To obtain the one-particle impurity Green's function we show how initial Krylov vectors can be computed via mid-circuit measurement and how Lanczos iterations can be computed using the symmetry-preserving ansatz. For a single-impurity Anderson model with an increasing number of bath sites, we show using numerical emulation that the ease of variational ground-state preparation is suggestive of linear scaling in circuit depth and sub-quartic scaling in optimizer complexity. Hence, combined with time-dependent methods for Green's function computation, our ansatz provides a useful tool to account for electronic correlations on early fault-tolerant processors. Finally, with a view towards computing real materials properties like magnetic susceptibilities and electron-hole propagators, we provide a method to compute many-body, time-dependent correlation functions using a combination of time evolution, mid-circuit measurement-conditioned operations, and the Hadamard test.

Read this article online: https://arxiv.org/abs/2405.15069

48. Simulating Plasma Physics with Qubit Lattice Algorithms

Presenting Author: Ilon Joseph, Laurence Livermore National Laboratory
Contributing Author(s): Buvanesh Sundar, Yuan Shi, Frank R. Graziani, Sheindel Gamerberg, Amy F. Brown, Vasily I. Geyko, Ivan Novikau, Roger W. Minich, Vinay Tripathi, Daniel Lidar, Yujin Cho, Bram Evert

Quantum computing may one day enable the direct simulation of the quantum phenomena in plasma physics applications. We have developed a series of benchmarks taken from the nonlinear dynamics of plasmas to track progress in the field. For example, the integrable dynamics of 3-wave interactions and the chaotic dynamics of combined 3- and 4-wave processes can achieve exponential speedup for calculating certain quantities of interest. Discrete models of the quantum physics of plasma waves can be constructed using lattices of qubits, which are equivalent to spin ½ systems. The ion acoustic wave can be simulated using the XX Hamiltonian, while the electron plasma wave can be simulated by adding a staggered magnetic field to the XX model to generate a mass term in the dispersion relation. Nonlinear electron plasma waves, which are often modeled using the nonlinear Schrodinger equation (NLSE), can be simulated by adding a ZZ term to form the Heisenberg model which is known to be gauge-equivalent to the NLSE. Nonlinear ion acoustic waves can be simulated using a generalization of the Heisenberg model. Benchmarks include calculation of the correct eigenvectors and eigenvalues as well as performing simple scattering experiments. In addition to the qubit lattice algorithms, we also report on the progress of our efforts to simulate these models on present-day quantum hardware platforms.

49. What ensures approximation hardness for quantum algorithms?

Presenting Author: Eliot Kapit, Colorado School of Mines and Atom Computing
Contributing Author(s): Brandon A. Barton, Sean Feeney, George Grattan, Pratik Patnaik, Jacob Sagal, Lincoln D. Carr, Vadim Oganesyan

Constraint satisfaction problems are an important area of computer science. Many of these problems are in the complexity class NP which is exponentially hard for all known methods, both for worst cases and often typical. Fundamentally, the lack guided local minimum escape ensures both exact and approximate classical optimization are hard, but the intuitive mechanism(s) for quantum approximation hardness are poorly understood. For algorithms simulating Hamiltonian time evolution, we explore this question for MAX-3-XORSAT. We conclude that the mechanisms for quantum exact and approximation hardness are fundamentally distinct. We qualitatively identify why traditional methods such as AQC are not good approximation algorithms. We propose new spectral filtering optimization methods to escape these issues, for random hypergraphs including extremal planted solution instances that are approximation hard in practice. We show that our new methods can return a constant fraction approximation in roughly $O (N)$ time (classical methods and AQC both take exponential time), or can find the ground state directly in roughly $O ( N^{3/2})$ time. This performance is predicted with approximate analytical calculations and confirmed numerically. We thus conjecture that random hypergraph instances, including extremal instances with planted partial solutions, are efficiently approximable with high probability. We do not claim that this approximation guarantee holds for all possible hypergraphs, but at present do not know what hypergraph properties would break spectrally filtered quantum optimization. These results suggest that quantum computers are more powerful for approximate optimization than had been previously assumed.

Read this article online: https://arxiv.org/abs/2312.06104

50. Phase transitions in (2+1)D subsystem-symmetric monitored quantum circuits

Presenting Author: Cole Kelson-Packer, University of New Mexico CQuIC
Contributing Author(s): Akimasa Miyake

The interplay of unitary evolution and projective measurements is a modern interest in the study of many-body entanglement. On the one hand, the competition between these two processes leads to the recently-discovered measurement-induced phase transition (MIPT). On the other, measurement-based quantum computation (MBQC) is a well-known computational model studying how measurements simulate unitary evolution utilizing the entanglement of special resources such as the 2D cluster state. The entanglement properties enabling MBQC may be attributed to symmetry-protected topological (SPT) orders, particularly subsystem symmetric (SSPT) orders. It was recently found that the 1D cluster state may be associated with an SPT phase in random circuits respecting a global Z₂ x Z₂ symmetry, and furthermore that all phase transitions in this scenario belong to the same universality class. As resources with greater computational power feature greater symmetry, it is fruitful to investigate further any relationship between levels of symmetry in MIPTs and MBQC. In this paper we investigate MIPTs on a torus with three levels of symmetry-respecting unitary evolution interspersed by measurements. Although we find two area-law phases and one volume-law phase with distinct entanglement structures for each ensemble, the phase transition from the volume-law phase to the area-law phase associated with the 2D cluster state has variable correlation length exponent ν. Whereas ν≈0.90 for unconstrained Clifford unitaries and ν≈0.83 for globally-symmetric Cliffords, subsystem-symmetric Cliffords feature a much smaller value ν≈0.38. It is speculated that the hierarchy of distinct transitions seen in these random monitored quantum circuit models might have consequences for computational universality in MBQC.

Read this article online: https://arxiv.org/abs/2407.18340

51. High-order dynamical decoupling under bandwidth constraints

Presenting Author: Leeseok Kim, University of New Mexico CQuIC
Contributing Author(s): Milad Marvian

We aim to design a high-order dynamical decoupling (DD) sequence using finite-width pulses that is robust against both free-evolution time and finite pulse width, using two different methods:

1. Symmetrization and concatenation
2. Trotter-Suzuki (TS) formula

52. Pulsed, multi-octave bandwidth quadrature measurements with calorimeters

Presenting Author: Emanuel Knill, National Institute of Standards and Technology, Boulder
Contributing Author(s): Ezad Shojaee, James R. van Meter, Karl Mayer, Scott Glancy

Pulsed homodyne detection is a standard technique for measuring quadratures of modes of light and requires high efficiency detectors whose output is proportional to photon number. The detectors are often implemented by integrating the signal from photodiodes over the duration of the pulse. When the shape of the mode is extremely broad in frequency, the lack of uniform response of the detectors across the spectrum prevents direct application of this technique. Calorimeters based on transition edge sensors have the potential for high efficiency and energy resolution over multiple octaves but output a signal that is proportional to energy, not photon number. We show that pulsed homodyne detection can be generalized to broadband pulsed (BBP) homodyne detection setups with detectors such as calorimeters that measure total energy instead of total number of photons. This generalization has applications in quantum measurements of femto-second pulses, and, speculatively, measurement of Rindler modes to verify the temperature of Unruh radiation. We analyze how the implemented measurement approaches an ideal quadrature measurement with growing LO amplitude. We prove that the moments of the measurement converge to the moments of the quadrature and quantify the convergence of post-measurement states to that obtained after an ideal quadrature measurement. Our quantification has applications in optical quantum information processing involving feedforward based on Gaussian measurements.

53. High-fidelity fully randomized benchmarking and testing for time-dependent errors

Presenting Author: Alex Kwiatkowski, National Institute of Standards and Technology, Boulder
Contributing Author(s): Laurent J. Stephenson, Hannah M. Knaack, Christina M. Bowers, Scott Glancy, Emanuel Knill, Dietrich Leibfried, Daniel H. Slichter

Randomized benchmarking (RB) is a widely used strategy to assess the quality of quantum gates in a computational context. One topic of recent interest in RB has been the detection and interpretation of time-dependent errors. Here, we present the design, implementation and analysis of single-qubit, fully randomized, Clifford RB experiments that are optimized for sensitivity to time-dependent errors. Full randomization requires that each random sequence is run only once for better signal to noise. For each experiment, we tested two hypotheses:

1. The success probability decays as a single exponential
2. The success probability decays as a mixture of exponentials.

54. The performance of Bosonic rotation and translation codes in one and two modes

Presenting Author: Akira Kyle, University of Colorado
Contributing Author(s): Noah Lordi, Josh Combes

Bosonic quantum error correcting codes encode a qubit into the Hilbert space of a harmonic oscillator and have seen recent experimental progress, including demonstrations of break even performance. Bosonic rotation codes are characterized by a discrete rotation symmetry in their Wigner functions and include the cat and binomial codes. Meanwhile, the Gottesman-Kitaev-Preskil (GKP) code is characterized by a discrete translation symmetry in its Wigner function. Rotation codes naturally protect against fock shift and dephasing errors while GKP naturally protects against displacement errors. We examine the performance of such codes against loss, amplification, and dephasing channels. By finding the best performing code in each noise regime for a given average energy constraint, we find the "phase" transition between rotation and translation codes as a function of dephasing and loss/amplification. In our study we consider most known single and two mode bosonic codes including square and hex GKP, binomial and pair-binomial, along with cat and pair-cat codes. Finally we introduce and study several novel two-mode rotation codes which demonstrate improved performance over their single mode counterparts.

55. Classical Shadows with Symmetries

Presenting Author: Martin Larocca, Los Alamos National Laboratory
Contributing Author(s): Frederic Sauvage

Classical shadows (CS) have emerged as a powerful way to estimate many properties of quantum states based on random measurements and classical post-processing. In their original formulation, they come with optimal (or close to) sampling complexity guarantees for generic states and generic observables. Still, it is natural to expect to even further lower sampling requirements when equipped with a priori knowledge regarding either the underlying state or the observables. Here, we consider the case where such knowledge is provided in terms of symmetries of the unknown state or of the observables. Criterion and guidelines for symmetric shadows are provided. As a concrete example we focus on the case of permutation invariance (PI), and detail constructions of several families of PI-CSs. In particular, building on results obtained in the field of PI quantum tomography, we develop and study shallow PI-CS protocol. Benefits of these symmetric CS are demonstrated compared to established CS protocols showcasing vastly improved performances.

Read this article online: https://arxiv.org/abs/2408.05279

56. Hot atoms and light cooperating

Presenting Author: Braden Larsen, University of Colorado JILA
Contributing Author(s): Hagan Hensley, James Thompson

Cavity quantum electrodynamics (cavity QED) systems have become an extremely powerful experimental tool for quantum sensing and many-body physics. Optical cavities play a pivotal role in enhancing the strength of atom-light interactions as they confine light to smaller mode volumes.

To improve the interaction time within these mode volumes, atoms are typically optically cooled and trapped. Despite this, there has been a recent push towards the use of thermal atomic vapors as they require less laser infrastructure. Here, we demonstrate an atomic vapor cavity QED platform by probing transits of thermal Rb atoms through a narrow optical cavity. Our system interfaces an atomic beam source developed by Martinez et al. [1] with a standing-wave optical cavity of 80 μm in length. This platform can read out transits of the cavity mode with as few as 4 atoms and has an interaction strength characterized by the single-atom cooperativity of C=1.53. Improvements to cavity technology will allow us to resolve single-atom transits and produce a non-classical light source by pushing the atom-light coupling strength into previously unexplored levels.

1. Martinez, et al. "A chip-scale atomic beam clock," Nat. Commun. 14, 3501 (2023).

57. Anomalous Scaling and Multiscale Structures in Fractional Quantum Ising Models

Presenting Author: Joshua Lewis, Colorado School of Mines
Contributing Author(s): Lincoln D. Carr

This work examines the impact of fractional derivatives on quantum phase transitions within a one-dimensional (1D) Ising model. We show that long-range interactions, induced by fractional derivatives, alter both the critical value of the phase transition and the associated critical exponents. A direct connection is established between the fractional order of the model and its scaling dimension through the anomalous dimension critical exponent η, alongside calculations of other critical exponents (α, β, γ, δ, ν) to a precision of 2 to 3 decimal places. Our analysis covers classical and quantum phase transitions, revealing a continuous transformation of critical exponents with varying fractional orders. The fractional Laplacian in the equations of motion, both classical and quantum, drives unique physical behaviors, such as enhanced tunneling, increased information speed, and unique resonant effects in periodic quantum potentials. Multiscale structures within the model induce anomalous scaling, distinguishing them from the standard second-order derivative behavior observed at the limiting case of the fractional order of 2, where a Gaussian fixed point drives the phase transition. These findings suggest that quantum computers can simulate and exploit these fractional dynamics driven by long-range, multiscale interactions, enabling materials design with novel tailored critical properties.

58. Engineering better measurements for quantum frequency comb metrology

Presenting Author: Noah Lordi, University of Colorado
Contributing Author(s): Eugene Tsao, Alexander Lind, Scott Diddams, Joshua Combes

Frequency combs are used to make the most precise measurements. They have found uses in spectroscopy, timekeeping, and frequency distribution among many other applications, pushing these fields to new frontiers. While frequency combs have extended the precision of many measurements, even combs have ultimate precision limits dictated by quantum mechanics. Several recent experiments have begun to probe the shot-noise limit, opening the door for quantum advantages with comb measurements. We present several ways to improve the performance of frequency combs measurements involving quantum light, filtering, and unique frequency profiles. These new techniques present a path towards a real quantum advantage for frequency comb metrology.

Read this article online: https://journals.aps.org/pra/abstract/10.1103/PhysRevA.109.033722

59. High optical depth atomic ensemble for spin squeezing based on measurement backaction

Presenting Author: Hariprasad Madathil, University of New Mexico CQuIC
Contributing Author(s): Francisco Elohim Becerra

Atomic ensembles provide a suitable testbed for investigating light-matter interactions. These interactions can allow for the generation of Spin Squeezed States (SSS), which enables the reduction of quantum noise in one spin direction below the Standard Quantum Limit (SQL), at the expense of increasing the noise along the conjugate direction. Among different light matter interactions, the Faraday effect can be used to generate spin squeezing based on quantum non-demolition (QND) measurements of the collective hyperfine spin-angular momentum in alkali atoms ¹. In our work, we investigate the birefringent light matter interactions from the coupling of the light polarization with the pseudospin formed by the clock states in the ground state manifold of the cesium (Cs) atoms. Combining this interaction with QND measurements in principle allows for the generation of SSS from quantum measurement backaction [2].

Among different experimental parameters, the optical depth (OD) of the atomic ensemble plays a crucial role in enhancing the light matter birefringence interaction [3]. Our first step towards spin squeezing is to prepare an atomic ensemble with very high OD. We trap Cs atoms in a Magneto Optical Trap (MOT) and prepare the atoms in the pseudospin formed by the clock states in the Cs ground-state manifold, |6𝑆1/2, 𝐹=3, 𝑚𝑓=0⟩ and |6𝑆1/2, 𝐹=4, 𝑚𝑓=0⟩.

References

1. A. Kuzmich, L. Mandel, and N. P. Bigelow, PRL 85, 1594 (2000)
2. Souma Chaudhury, Greg A. Smith, Kevin Schulz, and Poul S. Jessen, PRL 96, 043001 (2006)
3. Collin M. Trail, Poul S. Jessen, and Ivan H. Deutsch, PRL 105, 193602 (2010)

60. Designing Quantum Control Sequences Using Reinforcement Learning

Presenting Author: Charles Marrder, University of Colorado JILA
Contributing Author(s): William Schenken, Jarrod Reilly, Shuo Sun, Graeme Smith, Murray Holland

Quantum optimal control theory aims to manipulate noisy quantum state evolution such that some desired quantity is optimized, such as quantum Fisher information or fidelity. This has applications in quantum metrology and quantum computing, allowing experiments to realize desired state evolution with more accuracy. One control technique, known as dynamical decoupling, aims to minimize decoherence by applying a judiciously chosen sequence of effectively instantaneous electromagnetic pulses. While there exist analytic solutions for pulse timings which are optimal for different noise regimes, it is difficult to determine the optimal pulse timings for a given realistic noise spectrum. We propose a method for designing pulse sequences on qubits using reinforcement learning (RL). Our simulations show an RL agent can learn pulse sequences which minimize decoherence without needing any explicit information about the noise spectrum.

61. The theory of quantum circuits with symmetry-respecting gates

Presenting Author: Iman Marvian, Duke University
Contributing Author(s):

Quantum circuits with symmetry-respecting gates have attracted broad interest in quantum information science. However, despite their wide range of applications, some fundamental properties of these circuits remain poorly understood. Recent studies have shown that, in the presence of symmetries, the locality of gates can severely restrict the set of realizable unitaries. Interestingly, the nature of these restrictions depends significantly on the properties of the symmetry. In particular, for Abelian symmetries, recent work has provided a simple characterization of the group of all realizable unitaries with k-qudit gates, showing that certain restrictions that appear in the case of non-Abelian symmetries, such as SU(d) symmetry, do not arise for Abelian symmetries.

In this talk, I present an overview of recent progress on this problem. In addition to the theory of Abelian quantum circuits, I also describe new, powerful methods that are particularly useful for understanding circuits with non-Abelian symmetries. Using these methods, we settle an open question on quantum circuits with SU(d) symmetry. Namely, we show that 3-qudit SU(d)-invariant gates are semi-universal, i.e., they generate all SU(d)-invariant unitaries, up to certain constraints on the relative phases between sectors with inequivalent irreducible representations of the symmetry. I also discuss new methods for synthesizing U(1)-invariant unitaries with XY interaction.

Read this article online: https://arxiv.org/abs/2407.21249, https://arxiv.org/abs/2309.11051, https://arxiv.org/abs/2302.12466https://arxiv.org/abs/2407.21249, https://arxiv.org/abs/2309.11051, https://arxiv.org/abs/2302.12466

62. A Fast Renormalization-Group Tensor-Network Decoder for Planar Quantum Low-Density Parity Check Codes

Presenting Author: Cole Maurer, University of New Mexico CQuIC
Contributing Author(s): Jalan Ziyad, Andrew Landahl

We present a fast and Bayes-optimal-approximating tensor network decoder for planar quantum LDPC codes based on the tensor renormalization group algorithm, originally proposed by Levin and Nave. By precomputing the renormalization group flow for the null syndrome, we need only recompute tensor contractions in the causal cone of the measured syndrome at the time of decoding. This allows us to achieve an overall runtime complexity of O(pnχ6) where p is the depolarizing noise rate, and χ is the cutoff value used to control singular value decomposition approximations used in the algorithm. We apply our decoder to the surface code in the code capacity noise model and compare its performance to the original matrix product state (MPS) tensor network decoder introduced by Bravyi, Suchara, and Vargo. The MPS decoder has a p-independent runtime complexity of O(nχ3) resulting in significantly slower decoding times compared to our algorithm in the low-p regime.

63. Benchmarking logical three-qubit quantum Fourier transform circuits encoded in the Steane code

Presenting Author: Karl Mayer, Quantinuum
Contributing Author(s): Ciaran Ryan-Anderson, Natalie Brown, Elijah Durso-Sabina, Charles Baldwin, David Hayes, Joan Dreiling, Cameron Foltz, John Gaebler, Thomas Gatterman, Justin Gerber, Kevin Gilmore, Dan Gresh, Nathan Hewitt, Chandler Horst, Jacob Johansen, Tanner Mengle, Michael Mills, Steven Moses, Peter Siegfried, Brian Neyenhuis, Juan Pino, Russell Stutz

We implement logically encoded circuits for the three-qubit quantum Fourier transform (QFT), using the [[7,1,3]] Steane code, and benchmark the circuits on the Quantinuum system model H2-1. The circuits require multiple logical two-qubit gates, which are implemented transversally, as well as logical non-Clifford single-qubit rotations, which are performed by non-fault-tolerant state preparation followed by a teleportation gadget. First, we benchmark individual logical components using randomized benchmarking for the logical two-qubit gate, and a Ramsey-type experiment for the logical T gate. We then implement two versions of the full QFT circuit, using two methods for performing a control-T, and benchmark the circuits by applying them to each basis state in a pair of bases that are sufficient to lower bound the process fidelity. We compare the logical QFT benchmark results to predictions based on the logical component benchmarks, and assess the improvements needed to outperform the corresponding physical-level circuits.

Read this article online: https://arxiv.org/pdf/2404.08616

64. Brownian ratcheting of cold atoms in an arbitrary direction on a plane via quasi-periodic modulation of a two-dimensional dissipative optical lattice

Presenting Author: Caden McCollum, Miami University
Contributing Author(s): Stone Oliver, Henri Balla, Chanakya Pandya, Dr. Samir Bali

Brownian ratcheting refers to the harnessing of energy from surrounding noise fluctuations, converting these random fluctuations into useful directed motion. In our experiments, 87Rb atoms are cooled in a two-dimensional (2D) dissipative lattice formed by superposing three near-resonant laser beams, so that these confined atoms diffuse around owing to random photon recoils. Atomic ratcheting is induced by creating AC driving forces via biharmonic frequency modulation of the beams, which breaks the temporal symmetry of the system and produces directed motion without imparting any net force on the atoms. Two distinct ratcheting mechanisms are observed, namely, harmonic mixing and resonant activation, and their interplay is examined. When driving along both axes of the 2D lattice, these drivings interfere with one another to produce unwanted cross coupling between the axes occurs as a transient effect. This highly nonlinear coupling between the driving along one axis and the resulting motion along the other causes 2D motion to be difficult to control or predict. However, we observe that this cross-coupling may be suppressed through the use of quasi-periodic driving, where the driving frequencies applied along each axis are made incommensurate (i.e., the ratio ωX / ωY is made irrational To verify this quasiperiodicity we utilized split-biharmonic driving, a form of driving in which a temporally-symmetric driving force is applied along each axis to measure the cross-coupling.

Read this article online: http://rave.ohiolink.edu/etdc/view?acc_num=miami1721909349814025

65. Enhancing Quantum Algorithms Through Integration with Model Predictive Control Concepts

Presenting Author: Dominic Messina, Sandia National Laboratories
Contributing Author(s): Helen Durand, Alicia Magann, Mohan Sarovar

There is currently significant interest in developing new algorithms and applications for quantum computers. Over the last decade, hybrid quantum-classical algorithms based on parameterized quantum circuits, including variational quantum algorithms (VQAs) and, more recently, feedback-based quantum algorithms (FQAs), have been developed for broad applications ranging from combinatorial optimization to quantum simulation. These classes of hybrid quantum-classical algorithms have ties to quantum optimal control and quantum Lyapunov control, respectively. Here, we present a new type of hybrid quantum-classical algorithm that is inspired by concepts from model predictive control (MPC). The underlying premise is to relate parameterized quantum circuits to quantum control systems, and then assign values to quantum circuit parameters in a manner that is analogous to setting the values of control variables in MPC. In MPC, a combination of feedback and model-based, moving-horizon optimization are used to achieve a desired control objective. In the present work, feedback is provided via classical shadows, and the model-based optimization is facilitated by approximate, classically-tractable dynamic models of the quantum system. We discuss the benefits and limitations of the MPC-based approach compared with the Lyapunov-based approach utilized in FQAs, evaluate the reduced-order modeling effort in terms of accuracy and efficiency, and investigate how the selection of design parameters such as prediction horizon length impact the algorithm. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. SAND2024-10666A

66. Compressed Linear Randomized Gate Set Tomography

Presenting Author: Ashe Miller, Sandia National Laboratories
Contributing Author(s): Jordan Hines, Timothy Proctor

Characterizing errors in the set of gate operations in quantum devices is a necessary step in assessing their performance. Currently, this task currently handled by Gate Set Tomography (GST) which allows us to understand errors while not having to assume perfect state preparation and measurement. However, GST is computationally expensive, making it viable for only a small number of qubits which makes it insufficient to fully understand the behavior of modern-day systems. Here we propose an extension of GST which takes data for a variety of random short circuits and applies techniques from compressed sensing to analyze the errors on each gate. We test the viability of this technique when gates are modeled exclusively with intrinsic errors, creating a sparse error model to learn. We then use this model to provide an initial assessment of our technique's performance, ultimately demonstrating a method that promised the ability to characterize many qubits. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

67. Colocating a directional amplifier and a qubit

Presenting Author: Benton Miller, University of Colorado
Contributing Author(s): Lindsay Orr, Anja Metelmann, Florent Lecocq

Quantum mechanics requires that information acquired about the state of a qubit must be accompanied by qubit dephasing. Experimentally, nonidealities in the measurement reduce the qubit information acquisition rate and/or introduce additional dephasing, an effect quantified by the measurement efficiency. In the dispersive readout of a superconducting qubit, microwave circulators and isolators are typically placed after the qubit's readout cavity in the measurement chain to avoid backaction from subsequent amplifiers, but the losses intrinsic to these components and their associated wiring limit the measurement efficiency. To overwhelm the effect of these losses while introducing minimal qubit backaction, we merge a qubit its readout cavity with a directional preamplifier using programmable parametric interactions between multiple resonant modes¹. We demonstrate progress in the theoretical understanding and experimental characterization of such an embedded system that has potential to push state-of-the-art measurement efficiency closer to the quantum limit.

¹ F. Lecocq, L. Ranzani, G. A. Peterson, K. Cicak, X. Y. Jin, R. W. Simmonds, J. D. Teufel, and J. Aumentado, Physical Review Letters 126 (2021).

68. Efficient multiparty entanglement distribution with DODAG-X protocol

Presenting Author: William Munizzi, University of California, Los Angeles
Contributing Author(s): Roberto Negrin, Nicolas Dirnegger, Jugal Talukdar, Prineha Narang

In this work we introduce the DODAG-X protocol for multipartite entanglement distribution in quantum networks. Leveraging the power of Destination Oriented Directed Acyclic Graphs (DODAGs), our protocol optimizes resource consumption and enhances robustness to noise in dynamic and lossy networks. Implementing a variation on the X-protocol within the DODAG, we minimize graph verification and path-finding calculations, significantly reducing computational overhead when compared to other entanglement routing schemes. Additionally, our benchmarks on grid lattice and Small-World topologies reveal substantial measurement reduction compared to existing protocols. We demonstrate the success of DODAG-X for generating maximal three-party entanglement in arbitrary networks, and describe the potential for scaling to generic n-party entanglement. The DODAG-X protocol provides a scalable and efficient solution for entanglement routing, advancing current techniques for reliable quantum communication and network applications.

Read this article online: https://arxiv.org/abs/2408.07118

70. A magic-state-free architecture for early fault-tolerant quantum computers

Presenting Author: Jacob Nelson, University of New Mexico CQuIC
Contributing Author(s): Andrew Landahl, Andrew Baczewski

We present an architecture for early fault-tolerant quantum computers that avoids the need for qubit-costly magic-state distillation during runtime. It uses only transversal gates, boot-time preparations of the +1 eigenstate of the single-qubit Hadamard operator, and state teleportation between [[4,2,2]] (2D) and [[8,3,2]] (3D) error detecting color codes. Our architecture realizes the universal logical gate basis consisting of measurement and preparation of qubits in the Z-basis, single-qubit Hadamard gates, and controlled-controlled-Z gates. We provide explicit transpilations from the widely used Clifford = T gate basis to this one, allowing for optimal gate synthesis up to a constant prefactor. We characterize our architecture with two performance metrics and propose empirical tests based off mirror circuit fidelity estimation to estimate these performance metrics at various computational volumes.

71. Progress toward a continuous superradiant laser

Presenting Author: Zhijing Niu, University of Colorado JILA
Contributing Author(s): Cameron Wagner, Vera M. Schafer, Julia R.K. Cline, Dylan J. Young, Eric Yilun Song, James K. Thompson

Superradiant lasers are a promising path towards realizing a narrow-linewidth, high-precision and high bandwidth active frequency reference ¹. They shift the phase memory from the optical cavity, which is subject to technical and thermal vibration noise, to an ultra-narrow optical atomic transition of cold atoms trapped inside the cavity. Our previous demonstration of pulsed superradiance on the mHz transition in 87Sr ^{2, 3} achieved a fractional Allan deviation of 6.7×10^(−16) at 1s of averaging. Continuous-wave superradiance will improve the shortterm frequency stability by orders of magnitude. We achieve continuous loading of 2.1(3)×10^7 88Sr atoms/s into a high-finesse ring cavity in the strong collective atom-cavity coupling regime, and subsequent transport of the atoms in a moving intracavity lattice to a region free of laser light where the atoms experience lower decoherence. Our continuous, high flux apparatus is an excellent starting point for a continuous wave superradiant laser¹, dead-time free atom interferometers ⁴, and high-precision atomic clocks ⁵.

¹ D. Meiser, Jun Ye, D. R. Carlson, and M. J. Holland. Phys. Rev. Lett. 102, 163601 (2009)
² M. A. Norcia, M. N. Winchester, J. R. K. Cline, J. K. Thompson. Science Advances 2, e1601231 (2016)
³ M. A. Norcia, J. R. K. Cline, J. A. Muniz, J. M. Robinson, R. B. Hutson, A. Goban, G. E. Marti, J. Ye, and J. K. Thompson. Phys. Rev. X 8, 021036 (2018).
⁴ I. Dutta, D. Savoie, B. Fang, B. Venon, C.L. Garrido Alzar, R. Geiger, and A. Landragin. Phys. Rev. Lett. 116, 183003(2016).
⁵. M. Schioppo, R. C. Brown, W. F. McGrew, N. Hinkley, R. J. Fasano, K. Beloy, T. H. Yoon, G. Milani, D. Nicolodi, J. A. Sherman, N. B. Phillips, C. W. Oates, and A. D. Ludlow. Nature Photonics 11, 48-52 (2017)

Read this article online: https://arxiv.org/abs/2211.00158

72. Stimulated Raman 2-qubit logic gates in metastable trapped-ion qubits

Presenting Author: Jameson O'Reilly, University of Oregon
Contributing Author(s): Alex Quinn, Gabriel Gregory, I. Daniel Moore, Sean Brudney, Jeremy Metzner, David Wineland, David Allcock

The omg architecture [1] for trapped-ion quantum computing makes use of multiple qubit encodings to avoid crosstalk between coherent and dissipative operations. One type of qubit this scheme employs is the metastable (m) qubit, which has not been widely studied. We have implemented m qubits in the D5/2 manifold of 40Ca+ and performed one- and two-qubit stimulated Raman gates with integrated erasure detection. We perform these gates using laser beams far red detuned of the 854 nm D5/2 to P3/2 and find that the spontaneous Raman scattering error rates can be lowered such that they are no longer a limiting factor in achieving fidelities needed for fault-tolerance. [1] D. T. C. Allcock et al., J. Appl. Phys. Lett. 119, 214002 (2021)

73. Optimizing Dynamical Decoupling Sequence using Real-time Noise Sensing

Presenting Author: Ankur Pal, Louisiana State University
Contributing Author(s): Arshag Danageozian, Bran Purvis

Nitrogen Vacancy (NV) Centers are a promising candidate for the implementation of quantum technologies. This is partly due to the high coherence times of the nuclear spin, even at room temperature, that can be extended with dynamical decoupling (DD). However, in an experimental setting the noise can fluctuate and to accommodate for that we can use the NV center as a spectator qubit. This choice is motivated by the contrasting gyromagnetic ratio of the electrons corresponding to the spectator qubit, with the very low gyromagnetic ratio of nuclei corresponding to the memory qubit. The dynamics of the spectator qubits are therefore much faster than the memory qubits. By using Coherent Population Trapping (CPT), a 𝝺-type system trapped in the dark-state can be used to sense the noise in real-time. The noise causes a non-zero population of the excited state, leading to photon emission. The time series of the photon emissions has been used to sense the noise in real-time. We use this to obtain a probability distribution of the characteristic time of the noise. Since the pulses used in a DD sequence themselves have an error associated to them, it may sometimes be better to have more or less frequency of these pulses depending on the magnitude of noise at that moment. Consequently, we show that the optimal time between two DD pulses (𝛕) can be determined. By continuously updating 𝛕, we can actively tailor the pulse sequence in order to have a longer coherence time.

74. Towards efficient qubit phase estimation with single photons

Presenting Author: Sujeet Pani, University of New Mexico CQuIC
Contributing Author(s): Marco A. Rodríguez-García, Isaac Pérez Castillo, P. Barberis-Blostein and Francisco Elohim Becerra

Optical phase estimation is a central task in quantum metrology, where the phase encoded in an optical probe, such as a single photon, carries information about a certain parameter of interest in a physical system. The optimal measurement of the probe then allows for phase estimation with quantum-limited precision, given by the quantum Cramer-Rao bound (QCRB), which can be used to infer the parameter of interest. For the general problem of phase estimation over the full phase range [0, 2π), the canonical phase measurement yields estimators with variances approaching the QCRB, but requires complex quantum operations ¹. Alternatively, adaptive strategies optimizing over locally optimal measurements ² can approach the QCRB, but within a limited phase range [0, π) or smaller. Here, we theoretically and experimentally investigate a two-step adaptive phase estimation strategy for single photon qubits that combines fixed near optimal measurements to obtain a rough estimate of the phase within [0,2π), and adaptive quantum state estimation techniques, yielding estimations very close to the QCRB within the full parametric space. In our experimental implementation, we use a spontaneous parametric down-conversion source to generate heralded single-photon probes, and adaptive polarization optics and single-photon detectors to implement the adaptive strategy.

¹ M. A. Rodríguez-García, et al., Quantum 5, 467 (2021).
² R. Okamoto, et al., Phys. Rev. Lett. 109, 130404 (2012).

75. Increased connectivity and fast gates for superconducting qubits with common SQUID coupler

Presenting Author: Zachary Parrott, National Institute of Standards and Technology, Boulder
Contributing Author(s): Sudhir K Sahu, Kaixuan Ji, José A Estrada, Trevyn Larson, Akash Dixit, Anthony McFadden, Raymond W Simmonds

The majority of popular superconducting qubit system architectures employ pairwise nearest-neighbor (either fixed or tunable) couplings between qubits and individualized readout resonators. In these systems the localized nearest-neighbor connectivity can create overhead for algorithm circuit decomposition requiring numerous SWAP operations. However, there are feasible alternative options to add increased qubit connectivity and shared control and readout resources benefiting scalability. With appropriate parameter choice and frequency allocation, groups of qubits with all-to-all connectivity can be achieved despite the challenge of the quadratic increase in the number of pairwise interactions with increasing qubit count.

Our group has developed a design approach of this style consisting of multiple resonant modes, acting as either qubits or cavities, all galvanically coupled to a common tunable SQUID element. This enables high on/off coupling ratios and fast parametrically driven interactions for gates and readout. We will review our design and simulation tools for this approach and discuss an example of a three qubit system with all-to-all connectivity. We will highlight some of the design and operational challenges and advantages of this system and preliminary experimental results.

76. Strength of statistical evidence for genuine tripartite nonlocality

Presenting Author: Soumyadip Patra, University of New Orleans
Contributing Author(s): Peter Bierhorst

Recent advancements in network nonlocality have led to the concept of local operations and shared randomness-based genuine multipartite nonlocality (LOSR-GMNL). In this paper, we consider two recent experimental demonstrations of LOSR-GMNL, focusing on a tripartite scenario where the goal is to exhibit correlations impossible in a network where each two-party subset shares bipartite resources and every party has access to unlimited shared randomness. Traditional statistical analyses measuring violations of witnessing inequalities by the number of experimental standard deviations do not account for subtleties such as memory effects. We demonstrate a more sound method based on the prediction-based ratio (PBR) protocol to analyse finite experimental data and quantify the strength of evidence in favour of genuine tripartite nonlocality in terms of a valid p-value. In our work, we propose an efficient modification of the test factor optimisation using an approximating polytope approach. By justifying a further restriction to a smaller polytope we enhance practical feasibility while maintaining statistical rigour.

Read this article online: https://arxiv.org/abs/2407.19587

77. Diagnosing hardware errors in trapped ion qubits using reduced Choi-matrix tomography

Presenting Author: Elia Perego, University of California Berkeley
Contributing Author(s): Bharath Hebbe Madhusudhana; Abdrea Rodrigiuez-Blanco, Brigitta Whaley

Identifying and understanding hardware errors is crucial for assessing the performance of an experimental apparatus and enhancing the fidelity of the quantum circuits implemented on it. The initial step towards this goal involves the characterization of both the quantity and quality of error sources. Based on their physical origin the error sources can be classified into, coherent, incoherent Markovian and non-Markovian. The coherent errors are caused by calibration errors or systematics in the quantum control. The incoherent Markovian errors are caused by coupling to environment and the non-Markovian errors are caused by random errors and spurious interactions with the environment. To accomplish this task, we introduce a benchmarking technique based on reduced Choi-matrix tomography [1, 2], the properties of which provide valuable insights into unknown quantum processes affecting the target unitary. Remarkably, this method eliminates the need for exhaustive knowledge of all noise sources affecting the system.  We tested and validated the effectiveness of this technique on a single qubit gate implemented using a Calcium trapped ion [3]. We performed an RCM tomography, i.e., complete characterization of the RCM and used its mathematical properties to classify and characterize the errors present. Furthermore, intentional noise injection served to confirm the method's reliability for error detection and identification. This approach can be easily extended to cover the two-qubit gate scenario, and seamlessly integrated into the regular calibration and maintenance procedures for the experimental hardware. 

[1] B. H. Madhusudhana. Benchmarking multi-qubit gates - I: Metrological aspects, 2023, arXiv: 2210.04330

[2] B. H. Madhusudhana. Benchmarking multi-qubit gates - II: Computational aspects, 2023, arXiv: 2301.07109

[3] E. Perego, A. Rodriguez-Blanco, B. Whaley and B. H. Madhusudhana, manuscript under preparation.

Read this article online: https://arxiv.org/abs/2210.04330

78. Device-Independent Certification of Multipartite Distillable Entanglement

Presenting Author: Aby Philip, Cornell University
Contributing Author(s): Mark M. Wilde

Quantum networks consist of various quantum technologies, spread across vast distances, and involve various users at the same time. Certifying the functioning and efficiency of the individual components is a task that is well studied and widely used. However, the power of quantum networks can only be realized by integrating all the required quantum technologies and platforms across a large number of users. In this work, we demonstrate how to certify the distillable entanglement available in multipartite states produced by quantum networks, without relying on the physical realization of its constituent components. We do so by using the paradigm of device independence.

Read this article online: https://arxiv.org/abs/2408.01357

79. Towards practical quantum position verification

Presenting Author: Damian Pitalua Garcia, University of Cambridge
Contributing Author(s): George Cowperthwaite, Adrian Kent

We discuss that any useful quantum position verification (QPV) scheme requires to make physical assumptions to be protected against dislocation and replacement attacks. We thus motivate a standard assumption in quantum cryptography (for example, used in quantum key distribution (QKD)), that a tagging device can keep classical data secure. We discuss QPV schemes based on this assumption. Our schemes are practical with current technology and allow for errors and losses. We describe how a proof-of-principle demonstration might be carried out. QPV schemes involving quantum communications typically use photons to encode quantum states. This poses challenges, including errors in state preparation, processing, and measurement, and losses. The problem of losses is particularly challenging in schemes with large distances between the tagging device and the verifiers. An advantage of our schemes is that the queries and responses are purely classical. Quantum communications are needed only to replenish the key via QKD. Moreover, the QKD communications, unlike the position verification queries and responses, are not tightly time constrained. Given our assumptions, our schemes are secure against arbitrarily powerful quantum spoofers, who may share an arbitrary amount of entanglement. This is also an advantage compared to the best-known quantum schemes, which have only been proved secure against spoofers that share an amount of entanglement linear in the classical information).

Read this article online: https://arxiv.org/abs/2309.10070v2

80. Digital quantum simulation of controlled molecular dynamics in first quantization

Presenting Author: Max D. Porter, Sandia National Laboratories
Contributing Author(s): Andrew D. Baczewski, Alicia B. Magann

A longstanding goal is to use laser fields to coherently control the dynamics of quantum systems, such as atoms and molecules. In pursuit of this goal, simulations are essential for designing laser fields that achieve a desired control outcome. In this talk, we investigate the viability of quantum computers for performing these simulations in the presence of low-probability logical errors. We specifically consider simulating controlled molecular dynamics on quantum computers using Trotterized, time-dependent Hamiltonian simulation algorithms within a grid-based, first-quantized representation. We discuss the algorithm formulation, its asymptotic costs, and its compilation into Clifford + T gates. We then present numerical results for simulations of a controlled hydrogenic system. These numerical illustrations explore the impact of uncorrected logical errors, as well as Trotter error, on the simulation outcomes.

81. Spectral bunching and quantum dynamics in the kicked top

Presenting Author: Ryan Quinn, Carleton College
Contributing Author(s): Alex Gran, Noah Pinkney, Alex Kiral, Sudheesh Srivastava, Arjendu Pattanayak

We examine the dynamics of the quantum kicked top (QKT) using the reciprocal of the average difference between energy eigenvalues at a given kick strength K, a measure we have termed "spectral bunching." We find that the system displays unusual behaviors including rapid changes with K, at K values corresponding to local bunching minima, which we demonstrate for the four-qubit QKT via measurements of linear entanglement entropy and tunneling dynamics as well as eigenfunction shapes in angular momentum phase space at and around these K (4𝜋/3, 2𝜋, ~2.76𝜋, 4𝜋). Although most of these local bunching minima correspond to degeneracies in the energy spectrum, we ultimately find spectral bunching to be a more powerful tool than spectral degeneracies for identifying K values that correspond to rapidly changing dynamics. In addition, we report K-periodicities of 4𝜋 in linear entanglement entropy, 8𝜋 in tunneling and bunching, and 16𝜋 in the energy spectrum. The differing periodicities in bunching and the energy spectrum are the result of a reordering of states in the middle of the energy spectrum's cycle, which is then reversed in the second half. We also demonstrate that as we approach the semiclassical limit, the density of local bunching minima increases.

82. Hamiltonian simulation in Zeno subspace

Presenting Author: Kasra Rajabzadeh Dizaji, Arizona State University
Contributing Author(s): Ariq Haqq, Alicia B. Magann, Christian Arenz

We investigate the quantum Zeno effect as a framework for designing and analyzing quantum algorithms for Hamiltonian simulation. We show that frequent projective measurements of an ancilla qubit register can be used to simulate quantum dynamics on a target qubit register with a circuit complexity similar to randomized approaches. The classical sampling overhead in the latter approaches is traded for ancilla qubit overhead in Zeno-based approaches. A second-order Zeno sequence is developed to improve scaling and implementations through unitary kicks are discussed. We show that the circuits over the combined register can be identified as a subroutine commonly used in postTrotter Hamiltonian simulation methods. We build on this observation to reveal connections between different Hamiltonian simulation algorithms.

Read this article online: https://arxiv.org/abs/2405.13589

83. Noise induced regression to classical dynamics in the single mode Kerr effect

Presenting Author: Mohsin Raza, University of New Mexico CQuIC
Contributing Author(s): John DeBrota, Ivan Deutsch

Quantum information processing devices promise speed-ups in certain tasks over their classical counterparts, especially as the system size grows. However, it is well known from the seminal work of Zurek and others that a quantum system undergoing decoherence regresses to its classical counterpart as time progresses, and does so more rapidly as the system becomes more macroscopic. Progress in the Noisy Intermediate Scale Quantum (NISQ) computing era motivates us to better understand this transition in a quantitative way. In this study, we quantify quantum-to-classical transition of single mode Kerr Hamiltonian in the presence of noise. We study the interplay of generation of coherence and the degradation of this coherence with the increasing system size. It is shown that, as we increase the system size, the generation of so-called kitten states is severely restricted even in the presence of modest photon-loss. We show that the expectation values of observables coincide with the classical expectation values in this regime. Given that the generation of kitten states is severely restricted, we further ask, is there any "quantumness" generated in the system at early times by studying the early time behavior based on a mean-field approximation. Our results quantify the effect of noise on the quantum resources generated in the system and cast doubt on the effectiveness of NISQ devices.

84. Fault-tolerant measurement-based state preparation of logical states of the [[7,1,3]] code in the square and heavy-hexagonal lattices

Presenting Author: Andrea Rodriguez-Blanco, University of California Berkeley
Contributing Author(s): Ho Nam Nguyen, and Birgitta.K. Whaley

Conventionally, 2D topological quantum codes, such as the surface code, have been the preferred choice for quantum error correction (QEC) in protecting quantum information from errors. Their high tolerance to errors and the requirement of only local measurements of low-weight parity-checks have led to significant efforts in recent years to implement surface codes on current architectures. However, several issues limit the use of 2D topological codes for large-scale fault-tolerant quantum computation. One major limitation is that these codes require many physical qubits $n$ to encode a logical qubit $k$, and exponential logical error suppression by increasing $n$ lead to an enormous space overhead and a vanishing encoding rate $k/n$. Another challenge is the spatial locality barrier, with the code distance bounded by $d \leq n^{1/2}$. Alternatively, non-local QEC codes, such as quantum low-density parity-check (qLDPC) codes, can achieve a constant encoding rate $k\sim n$, and forgood qLDPC codes, even overcome the locality barrier with linear distance scaling $d \sim n$. However, a current challenge with these codes is the design and efficient implementation of universal logical operations required for fault-tolerant quantum computation. Another recent proposal for a non-local code that achieves fault-tolerant quantum computation with constant space and quasi-polylogarithmic time overhead is based on the concatenation of quantum Hamming codes of growing size. Both high-rate qLDPC codes and the new concatenated code protocol require long-range interactions, such as two-qubit gates between non-neighboring qubits, to achieve low overheads. These long-range connections can be engineered through the physical movement of qubits, as seen in trapped ions and neutral atoms, or through more advanced wiring of multiple 2D layouts in superconducting architectures. However, such long-range interactions may not always be advantageous, as they inevitably introduce time overhead in circuit implementation and may induce decoherence effects during qubit transport. Therefore, it is important to explore approaches where non-local interactions can be limited. In this work, we assume that high-rate concatenated codes can be embedded in 2D local architectures, with gates implemented only between neighboring qubits. We evaluate the performance of different level-1 fault-tolerant encoding circuits of the [[7,1,3]] quantum Hamming code mapped into two types of graphs: the square and heavy-hexagonal lattices. We present various fault-tolerant embedding strategies to overcome the connectivity constraints in these two lattices, analyze the trade-offs between space and time overheads for each approach and assess the logical fidelity of the resulting encoded states.

85. Quantum algorithms for calculating the expected value of #P-Hard stochastic programming problems

Presenting Author: Caleb Rotello, National Renewable Energy Laboratory
Contributing Author(s): Peter Graf, Matthew Reynolds, Eric B. Jones, Cody James Winkleblack, Wesley Jones

Two-stage stochastic programming is a problem formulation for decision-making under uncertainty. In the first stage, the actor makes a best "here and now" decision in the presence of uncertain quantities that will be resolved in the future, represented in the objective function as the expected value functiobn. This function is a multi-dimensional integral of the second stage optimization problem, which must be solved over all possible future scenarios. In practice, even approximating this expected value function is #P-Hard. This work uses a quantum algorithm to estimate the expected value function with a polynomial speedup. Our algorithm gains its advantage through the two following observations. First, by encoding the probability distribution as a quantum wavefunction in an auxilliary register, and using this register as control logic for a phase-separation unitary, Digitized Quantum Annealing (DQA) can converge to the minimium of each scenario in the random variable in parallel. Second, Quantum Amplitude Estimation (QAE) on DQA can calculate the expected value of this per-scenario optimized wavefunction, producing an estimate for the expected value function. Assuming the probability distribution wavefunction can be prepared efficiently, we conclude our method has a polynomial speedup over classical methods for estimating the expected value function, and show this by demonstrating the speedup of each sub-component is preserved.

Read this article online: https://arxiv.org/abs/2402.15029

86. Dynamics and Shielding in Many-Body Systems with Long-Range Interactions

Presenting Author: Shreyas Sadugol, Tulane University
Contributing Author(s): Lev Kaplan, Fausto Borgonovi, Giuseppe Celardo

Long-range interacting 1D quantum chains offer a powerful platform for simulating complex quantum systems and investigating novel quantum phases and critical phenomena. In this poster, we focus on the 1d Ising chain with long-range interactions to examine how various Hamiltonian parameters-such as coupling strengths, directions, and lengths-impact the propagation velocity and localization of perturbations. Our study delves into both short-range and long-range interactions to understand their influence on propagation characteristics and the role of long-range couplings in thermalization, shielding, and confinement. A particular emphasis is placed on quantitatively analyzing how long-range couplings can suppress propagation. To support our findings, we propose a semi-analytical model to describe the "light cone" velocity across various Hamiltonians, enhancing our understanding of "light cone" dynamics and quantum phase transitions.

87. Quantum Property Preservation

Presenting Author: Kumar Saurav, University of Southern California
Contributing Author(s): Daniel Lidar

We initiate the study of quantum property preservation. This problem arises in the context of open quantum systems subject to continuous control via a time-dependent Hamiltonian. The goal is to maintain a target property of the system by means of a smoothly varying control Hamiltonian. We develop a general theory to analyze this problem and formulate the characteristics of proper- ties that can be time-locally preserved and their associated control Hamiltonians. The theory has an intuitive geometric interpretation in terms of the level sets of the target property. We present solutions for various noise channels and target properties, which we classify as either trivially con- trollable, uncontrollable, or controllable. In the controllable case, we demonstrate the existence of control singularities and associated breakdown times, beyond which property preservation fails. The property preservation approach we develop is complementary to quantum error correction as it does not involve any ancilla qubits, nor does it rely on measurement and feedback. From a control theory perspective, our work addresses the task of tracking control for open quantum systems.

88. Microtubule quantum memristance and the topological Hall effect

Presenting Author: Kenton Schroeder, Wichita State University
Contributing Author(s): Anusha Krishna-Murthy, Elizabeth C. Behrman, James E.Steck

We present evidence that neuronal microtubule memristance operates according to a topological spin lattice, based on analysis of new work in topological physics and chemistry and illustrative calculations. We establish relevance to past theory of spin and antiferromagnets. Our work indicates that microtubule memristance is switching generated by the topological Hall Effect due to the presence of molecular chirality and the Dyzaloshinskii-Moriya interaction.

89. Superfluid Acoustic Analogs of Fiber Optic and Ring Laser Gyroscopes

Presenting Author: Keith Schwab, California Institute of Technology
Contributing Author(s): K.C. Schwab and M.P. Freeman

Longitudinal acoustic waves in superfluid $^4$He (first sound) propagate at the speed of sound relative to the fluid, and with extremely low loss at low temperatures. We have recently demonstrated acoustic attenuation lengths exceeding $600km$ at 8kHz and 40mK.\cite{de2017ultra} Due to macroscopic quantization, superfluid contained in a ring will remain fixed to the non-rotating frame while the container rotates. As a result, acoutsic waves traveling round the ring which are co-rotating or counter-rotating will arrive at a detector with a phase shift, resulting in an acoustic Sagnac effect. This approach may form acoustic analogues of the fiber-optic gyroscope (FOG) and the ring laser gyroscope. The low value of the speed of first sound compared to the speed of light creates long Sagnac time delays, and the ultra-low acoustic loss may make long path length sensing coils possible, both are key properties for high sensitivity to rotation. These effects form a possible alternate route to ultra-sensitive rotation sensors which does not require a junction structure (such as a Josephson junction for the superfluid) or the measurement of small, low frequency mass currents.

Read this article online: https://www.overleaf.com/read/bpkjskpdkgdn#b3032d

90. Quantum Computing with Strontium-87: New Capabilities for Neutral Atom Systems

Presenting Author: Enrique Segura Carrillo, University of Colorado JILA
Contributing Author(s): Eric J Meier, Leonardo de Melo, Sivaprasad T Omanakuttan, Vikas V Buchemmavari, Anupam Mitra, Ivan H Deutsch, Michael J Martin

Ultracold neutral atoms are promising candidates for quantum information applications due to their long coherence times, scalability, and flexibility in encodings and connectivity. Strontium-87's nuclear spin provides with a 10-dimensional nuclear spin manifold in its ground state, subspaces of which have been leveraged to realize a qubit. We report on our ongoing experimental efforts to implement a Strontium-based quantum computing architecture, based on quantum control and entanglement of large d-dimensional (up to d=10) nuclear spin qudits. Our work targets mid-circuit quantum non-demolition (QND) erasure detection and QND cooling of qudits, as well as single- and two-qudit gates based on quantum control.

91. Tight lower bounds on learning observables

Presenting Author: Akshay Seshadri, University of Colorado Boulder
Contributing Author(s):

With growing interest in developing large-scale quantum devices, learning properties of quantum systems such as expectation values has become increasingly important. Many studies derive bounds on the sample complexity for the learning task at hand by allowing a large class of measurements (e.g., all entangled measurements), or find worst-case bounds (e.g., over all observables with a bounded operator norm). However, in practice, for the task of estimating expectation values, one is often interested in learning specific observables using measurement protocols that are easy to implement on the underlying quantum computing architecture. Here, we focus on this problem and derive a lower bound on the error of learning an observable using a given non-adaptive measurement protocol. Our lower bound is characterized by a seminorm on observables, and is tight to within a factor of order 1 for confidence levels greater than 90%, saturated by a constructive method called the minimax method. Using these results, we show that for randomized measurements, the minimax method not only matches the performance of classical shadows, but also that there are many observables for which it can do exponentially better than classical shadows when implementing random Pauli measurements. More generally, our results can be used to analyze the performance of different measurement protocols.

92. Measuring gravitational redshift with spin-spin coupling

Presenting Author: Ruhi Shah, University of Waterloo
Contributing Author(s):

Gravitational redshift measurements and tests of Einstein's Equivalence Principle (EEP) are conducted using clocks at different heights in Earth's gravitational field. However, these experiments usually use separate coherence measurements at different heights that are then subtracted. We propose a scheme that uses dipole-dipole coupling to create multi-body correlations between two spatially separated spin ensembles. First, we use quantum field theory in curved space to show that gravitational redshift appears as an energy shift in the Zeeman Hamiltonian. Then, the dipole-dipole coupling between two spin ensembles at different gravitational potentials is used to encode the energy shift due to gravity as a coherence measurement. There is an improvement in the phase shift that scales with the size of each ensemble and comes from a single measurement of coherence. The notion of using multi-body correlations to enhance sensitivity to the gravitational field is also extended to a single spin ensemble in a spatial superposition, allowing for sensitivity to measurements of Newtonian gravity.

93. Hamiltonian Simulation in the Interaction Picture using the Magnus Expansion

Presenting Author: Kunal Sharma, IBM
Contributing Author(s): Minh Tran

We propose an algorithm for simulating the dynamics of a geometrically local Hamiltonian~$A$ under a small geometrically local perturbation~$\alpha B$. In certain regimes, the algorithm achieves the optimal scaling and outperforms the state-of-the-art algorithms. By moving into the interaction frame of $A$ and classically computing the Magnus expansion of the interaction-picture Hamiltonian, our algorithm bypasses the need for ancillary qubits. In analyzing its performance, we develop a framework to capture the quasi-locality of the Magnus operators, leading to a tightened bound for the error of the Magnus truncation. The Lieb-Robinson bound also guarantees the efficiency of computing the Magnus operators and of their subsequent decomposition into elementary quantum gates. These features make our algorithm appealing for near-term and early-fault-tolerant simulations.

Read this article online: https://arxiv.org/abs/2404.02966

94. Real-time Quantum Monte Carlo Algorithm for Open Quantum Systems

Presenting Author: Tong Shen, University of Southern California
Contributing Author(s): Daniel Lidar

We present a real-time stochastic approach based on density matrix quantum Monte Carlo (QMC) to simulate the dynamics of open quantum systems coupled to infinite-dimensional quantum baths. This approach stochastically samples the time-dependent density matrix of a many-body system evolving under a Markovian or non-Markovian master equation, enabling a comprehensive investigation of the dynamics of open quantum systems within the field of Hamiltonian quantum computing, including both gate-model quantum computing and quantum annealing. Through comparative analysis with exact solutions of quantum master equations and quantum trajectory methods, we demonstrate that QMC exhibits excellent agreement and showcases significant improvements in computational time and memory overhead. Additionally, the method's inherent ability to access the density matrix enables the efficient computation of various quantum information metrics, such as entanglement entropy and purity, during time evolution. As QMC is unconstrained by entanglement, this paves the way for larger scale simulations of various time-dependent system behaviors such as quantum quenches than is possible using alternative methods.

95. Security Assumptions in Dispersive-Optics QKD

Presenting Author: Ariel Shlosberg, University of New Mexico CQuIC
Contributing Author(s): Alex Kwiatkowski, Akira Kyle, Graeme Smith

Quantum key distribution (QKD) seeks to provide a method of generating cryptographically-secure keys between remote parties while guaranteeing unconditional security. Implementations of high-dimensional QKD using dispersive-optics (DO-QKD) have been proposed to allow for multiple secure bits to be transmitted per photon while remaining cost-effective and scalable using existing telecommunication technology [1]. In the recent literature, there have been a number of experimental realizations of DO-QKD systems [2-6], with security analysis based on the treatment in Ref. [1]. Here we demonstrate that in the case of finite dispersion, the model assumed for the eavesdropper's attack in Ref. [1] is non-optimal for the eavesdropper, which leads to a significant overestimation of the secure key rate between parties. We consider an alternative attack model that Alice and Bob find indistinguishable from the Ref. [1] model, as long as they are restricted to making the measurements typical in DO-QKD. We provide concrete examples where a significant gap exists between the Holevo information, and therefore the secret key rate, predicted by the two models. We further analyze the experiment in Ref. [2] as an example of a case where secure key is predicted according to the Ref. [1] model, but where in fact there is zero secure key rate when considering the full set of collective attacks that an eavesdropper may perform.

Read this article online: https://arxiv.org/pdf/2403.08992

96. Geometry of two-body correlations in three-qubit states

Presenting Author: Shravan Shravan, University of New Mexico CQuIC
Contributing Author(s): Simon Morelli, Otfried Guehne, Satoya Imai

The Bloch sphere provides a highly useful geometric characterization for single qubits. However, such a characterization for multipartite systems generally involves complex higher-dimensional objects that are not fully understood. In recent years, considerable efforts have been devoted to characterizing higher-dimensional state spaces using a restricted and accessible set of parameters. In our work, we study the specific case of the geometry of admissible two-body correlations in a three-qubit system using sector lengths, which are three local invariant coordinates based on the Bloch vector lengths of the marginal states. This approach relates to the quantum marginal problem, where properties of a global multipartite state can be inferred from the local properties of the parties and the correlations between a reduced number of parties. In our results, we first establish tight and optimal nonlinear bounds that are satisfied by all pure states and extend this by including three-body correlations. Second, we consider mixed states and conjecture a tight and optimal nonlinear bound for all three-qubit states. Finally, within the framework of sector lengths, we provide criteria for detecting different types of multipartite entanglement and compare these against similar witnesses. We also characterize the rank of the quantum state using the coordinates of our framework.

Read this article online: https://arxiv.org/abs/2309.09549

97. Scalable application-oriented benchmarking of quantum computers

Presenting Author: Noah Siekierski, Sandia National Laboratories
Contributing Author(s): Stefan Seritan, Timothy Proctor

Benchmarking quantum applications is an essential part of tracking progress towards quantum advantage and comparing different quantum devices on real-world problems. However, current application-oriented benchmarking approaches often lack scalability due to the need for expensive classical computation or are limited to testing full problem instances that may be too large to run on current devices. Here, we show how subcircuit volumetric benchmarking using mirror circuit fidelity estimation can be used to design scalable benchmarks from any quantum algorithm that uses only unitary operations. We demonstrate how to generate a subcircuit volumetric benchmark from an existing application-oriented benchmarking suite, compare the performance of real quantum devices on benchmarks created using our approach, and test the predictive power of our benchmarking protocol by using it to estimate the success of untested problem instances. This material is based upon work supported by the U.S. Department of Energy, Office of Science, National Quantum Information Science Research Centers. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.SAND2024-10400A

98. Demonstration of Algorithmic Quantum Speedup for an Abelian Hidden Subgroup Problem

Presenting Author: Phattharaporn Singkanipa, University of Southern California
Contributing Author(s): Victor Kasatkin, Zeyuan Zhou, Gregory Quiroz, and Daniel A. Lidar

Simon's problem is to find a hidden period (a bitstring) encoded into an unknown $2$-to-$1$ function. It is one of the earliest problems for which an exponential quantum speedup was proven for ideal, noiseless quantum computers, albeit in the oracle model. Here, using two different $127$-qubit IBM Quantum superconducting processors, we demonstrate an algorithmic quantum speedup for a variant of Simon's problem where the hidden period has a restricted Hamming weight $w$. For sufficiently small values of $w$ and for circuits involving up to $58$ qubits, we demonstrate an exponential speedup, albeit of a lower quality than the speedup predicted for the noiseless algorithm. The speedup exponent and the range of $w$ values for which an exponential speedup exists are significantly enhanced when the computation is protected by dynamical decoupling. Further enhancement is achieved with measurement error mitigation. This constitutes a demonstration of a bona fide quantum advantage for an Abelian hidden subgroup problem.

Read this article online: https://arxiv.org/abs/2401.07934

99. A dissipation-induced superradiant transition in a strontium cavity-QED system

Presenting Author: Eric Song, University of Colorado JILA
Contributing Author(s): Diego Barberena, Dylan J. Young, Edwin Chaparro, Anjun Chu, Sanaa Agarwal, Zhijing Niu, Jeremy T. Young, Ana Maria Rey, James K. Thompson

Superradiance refers to the emission phenomenon where the collective emission rate is faster than the simple sum of individual ones'. Today, superradiance has been demonstrated in a variety of platforms ranging from solid-state platforms to cold atoms. A natural next step would be to explore what happens when one coherently continuously drives such a system. This was investigated in the Cooperative Resonance Fluorescence (CRF) model about 45 years ago [1], and a continuous superradiant transition was predicted arising from the competition between the drive and collective dissipation - superradiance. Here we provide a clean implementation of the model in a strontium (Sr) cavity-QED system. We trap 88Sr atoms in a high-finesse optical cavity with the closest TEM00 mode tuned to resonance with the narrow-linewidth 1S0-3P1 transition and drive the atoms on resonance with the same optical transition. We show that at low incident fields (i.e. the superradiant phase), the superradiant field cancels the incident drive field, and atoms sustain a non-zero dipole moment with a zero intra-cavity field; at high incident fields (the normal phase), atoms start to Rabi flop and have a zero time-averaged dipole moment. We find that spontaneous emission can melt the continuous transition to a first-order transition and lead the system to a different steady state. Utilizing the flexible tunability of our cavity-QED system, we also explore how elastic interactions between atoms can change the properties of the phase transition. This opens the door for exploring new symmetries in open quantum systems and for new ways of generating spin squeezing. [1] Carmichael, H. J. (1980). Analytical and numerical results for the steady state in cooperative resonance fluorescence. Journal of Physics B: Atomic and Molecular Physics, 13(18), 3551.

100. Quantum neural network training of a repeater node

Presenting Author: James Steck, Wichita State University
Contributing Author(s): Diego Fuentealba, Jackson Dahn, Elizabeth Behrman

The construction of robust and scalable quantum gates is a uniquely hard problem in the field of quantum computing. Real-world quantum computers suffer from many forms of noise, characterized by the decoherence and relaxation times of a quantum circuit, which make it very hard to construct efficient quantum algorithms. One example is a quantum repeater node, a circuit that swaps the states of two entangled input and output qubits. Robust quantum repeaters are a necessary building block of long-distance quantum networks. A solution exists for this problem, known as a swap gate, but its noise tolerance is poor. Machine learning may hold the key to efficient and robust quantum algorithm design, as demonstrated by its ability to learn to control other noisy and highly nonlinear systems. Here, a quantum neural network (QNN) is constructed to perform the swap operation and compare a trained QNN solution to the standard swap gate. The system of qubits and QNN is constructed in MATLAB and trained under ideal conditions before noise is artificially added to the system to test robustness. We find that the QNN easily generalizes for two qubits and can be scaled up to more qubits without additional training. We also find that as the number of qubits increases, the noise tolerance increases with it, meaning a sufficiently large system can produce extremely noise-tolerant results. This begins to explore the ability of neural networks to construct those robust systems.

101. Efficient quantum linear solver algorithm with detailed running costs

Presenting Author: Yigit Subasi, Los Alamos National Laboratory
Contributing Author(s): David Jennings, Matteo Lostaglio, Sam Palister, Andrew T Sornborger

As we progress towards physical implementation of quantum algorithms it is vital to determine the explicit resource costs needed to run them. Solving linear systems of equations is a fundamental problem with a wide variety of applications across many fields of science, and there is increasing effort to develop quantum linear solver algorithms. Subasi et al. proposed an algorithm inspired by adiabatic quantum computing, based on a sequence of random Hamiltonian simulation steps, with scaling $O(\kappa \log(\kappa)/\epsilon)$ in the condition number $\kappa$ of the linear system and the target error $\epsilon$. This algorithm has recently been improved to have a scaling linear in the condition number and logarithmic scaling in the error by Cunningham and Roland, which is known to be optimal. We optimize many aspects of this algorithm to minimize the running cost as much as possible. In addition, we present a variant that avoids Hamiltonian simulation, which should be advantageous when the matrix is accessed using a block-encoding. A thorough non-asymptotic analysis of all cost contributions leads to a closed formula for the expected query complexity $Q$ of the algorithm as a function of $\kappa$, $\epsilon$ and the block-encoding scaling factor $\alpha$. Finally, we analyze the cost numerically for low-dimensional instances and compare the performance of our algorithm with other proposals in the literature.

Read this article online: https://arxiv.org/abs/2305.11352

102. Indirect initialization and readout of trapped-ion qubits using a magnetic gradient

Presenting Author: Tyler Sutherland, Oxford Ionics
Contributing Author(s): None

We propose a mixed-species geometric phase gate that maps the magnetic quantum number mf of a spin non-zero 'qubit' ion onto the spin-flip probability of a spin-zero 'readout' ion. Repeatedly implementing the gate then measuring the readout ion allows us to determine mf for the qubit. Importantly, the implementation involves no transitions on the qubit. This means the probability that the gate results in a qubit state-change error can be made vanishingly small, giving the scheme significant potential for high fidelity operation.

103. Quantum Fisher Information Compression

Presenting Author: Rui Jie Tang, University of Toronto
Contributing Author(s): Jeremy Marcus, Noah Lupu-Gladstein, Arthur Pang, Giulio Chiribella, Aephraim Steinberg, Batuhan Yilmaz

Quantum metrology aims to optimize the measurement of unknown parameters encoded in quantum states through their interaction with quantum systems. When multiple copies of such states are available, quantum state tomography can be employed to analyze measurement statistics and accurately infer these parameters. The precision of this inference is fundamentally limited by the Cramér-Rao Bound, which dictates the minimum possible variance of the estimate. Measurement uncertainty in quantum systems is determined by the sensitivity to the unknown parameter, quantified by the Quantum Fisher Information (QFI). In this work, we report two linear optical experiments that demonstrate a significant breakthrough: for a qubit in an equatorial state with an unknown phase parameter, the entire sensitivity and QFI from multiple copies of these states can be compressed into a single qubit, along with a logarithmic amount of classical information. This discovery reveals the ultimate limit of information compression in quantum metrology, with implications for the efficiency of quantum measurements and data processing.

104. Theoretical Approximation Ratios for QAOA on 3-Regular Max-Cut Instances at Depth p=1

Presenting Author: Reuben Tate, Los Alamos National Laboratory
Contributing Author(s): Stephan Eidenbenz

We generalize Farhi et al.'s 0.6924-approximation result technique of the Max-Cut Quantum Approximate Optimization Algorithm (QAOA) on 3-regular graphs to obtain provable lower bounds on the approximation ratio for warm-started QAOA. Given an initialization angle theta, we consider warm-starts where the initial state is a product state where each qubit position is angle theta away from either the north or south pole of the Bloch sphere; of the two possible qubit positions the position of each qubit is decided by some classically obtained cut encoded as a bitstring b. We illustrate through plots how the properties of b and the initialization angle theta influence the bound on the approximation ratios of warm-started QAOA. We consider various classical algorithms (and the cuts they produce which we use to generate the warm-start). Our results strongly suggest that there does not exist any choice of initialization angle that yields a (worst-case) approximation ratio that simultaneously beats standard QAOA and the classical algorithm used to create the warm-start. Additionally, we show that at theta=60 degrees, warm-started QAOA is able to (effectively) recover the cut used to generate the warm-start, thus suggesting that in practice, this value could be a promising starting angle to explore alternate solutions in a heuristic fashion.

Read this article online: https://arxiv.org/abs/2402.12631

105. A hybrid network link between source and memory node at telecom wavelength

Presenting Author: Nayana Tiwari, University of Chicago
Contributing Author(s): Dahlia Ghoshal, Yuzhou Chai, Alexander Kolar, Tian Zhong, Hannes Bernien

As quantum platforms reach scalability limitations, distributed computing and photonic interconnects become necessary for larger system sizes. However, interfacing various quantum nodes over large distances can require lossy quantum frequency conversion processes to telecom wavelengths where optical fibers have the lowest loss. Here take an alternative approach and design a hybrid network architecture that leverages the advantages of different quantum platforms while communicating at the same telecom wavelength. We are simultaneously developing a warm atomic ensemble node to produce heralded single photons and a rare-earth ion-doped solid state crystal node to coherently store photons through an atomic frequency comb protocol. Due to our engineered choice of atom and crystal, we can match an excited state telecom transition in the rubidium source by tuning the resonance of the erbium-doped yttrium orthovanadate memory using a low external magnetic field. We report on our recent progress towards photon storage and retrieval. Additionally, we describe plans to demonstrate the hybrid connection with entangled photons and extend the link over the Chicago-area quantum network.

106. Loss-tolerant implementations of controlled-phase gates in photonic quantum computing through dynamical squeezing

Presenting Author: Ankit Tiwari, Arizona State University
Contributing Author(s): Saikat Guha, Christian Arenz.

Photonic quantum systems are among the most promising platforms for realizing a universal quantum computer. However, the non-linear processes required to implement controlled phase gates that are key for universal quantum computing are typically weak compared to the characteristic energy scales of photonic systems. As a result, the deterministic implementation of controlled phase gates remains challenging in such systems before photon losses and decoherence take over. We address this issue by utilizing a recently developed squeezing protocol to speed up the implementation of controlled phase gates based on amplifying cross-Kerr interactions. We show that by alternating between squeezing along different quadrature, controlled phase gates can be implemented several orders of magnitude faster while simultaneously outperforming photon losses and decoherence. We develop bounds for the associated gate error and discuss potential implementations in nano photonic waveguides.

107. Development of a rotational ramsey interferometer for measuring symmetrization of 40 Ca+

Presenting Author: Ryan Tollefsen, University of California Berkeley
Contributing Author(s): Neil Glikin, Yu-Lung Tang, Neha Yadav, Hartmut Haeffner

We present a specialized surface-electrode Paul trap which generates a cylindrically symmetric pseudopotential. When two ions are loaded into this trap, they form a small Coulomb crystal which behaves like a planar quantum rotor. By preparing the rotor in a state of high angular momentum, we are able to singly address rotational sidebands and create coherent superpositions of angular momentum quanta. We propose this system can be used as a rotational Ramsey interferometer. By cooling the correct motional modes of the rotor, symmetrization becomes observable in the rotational degree of freedom. This results in rephasing after the rotational states in superposition undergo a pi relative rotation, contingent on the ions' indistinguishability. In other words, we can physically realize a particle exchange operation. If the phase of recombination is measurable, this experiment could verify that 40 Ca+ are fermions. The current challenge lies in achieving a pi-relative rotation within the superposition's short coherence time. Here, we present new forms of control over our ion-rotor and progress in decreasing the amount of time necessary for ion recombination.

108. Empirical optimization of dynamical decoupling on quantum processors

Presenting Author: Christopher Tong, Massachusetts Institute of Technology
Contributing Author(s): Helena Zhang, Swarnadeep Majumder, Derek Wang, Luke Govia, Bibek Pokharel

Dynamical decoupling (DD) is a low-overhead error suppression method for quantum computers that has become an essential part of the experimentalist toolkit. While there is mature literature on theoretically derived DD sequences, tailoring sequences to the quantum device and task at hand is non-trivial. We use a genetic algorithm to empirically learn DD strategies, from an exponentially large search space, that significantly outperform canonical DD sequences on the 27-qubit Bernstein-Vazirani algorithm, the 50-qubit GHZ state preparation circuit, and the 80-qubit mirrored randomized benchmarking protocol, with the advantage persisting over long periods of time. Our method is scalable due to its quick convergence and constant runtime independent of circuit size and adaptable to different classes of problems whether or not the solution is known a priori. Finally, we demonstrate the effectiveness of empirical DD optimization on the semiclassical quantum Fourier transform algorithm, which involves mid-circuit measurements followed by classical information processing, and show that our method successfully suppresses errors on dynamic circuits with mid-circuit measurement randomized benchmarking.

Read this article online: https://arxiv.org/abs/2403.02294

109. Shaded lightcones for classically accelerated quantum error mitigation

Presenting Author: Minh Tran, IBM
Contributing Author(s): Patrick Rall, Andrew Eddins

Quantum error mitigation (QEM) can recover accurate expectation values from a noisy quantum computer by trading off bias for variance, such that an averaged result is more accurate but takes longer to converge. Probabilistic error cantcellation (PEC) stands out among QEM methods as a robust means of controllably eliminating bias. However, PEC often exhibits a much larger variance than other methods, inhibiting application to large problems. Recent analyses have shown that the variance of PEC can be reduced by not mitigating errors lying outside the causal lightcone of the desired observable. Here, we improve the lightcone approach by classically computing tighter bounds on how much each error channel in the circuit can bias the final result. This set of bounds, which we refer to as a "shaded lightcone," enables a more targeted application of PEC, improving the tradespace of bias and variance. Although a tight shaded lightcone is exponentially hard to compute, we present an algorithm providing a practical benefit even with modest classical resources, leveraging the ease of evolving an error instead of the state or observable. The algorithm reduces the time that would be needed to apply PEC in an example 127-qubit Trotter circuit by more than an order of magnitude compared to standard lightcone-PEC, unlocking the possibility of computing classically-difficult circuits on noisy quantum hardware with controlled error bounds.

110. Magnetic control of quantum systems towards quantum states of motion

Presenting Author: Jason Twamley, Okinawa Institute of Science and Technology
Contributing Author(s): S. Bose, GK. Brennen, C. Cusicanqui, JE Downes, M. Hatifi, GC Hermosa, A Hodges, D. Kim, K. Jadeja, R. Lecamwasam, A. Nayak, S. Raman-Nair, S. Tian

We show how magnetic forces can be used to engineer mechanical degrees of freedom towards the quantum regime. We describe recent experimental work where we developed a highly diamagnetic, electrically insulating material, which, when levitated, exhibits exceptionally high Q-factors due to minimal eddy current damping. We perform feedback cooling using magnetic actuation to reduce the centre of mass temperature by x1000 [1]. This paves the way towards cooling macroscopic (~mm), objects close to the quantum regime and to engineer macroscopic quantum motional states which can be useful for quantum sensing or testing gravitationally induced entanglement. For the latter we also propose two schemes to produce ultra-large macroscopic quantum superpositions using spin-mechanical coupling actuated by quantum magnetic forces produced by superconducting flux qubits acting on a magnetically levitated YIG particle or when the entire flux qubit is levitated [2]. Finally, we propose a new platform for quantum thermodynamic engines based on magnetic spin-mechanical coupling of NV defects in diamond where the working fluid is the motion, permitting real work to be extracted and energy can be stored [3]. [1] Feedback cooling of an insulating high-Q diamagnetically levitated plate, S. Tian, K. Jadeja, D. Kim, A. Hodges, GC. Hermosa, C. Cusicanqui, R. Lecamwasam, JE. Downes, J. Twamley, Appl. Phys. Lett. 124, 124002 (2024). [2] Massive quantum superpositions using magneto-mechanics, S. Raman-Nair, S. Tian, GK. Brennen, S. Bose, J. Twamley, arXiv: 2307.14553 [3] Diamond quantum heat engines, A. Nayak, M. Hatifi, J. Twamley, in preparation.

Read this article online: https://pubs.aip.org/aip/apl/article/124/12/124002/3275770/Feedback-cooling-of-an-insulating-high-Q?searchresult=1

111. Direct characterization of Gaussian quantum processes with Gaussian resources

Presenting Author: Kevin Valson Jacob, Wheaton College, IL
Contributing Author(s): Logan Grove, Pratik Barge

We develop an efficient procedure to fully characterize arbitrary Gaussian quantum processes. This is done by directly obtaining all elements of the symplectic matrix and of the displacement vector that describe the process. The procedure utilizes a well-characterized coherent probe and employs quadrature detection methods. The procedure involves $O(N^2)$ steps to characterize an $N$-mode Gaussian quantum process. We show how the method naturally simplifies in the case of linear optics. We further demonstrate that the procedure is resilient to loss, and that such loss can be characterized without any modification to our proposed method. We simulate this procedure using the Python package Strawberry Fields, and demonstrate that the reconstructed symplectic matrix closely approximates the actual matrix.

112. Virtual Z gates and symmetric gate compilation

Presenting Author: Arian Vezvaee, University of Southern California
Contributing Author(s): Vinay Tripathi, Daria Kowsari, Eli Levenson-Falk, Daniel A. Lidar

The virtual Z gate is a crucial tool for performing quantum gates across various platforms, including superconducting systems. where it is often combined with a set of basis-set gates (e.g., X gate) to compile other types of gates (such as Y gate). In this work, we show that the compilation method significantly impacts outcomes in open quantum systems. We experimentally demonstrate the importance of symmetric compilation with respect to virtual Z gates. Our findings indicate that asymmetric compilation-often being the default choice in cloud-based quantum devices-leads to an asymmetry in the fidelity between orthogonal quantum states in the (x,y) plane of the Bloch sphere. Moreover, symmetric compilation is critical for correct implementation of dynamical decoupling (DD) sequences, as asymmetric compilation yields DD sequences that differ from the intended ones. Our findings call for caution when interpreting previous studies that did not use symmetric compilation. Additionally, by using the correct experimental implementation of DD sequences, we uncover an important effect: pulse interference. We show that, beyond the well-known coherent errors that affect DD sequences, the overlap of consecutive pulses can also cause oscillations in the fidelity of qubits subject to DD. We further demonstrate a simple and easily implementable solution to mitigate this effect.

Read this article online: https://arxiv.org/abs/2407.14782

113. Searching weighted barbell graphs with Laplacian and adjacency quantum walks

Presenting Author: Thomas Wong, Creighton University
Contributing Author(s): Jonas Duda

A quantum particle evolving by Schrödinger's equation in discrete space constitutes a continuous-time quantum walk on a graph of vertices and edges. When a vertex is marked by an oracle, the quantum walk effects a quantum search algorithm. Previous investigations of this quantum search algorithm on graphs with cliques have shown that the edges between the cliques can be weighted to enhance the movement of probability between the cliques to reach the marked vertex. In this paper, we explore the most restrictive form of this by analyzing search on a weighted barbell graph that consists of two cliques of the same size joined by a single weighted edge/bridge. This graph is generally irregular, so quantum walks governed by the graph Laplacian or by the adjacency matrix can differ. We show that the Laplacian quantum walk's behavior does not change, no matter the weight of the bridge, and so the single bridge is too restrictive to affect the walk. Similarly, the adjacency quantum walk's behavior is unchanged for most weights, but when the weight equals the size of a clique, the success probability is boosted from 0.5 to 0.820, independent of the size of the barbell graph.

114. Observing dynamical phases of BCS superconductors in a cavity QED simulator

Presenting Author: Dylan Young, University of Colorado JILA
Contributing Author(s): Anjun Chu, Eric Yilun Song, Diego Barberena, David Wellnitz, Zhijing Niu, Vera M. Schäfer, Robert J. Lewis-Swan, Ana Maria Rey, and James K. Thompson

Out-of-equilibrium dynamics in many-body systems can exhibit rich behavior not found in equilibrium systems. In particular, certain non-dissipative systems quenched out of equilibrium can experience distinct "dynamical phases" and corresponding phase transitions as either Hamiltonian parameters or initial conditions are tuned. The Bardeen-Cooper-Schrieffer (BCS) model of superconductivity provides a notable example: studies of this model predict the existence of three dynamical phases when a superconductor in its ground state experiences a sudden quench in interaction strength. Despite this, experimental observation of all three phases in condensed matter systems has yet to be realized. Following a previous theory proposal, we present results simulating the BCS model using an ensemble of 88Sr atoms subject to cavity-mediated spin-exchange interactions. By tuning both the interaction strength and the shape of the single-particle energy distribution, we observe three dynamical phases and identify them with the predicted dynamics in a quenched BCS superconductor. Additionally, we present preliminary work studying similar gap-protection physics in an extended system featuring three internal atomic levels. We show how this related physics can be interpreted as gap protection of the orientation of the collective electric dipole moment.

Read this article online: https://www.nature.com/articles/s41586-023-06911-x

115. A Dual-rail Qubit with Parametrically Coupled Transmons

Presenting Author: Tongyu Zhao, National Institute of Standards and Technology, Boulder
Contributing Author(s): Xiaoyue Jin, Katarina Cicak, Sudhir Sahu, John Teufel, Raymond Simmonds

Quantum error correction (QEC) is crucial to developing fault-tolerant quantum computers. Despite experiments on different platforms reporting physical error rates below the surface code threshold, it is still of great interest to explore QEC protocols with higher efficiency to reduce overhead. Erasure qubit is proposed in the light of such pursuit. The concept behind erasure qubits is the conversion of the dominant error of the physical qubits into leakage out of computational subspace, which can be efficiently detected and corrected with appropriate system design. To demonstrate the potential of such scheme, we introduce erasure qubit in which the logical qubit is encoded in the single-photon subspace of two parametrically coupled transmons, i.e. dual-rail encoding. In this talk, we will present how the design of our device as well as the dual-rail encoding largely improves the coherence of the erasure qubit, and we will show that parametric interaction between the transmons allows high fidelity single-qubit gate within the logical subspace.

116. Quantum Technology Master's Internship Program at University of Oregon: Hands-on Training for Future Quantum Engineers

Presenting Author: Nikolay Zhelev, University of Oregon
Contributing Author(s): Benjamin Aleman, Hailin Wang, Brian Smith, David Allcock, Steven van Enk

As Quantum Industry scales up on the path towards maturity, it is becoming increasing clear that there needs to be a more robust talent pipeline apart from the current one that relies primarily on PhD level workforce [1,2]. Following the successful model of the already established master's internship programs at University of Oregon [3,4], we have designed a new five academic quarters Quantum Technology Master of Science program. The program incorporates two academic quarters of hands-on practical classes tailored for the skills needed for the quantum industry and pairs the students with industry or national labs partners for internships for as much as three academic quarters. In this poster, we discuss the practical skills we focus our curriculum on based on our conversations and feedback from the quantum ecosystem leaders and outline the choices we have made in designing our curriculum.

1. M.F. J. Fox, B.M. Zwickl, and H. J. Lewandowski, Preparing for the quantum revolution: What is the role of higher education?, Phys. Rev. Phys. Educ. Res. 16, 020131 (2020).
2. C.D. Aiello et. al., Achieving a quantum smart workforce, Quantum Sci. Technol. 6 030501 (2021).
3. https://internship.uoregon.edu
4. https://electrochemistry.uoregon.edu

117. Emergent Non-Markovian Dynamics in Logical Qubit Systems

Presenting Author: Jalan Ziyad, University of New Mexico CQuIC
Contributing Author(s): Tzvetan Metodi, Robin Blume-Kohout, Kenneth Rudinger

Simulations of error-corrected logical qubits have demonstrated that logical qubit dynamics can exhibit non-Markovian behavior, even when the underlying physical noise is Markovian. Such non-Markovian dynamics present challenges to holistic logical qubit characterization and can also non-trivially degrade the performance of operational tasks performed on error-corrected quantum computers. To understand this emergent non-Markovianty, we construct a method for mapping arbitrary Markovian physical qubit dynamics to logical qubit dynamics. Examining a particular form of Markovianity relevant to gates, we provide illustrative examples in small quantum codes to explain these non-Markovian phenomena. Finally, we use measures of non-Markovianity to determine sufficient conditions for the use of gate-based characterization techniques (such as gate set tomography) in early fault-tolerant quantum devices.

118. Efficient classical simulation of quantum computation beyond Wigner positivity

Presenting Author: Michael Zurel, University of British Columbia
Contributing Author(s): Arne Heimendahl

We present the generalization of the CNC formalism, based on closed and noncontextual sets of Pauli observables, to the setting of odd-prime-dimensional qudits. By introducing new CNC-type phase space point operators, we construct a quasiprobability representation for quantum computation which is covariant with respect to the Clifford group and positivity preserving under Pauli measurements, and whose nonnegative sector strictly contains the subtheory of quantum theory described by nonnegative Wigner functions. This allows for a broader class of magic state quantum circuits to be efficiently classically simulated than those covered by the stabilizer formalism and Wigner function methods.

Read this article online: https://arxiv.org/pdf/2407.10349