Phase tracking for sub-shot-noise-limited receivers

Phase trackingNonconventional receivers for phase-coherent states based on non-Gaussian measurements such as photon counting surpass the sensitivity limits of shot-noise-limited coherent receivers, the quantum noise limit (QNL). These non-Gaussian receivers can have a significant impact in future coherent communication technologies.

However, random phase changes in realistic communication channels, such as optical fibers, present serious challenges for extracting the information encoded in coherent states.

While there are methods for correcting random phase noise with conventional heterodyne detection, phase tracking for non-Gaussian receivers surpassing the QNL is still an open problem.

Here we demonstrate phase tracking for non-Gaussian receivers to correct for time-varying phase noise while allowing for decoding beyond the QNL.
The phase-tracking method performs real-time parameter estimation and correction of phase drifts using the data from the non-Gaussian discrimination measurement, without relying on phase reference pilot fields.

This method enables non-Gaussian receivers to achieve higher sensitivities and rates of information transfer than ideal coherent receivers in realistic channels with time-varying phase noise.

This demonstration makes sub-QNL receivers a more robust, feasible, and practical quantum technology for classical and quantum communications.

See the article

Adiabatic rapid passage facilitates robust entangling gates for neutral atoms

Top: the sequence using spin echo and rapid adiabatic passage to implement a Mølmer-Sørensen gate. Bottom: loss of gate fidelity, measured as gate infidelity on a log scale for different levels of imperfections in the experiment; the flat surface shows the robustness to errors due to atomic motion, finite temperature and other sources of error.

Anupam Mitra, Pablo Poggi and Ivan Deutsch collaborating with Michael Martin, Grant Biedermann and Alberto Marino at Sandia National Laboratories, Los Alamos National Laboratory and the University of Oklahoma have published a Rapid Communication in the journal Physical Review A proposing a way to implement a robust two-qubit entangling gate, the Mølmer–Sørensen gate, for neutral atoms using rapid adiabatic Rydberg dressing.

Neutral atoms, like their charged ion counterparts, are considered to be a promising platform for scalable quantum computation, allowing for virtually perfect single qubit operations on extremely coherent neutral atom quantum bits, which are at the heart of ultra-precise atomic clocks.  On the other hand, generating entanglement between neutral atoms has proven to be more challenging. Proposals for achieving two-qubit entangling gates are based on accessing their highly excited Rydberg states, which typically have strong interactions with other atoms. However, exciting to Rydberg states leads to many challenges like finite radiative lifetime of Rydberg states and loss of quantum coherence due to motion induced dephasing.

In this work, the authors propose using a rapid adiabatic passage from a clock state to a Rydberg state and back to introduce a nonlocal two-atom dynamical phase rapidly compared to Rydberg radiative lifetimes. Moreover, they show that the resulting entangling gate is robust to imperfections in the experiment like laser frequency detuning, laser amplitude, finite atomic motion and imperfect Rydberg blockade. They show the dominant contribution to the errors in implementing a two-qubit entangling gate is from the single-qubit component of a two-qubit entangling gate. Using a simple spin-echo combined with rapid adiabatic passage, they show how a pure entangling gate, the Mølmer–Sørensen gate can be implemented with high fidelity over a large range of experimental imperfections.  Analogous to the use in atomic ion based quantum logic, the Mølmer-Sørensen gate for neutral atom based quantum logic can affectively mitigate the errors due atomic motion and finite temperature.

This serves as a milestone towards developing large-scale quantum computers with neutral atoms. The full article can be found online at https://link.aps.org/doi/10.1103/PhysRevA.101.030301

A Zhao Blurb Figure

Reducing the measurement complexity of variational quantum algorithms

A Zhao Blurb Figure

Andrew Zhao and Akimasa Miyake, with collaborators from Tufts University and Caltech, have published a paper in Physical Review A in which they propose a technique to reduce the number of circuit repetitions required to implement variational quantum algorithms. Such algorithms are designed for the hardware limitations of present-day quantum computers. Instead of performing one long, sophisticated quantum circuit, variational algorithms repeatedly run and measure the outputs of shorter, more feasible circuits, and then use classical optimization techniques to arrive at a solution. Typically, one uses these algorithms to calculate physical quantities, such as the energy of a chemical system.

However, the number of variational circuit repetitions required can be overwhelming, especially when simulating chemistry. To address this issue, the paper introduces a method to group together compatible measurements which may be performed simultaneously, in exchange for slightly longer circuits. To demonstrate the practical effectiveness of this technique, the authors present numerical calculations in the context of simulating various chemical molecules. They observe a roughly tenfold reduction in the number of required measurements, and they furthermore confirm this effectiveness in general with analytical proofs.

The full article can be found online at https://link.aps.org/doi/10.1103/PhysRevA.101.062322.

Simulation Method

Measurement-based feedback control enables quantum simulation of the chaotic quantum-to-classical transition

Feedback Control diagram

In classical mechanics chaos in a dynamical system is related to the unpredictability arising from high sensitivity to the initial configuration. The question of how this behavior, based on the notion of trajectories on phase space, is recovered from the macroscopic limit of the dynamics of quantum systems is a long standing question in theoretical physics. About 20 years ago, in pioneering work, a group of scientists at Los Alamos National Laboratories [1] investigated the role of quantum measurement as the mechanism enabling the definition of quantum trajectories on phase space, hence allowing  the emergence of chaos from quantum dynamics.

Recently a team from CQuIC, led by PhD student Manuel Muñoz and FRHTP postdoc Pablo Poggi, with Project Directors Poul Jessen and Ivan Deutsch, proposed a novel scheme which uses quantum measurement and feedback control to simulate complex dynamics, such as those of chaotic systems [2]. Using the proposed protocol to simulate the dynamics of a quantum kicked top and its mean-field version they showed how the scheme recovers the correct classical chaotic dynamics. The proposed scheme presents a suitable platform to explore the emergence of chaotic “quantum trajectories” featuring the correct classical limit, a problem which is still an open challenge. To strengthen this point the authors studied the proposed scheme in the context of a free space atom-light interface, and showed how in presence of model decoherence and experimental parameters as those in state of the art experiments, the proposed simulation scheme gives access to the correct chaotic classical limit. Thus, this proposal paves the way for the potential experimental observation of the chaotic quantum-to-classical transition.

Given the flexibility of the proposed simulation scheme they expect to see a broad range of applications in the study of quantum simulation of complex systems in the near future.This research was published in Physical Review Letters, and can be accessed in https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.11050

[1] Tanmoy Bhattacharya, Salman Habib, and Kurt Jacobs, Phys. Rev. Lett. 85, 4852 (2000)

[2] Manuel H. Muñoz-Arias,  Pablo M. Poggi, Poul S. Jessen, and Ivan H. Deutsch, Phys. Rev. Lett. 124, 110503 (2020)