Optical phase estimation approaching the quantum limit in a single shotOptical phase estimation approaching the quantum limit in a single shot

Optical phase estimation approaching the quantum limit in a single shot

Optical phase estimation is at the center of many metrological tasks where the value of a physical parameter of a system is mapped to the phase of an electromagnetic field, and single-shot measurements of this phase retrieve the information of this parameter. In this article, we demonstrate optimized estimation strategies for single-shot measurements for the optical phase of coherent states, which achieve sensitivities surpassing the heterodyne limit and potentially approaching the quantum limit, the Cramer-Rao lower bound (CRLB). These strategies are based on optimized photon counting, coherent displacement operations, and fast feedback. Our demonstration uses fast processing for optimizing the single-shot measurement during the optical mode, and enables surpassing the heterodyne limit for a wide range of optical powers without correcting for detection efficiency of our system. This is, to our knowledge, the most sensitive single-shot measurement of an unknown phase encoded in optical coherent states

This research has been published in Physical Review Letters, and can be accessed in https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.120505

Optical phase estimation approaching the quantum limit in a single shot

(a) The input state and optimized local oscillator (LO) interfere on a beam splitter such that the input is displaced in phase space. The probability distribution for the unknown phase is then updated according to Bayes rule and new optimal values for the LO are applied. (b) Experimental data (points with error bars) shows that adaptive non-Gaussian measurements surpass the limits of ideal heterodyne detection (dashed red) and approach the fundamental sensitivity limit given by the CRLB for our detection efficiency of 70% (solid black)

Dynamical Phase Transitions

Using measurement and feedback control to simulate complex mean field spin dynamics in quantum systems

In physical systems composed of many parts, like ultracold atomic ensembles, electrons in a superconductor, or arrays of superconducting circuits, complex dynamics emerges when the different parts of the system interact with each other. Even though typical low-energy interactions found in nature are of two-body nature, considering higher order interactions can lead to novel complex phenomena which could be studied in quantum simulators. In fact, achieving programmable simulation of many-body interactions is a central goal of near-term quantum information processing devices.

Recent work from CQuIC led by PhD student Manuel Muñoz-Arias and FRHTP postdoc Pablo Poggi together with Professors Poul Jessen and Ivan Deutsch, demonstrated the viability of combining quantum measurements and feedback control to program the simulation of a family of spin systems called “p-spin models” which exhibit complex nonlinear dynamics related to the mean-field interaction of p bodies. [1].  In particular it was shown how this simulation scheme can be used to investigate the emergence of phenomena such as dynamical phase transitions, which are drastic changes on the macroscopic motion of a system as a single parameter is varied, and spontaneous symmetry breaking in adiabatic evolution induced by measurement. The proposal is particularly suited to explore such signatures of critical phenomena in simple systems such as ensembles of utlracold atoms subject to global measurements and control. The work gives a fresh twist to the well-established toolbox of quantum feedback control, previously studied in quantum optics, and extend this tool to explore a broad scope of physical phenomena.

This research has been published in Physical Review A, and can be accessed in https://journals.aps.org/pra/abstract/10.1103/PhysRevA.102.022610

Dynamical Phase Transitions
Simulation of dynamical phase transitions in the mean field dynamics using quantum measurements and feedback (black and orange symbols). (a) and (c) show long-time average magnetization as a function of the external field. Sharp change indicates the phase transition. (b) and (d) show the same feature for a long-time average expectation value of a two-body operator. Results are to be comparead with the exact mean-field solution (solid red line)

[1] Manuel H. Muñoz-Arias, Ivan Deutsch, Poul Jessen, Pablo Poggi, “Simulation of the complex dynamics of mean-field p-spin models using measurement-based quantum feedback control”, Phys. Rev. A 102, 022610 (2020)

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