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)

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

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)

Deutsch receives APS Five Sigma Physicist Award

In April, 2019, the American Physical Society (APS) awarded Regents’ Professor and CQuIC Director, Ivan Deutsch the APS Five Sigma Physicist Award, “a pin awarded to APS members who exhibit outstanding advocacy work in addressing science policy issues.”  Prof. Deutsch was selected as an award recipient because of his “multiple advocacy actions” and communication with the APS Office of Government Affairs throughout the year.  Prof. Deutsch’s service was especially impactful in:

  • Meeting with U.S. Senator Tom Udall’s staff concerning problematic language in the original National Quantum Initiative (NQI) Act
  • Stating a willingness to volunteer for additional meetings to ensure that language in the NQI did not negatively impact other research programs

His advocacy contributes “a vital role” how the public views the (APS) discipline.” 

See the APS article.