Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session M27: Open Quantum SystemsFocus Live

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Sponsoring Units: DAMOP DQI Chair: Alp Sipahigil, Caltech 
Wednesday, March 17, 2021 11:30AM  12:06PM Live 
M27.00001: Tracking evaporative cooling of an atomic quantum gas in real time Invited Speaker: Johannes Zeiher Ultracold atomic gases provide a clean setting for the study of mesoscopic systems, which are characterized by the importance of fluctations of their constituent particles. However, detection of ultracold atomic gases is typically destructive, precluding repeated measurements on the same sample. In our experiment, we overcome this limitation by utilizing the enhanced lightmatter coupling in a highfinesse optical cavity. We use a noninvasive measurement scheme to record realtime traces of the atom number dynamics in a mesoscopic quantum gas undergoing evaporative cooling. Extracting twotime correlation functions from our measurements, we reveal the nonlinear dynamics of the evaporating gas. This allows for exploring the intriguing interplay between atom number and temperature as well as their fluctuations. Furthermore, by closing a classical feedback loop, we demonstrate the preparation of atomic ensembles with subpoissonian shottoshot atom number fluctuations. Our results provide a novel testbed for observing thermodynamics and transport phenomena in mesosopic cold atomic gases and pave the way for cavityassisted feedback stabilization of atom number and temperature in atomic quantum simulators. 
Wednesday, March 17, 2021 12:06PM  12:18PM Live 
M27.00002: TimeEvolution of Open Quantum Systems using Quantum and Classical Resources Kade HeadMarsden, Prineha Narang How an open quantum system evolves in the presence of its environment is crucial to better understanding and improving many processes including the communication of quantum information and the transfer of energy. In the dissipative Markovian regime, energy or information lost by the system is never recovered, however, in the nonMarkovian regime, recurrences of quantum properties such as coherences and entanglement can occur. Accurately modeling these recurrences could allow for improved experimental parameter estimation and for the potential control of noise processes in quantum technologies. Here, I will discuss reduced density matrix methods which extend the Kraus mapping formalism to capture nonMarkovian dynamics using both classical and quantum computational resources. I will discuss application of these methods to molecular systems, with supporting data from IBM’s Qiskit simulator and devices. 
Wednesday, March 17, 2021 12:18PM  12:30PM Live 
M27.00003: Hidden timereversal symmetry, quantum detailed balance and exactlysolvable drivendissipative quantum systems David Roberts, Andrew Lingenfelter, Aashish Clerk Drivendissipative quantum systems generically do not satisfy simple notions of detailed balance based on microscopic reversibility. We show that such systems can nonetheless have one or more hidden timereversal symmetries, a concept that we define in terms of correlation functions and thermofield double states. We demonstrate that the presence of hidden symmetry directly leads to an extremely efficient method for analytically solving for steady states of the dynamics, even if such states are nontrivial. This represents a generalization of the socalled coherent quantum absorber method [1, 2]. We discuss how hidden TRS is relevant to a variety of driven qubit and nonlinear cavity models, and how this symmetry has simple, experimentallyobservable consequences. We also discuss how this symmetry underlies somewhat opaque exact solution techniques in quantum optics that are based on phase space methods (i.e. the complexP representation). 
Wednesday, March 17, 2021 12:30PM  12:42PM Live 
M27.00004: Steadystate properties of quantum nonHermitian lattice models Alexander McDonald, Ryo Hanai, Aashish Clerk The physics of nonHermitian systems is an active area of research. In a quantum setting, it is tempting to directly define the system’s state (i.e. its density matrix) from the Hamiltonian. While various prescriptions have been proposed (e.g. exponentiating the nonHermitian Hamiltonian, occupying right eigenvectors), these are largely problematic. Further, a number of natural questions about nonHermitian steadystates, such as the role of right and left eigenvectors, particle statistics or the sensitivity to boundary conditions analogous to the nonHermitian skin effect have not been fully studied. Here, we address these issues in a more physical manner, using the fact that quantum nonHermitian dynamics almost always requires a coupling to external dissipative environments. We study quantum versions of the paradigmatic HatanoNelson model (i.e. a nonHermitian, nonreciprocal 1D tight binding model) that are realized using engineered dissipation, both for bosons and fermions. Our analysis reveals a number of basic and generic insights. In particular, we highlight the role of an emergent momentumdependent temperature, and the fact that the nonHermitian skin effect alone does not determine the steadystate sensitivity to boundary conditions. 
Wednesday, March 17, 2021 12:42PM  12:54PM Live 
M27.00005: Measurement and entanglement phase transitions in alltoall quantum circuits Adam Nahum, Sthitadhi Roy, Brian Skinner, Jonathan Ruhman Quantum manybody systems subjected to local measurements at a nonzero rate can be in distinct dynamical phases, with differing entanglement properties. We introduce theoretical approaches to measurementinduced phase transitions in "alltoall" quantum circuits with unitaries and measurements, in which any qubit can couple to any other. We first solve the simplest "minimal cut" toy model for entanglement dynamics in alltoall circuits. We then show that certain alltoall measurement circuits allow exact results by exploiting the circuit's local treelike structure, and we compare these results with numerics in alltoall circuits. We characterize the two different phases in alltoall circuits using observables that are sensitive to the amount of information propagated between the initial and final time. 
Wednesday, March 17, 2021 12:54PM  1:06PM Live 
M27.00006: Liouvillianity breaking in dissipative interacting Floquet systems under highfrequency drive Kaoru Mizuta, Kazuaki Takasan, Norio Kawakami Periodicallydriven (Floquet) systems have been attracted much interest as one of the most important class of nonequilibium systems. FloquetMagnus (FM) expansion is a powerful tool for Floquet systems under highfrequency drives. In closed Floquet systems, it gives an effective static Hamiltonian which describes their stroboscopic dynamics. However, in dissipative Floquet systems dominated by a Liouvillian, it remains an important problem whether the FM expansion gives a static effective Liouvillian (Liouvillianity). 
Wednesday, March 17, 2021 1:06PM  1:18PM Live 
M27.00007: Chiral quantum optics with giant atoms Ariadna Soro Álvarez, Anton Frisk Kockum In quantum optics, it is common to assume that atoms are pointlike objects compared to the wavelength of the electromagnetic field they interact with. However, this dipole approximation is not always valid, e.g., if atoms couple to radiation at multiple discrete points. Previous work has shown that superconducting qubits coupled to a 1D waveguide can behave as such ‘giant atoms’ and then can interact through the waveguide without decohering, a phenomenon that is not possible with small atoms. In the present work, we prove that this decoherencefree interaction is also possible when the coupling to the waveguide is chiral. Furthermore, we derive conditions under which the giant atoms in this architecture exhibit dark states. In particular, we show that unlike small atoms, giant atoms in a chiral waveguide can reach a dark state even outside the drivendissipative regime, i.e., without being excited by a coherent drive. 
Wednesday, March 17, 2021 1:18PM  1:30PM Live 
M27.00008: Transport and dynamics in the frustrated twobath spinboson model Ron Belyansky, Seth P Whitsitt, Rex Lundgren, Yidan Wang, Andrei Vrajitoarea, Andrew Houck, Alexey V Gorshkov We study the nonequilibrium dynamics, including transport properties, of photons in the twobath spinboson model, in which a spin1/2 particle is frustratingly coupled to two independent Ohmic bosonic baths. We show that the frustration in this model gives rise to rich physics in a wide range of energies, in contrast to the onebath spinboson model where the nontrivial physics occurs at an energy scale close to the renormalized spin frequency. The renormalized spin frequency in the twobath spinboson model is still important, featuring in different observables, including the elastic transport properties of a photon. The elastic scattering displays nonmonotonic behavior at high frequencies, and is very different in the two channels: intra and interbath scattering. The photon can also be inelastically scattered, a process in which it is split into several photons of smaller energies. We show that such inelastic processes are highly anisotropic, with the outgoing particles being preferentially emitted into only one of the baths. Moreover, the inelastic scattering rate is larger than in the onebath case, and can exceed the total elastic rate. Our results can be verified with stateoftheart circuit and cavity quantum electrodynamics experiments. 
Wednesday, March 17, 2021 1:30PM  1:42PM Live 
M27.00009: Timecoarsegrained dynamics of open quantum systems: The systemplusreservoir
approach and effective models Wentao Fan, Hakan E Tureci, Kanupriya Sinha In this work, we study temporal coarsegraining as a basis to derive accurate effective models for open quantum systems. In particular, we demonstrate that the coarsegraining time scale, when treated as a physical parameter, determines the validity of various commonly employed approximations such as the RWA and the Markov approximation. With a suitable choice of the coarsegraining time scale, we systemically derive the Lindblad dynamics without assuming the alltime factorizability of the full density matrix. Deviations from the Lindblad dynamics are quantified analytically and then compared to numerical results obtained from the full systemplusreservoir master equation. We further compare the systemplusreservoir approach to direct timecoarsegraining of the Lindblad master equation and to naïve timeaveraging of the dynamical variables, emphasizing on the sources of their differences as well as the corresponding physical interpretations. Finally, we address the initial condition problem of timecoarsegrained dynamics and argue that the choice of the initial condition depends on one's interpretation of the physical nature of timecoarsegraining. 
Wednesday, March 17, 2021 1:42PM  1:54PM Live 
M27.00010: Symmetry breaking and error correction in open quantum systems Simon Lieu, Ron Belyansky, Jeremy Young, Rex Lundgren, Victor Albert, Alexey V Gorshkov Symmetrybreaking transitions are a wellunderstood phenomenon of closed quantum systems in quantum optics, condensed matter, and high energy physics. However, symmetry breaking in open systems is less thoroughly understood, in part due to the richer steadystate and symmetry structure that such systems possess. For the prototypical open systema Lindbladiana unitary symmetry can be imposed in a "weak" or a "strong" way. We characterize the possible Z_n symmetry breaking transitions for both cases. In the case of Z_2, a weaksymmetrybroken phase guarantees at most a classical bit steadystate structure, while a strongsymmetrybroken phase admits a partiallyprotected steadystate qubit. Viewing photonic cat qubits through the lens of strongsymmetry breaking, we show how to dynamically recover the logical information after any gappreserving strongsymmetric error; such recovery becomes perfect exponentially quickly in the number of photons. Our study forges a connection between drivendissipative phase transitions and error correction. 
Wednesday, March 17, 2021 1:54PM  2:06PM Live 
M27.00011: Time Reversal Symmetry Breaking in Driven Dissipative Spin Systems Daniel Paz, Mohammad Maghrebi Timereversal symmetry is an intrinsic property of equilibrium systems that forbids currents under equilibrium conditions. On the other hand, timereversal symmetry is explicitly broken in driven systems, leading to new states of matter. In this talk, I discuss the impact of timereversal symmetry breaking on drivendissipative systems. I will focus on the drivendissipative Ising model, and show that a kind of spin current emerges in the nonequilibrium steady state. Interestingly, this current diverges at the critical point and shows nontrivial critical scaling with system size. While previous studies have concluded an effective equilibrium behavior at criticality, we find that the effective temperatures corresponding to two (longitudinal) components of the spin are equal in magnitude but opposite in sign, signaling the extreme nonequilibrium nature of the steady state. Finally, at the weakly dissipative critical point the current remains critical but exhibits new critical scaling. These features are immediately accessible in experimental platforms. 
Wednesday, March 17, 2021 2:06PM  2:18PM Live 
M27.00012: Superoperator structures and nogo theorems for dissipative quantum phase transitions Thomas Barthel, Yikang Zhang In the thermodynamic limit, the steady states of open quantum manybody systems can undergo nonequilibrium phase transitions due to a competition between Hamiltonian and dissipative terms. Here, we consider Markovian systems and elucidate structures of the Liouville superoperator that generates the dynamics. In many cases of interest, a nonorthogonal basis transformation can bring the Liouvillian into blocktriangular form, making it possible to assess its spectrum. The spectral gap sets the asymptotic decay rate. The superoperator structure can be used to bound gaps from below, showing that, in a large class of systems, dissipative phase transitions are actually impossible and that the convergence to steady states is exponential. Furthermore, when the blocks on the diagonal are Hermitian, the Liouvillian spectra obey Weyl ordering relations. The results are exemplified by various spin models. 
Wednesday, March 17, 2021 2:18PM  2:30PM Live 
M27.00013: Extending the quantum coherence of a qubit via engineering the noise spectrum of its environment Maxime Joos, Dolev Bluvstein, Yuanqi Lyu, David Minot Weld, Ania Claire Jayich Controlling the environment of a qubit to suppress decoherence is a key technique for modern quantum technologies. Such control can be either passive, via e.g. materials engineering, or active, via e.g. driving fields. Using a shallow defect center coupled to RFdriven surface spins, we demonstrate experimentally that spectral engineering of the spin bath enables improved qubit coherence. Results are in agreement with our quantitative model, and open the path to active decoherence protection using customdesigned waveforms applied to the environment rather than the qubit. 
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