Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K26: Open Quantum Systems IFocus

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Sponsoring Units: DAMOP DQI Chair: Andrew Daley, University of Strathclyde Room: LACC 404A 
Wednesday, March 7, 2018 8:00AM  8:36AM 
K26.00001: Superconducting Circuits: Controlling the Bath Invited Speaker: Irfan Siddiqi Superconducting circuits are inherently open quantum systems, and advances in surface science and circuit design to reduce spurious coupling to an uncontrolled environment have resulted in stateoftheart devices with coherence times in excess of 100 microseconds. Such longlived coherence affords another modality where the environment can be modified by way of microwave control pulses to engineer a dressed Hamiltonian with new types of lightmatter coupling. Alternatively, the bath can be monitored to tailor different measurement protocols. I will illustrate how bath engineering can use used to probe quantum uncertainty relations and realize realtime adaptive measurements, as well as present new methods for quantum state reconstruction based on machine learning techniques. 
Wednesday, March 7, 2018 8:36AM  8:48AM 
K26.00002: Frequency combs in a Josephson junction circuit Saeed Khan , Hakan Tureci We investigate the dynamics of a microwavedriven Josephson junction capacitively coupled to a linear oscillator [1]. In the driving regime where the Josephson junction can be approximated as a Kerr oscillator, this minimal nonlinear system has been previously shown to exhibit a bistability in phase and amplitude. We characterize the full phase diagram and show that besides a parameter regime exhibiting bistability, there is also a regime of selfoscillations characterized by a frequency comb in its spectrum. We discuss the mechanism of comb generation which appears to be different from those studied in microcavity frequency combs and modelocked lasers. We then address the fate of the comblike spectrum in the regime of strong quantum fluctuations, reached when nonlinearity becomes the dominant scale with respect to dissipation. We find that the nonlinearity responsible for the emergence of frequency combs also leads to its dephasing, leading to broadening and ultimate disappearance of sharp spectral peaks. Our study explores the fundamental question of the impact of quantum fluctuations for quantum systems which do not possess a stable fixed point in the classical limit. 
Wednesday, March 7, 2018 8:48AM  9:00AM 
K26.00003: Spin pumping under relaxation: Beyond the Markovian approximation Kazunari Hashimoto , Gen Tatara , Chikako Uchiyama The spin pumping is a method to generate spin polarized electron current (spin current) in ferromagnetnormal metal junctions, where the spin current is induced by precession of the magnetization in the ferromagnet into the normal metal. A standard model used in analyzing the spin pumping consists of a magnetic quantum dot attached to an electron reservoir. Most of its theoretical studies have been performed in an adiabatic regime, where the precession is sufficiently slow compared to the relaxation time of the dot by tunneling. However, since period of the precession (~10^{9}sec.) is comparable to or shorter than the relaxation time (~10^{8}sec.) in actual experiments, it is necessary to study the spin pumping under relaxation (it has been studied with the Markovian approximation in [K. Hashimoto, G. Tatara, and C. Uchiyama, Phys. Rev. B 96, 064439(2017)]). In the short time scale, the correlation time of the reservoir should also be considered to be finite. This requires to analyze electron dynamics beyond the Markovian approximation, since it fails in the short time scale. To this end, we study the electron dynamics in terms of the full counting statistics with the quantum master equation without using the Markovian approximation. 
Wednesday, March 7, 2018 9:00AM  9:12AM 
K26.00004: Nonlocal tuning of quantum current fluctuations in multiconnected lattices with and without external reservoirs Mekena Metcalf , ChenYen Lai , Massimiliano Di Ventra , ChihChun Chien Fluctuations arising from noncommutative operators in Heisenberg's uncertainty relation are fundamental in quantum mechanics. The current operator J_{i,i+1} measures current between two lattice sites and does not commute with the interconnected current operator J_{i+1,i+2}. This means the current fluctuations are correlated and proportional to J_{i,i+2}. Quantum fluctuations of current are affected by the tunneling strength between the conjoining nextnearestneighbor lattice sites. Open system and closed system methods are used to determine how the tunneling coefficient affects fluctuations for noninteracting and interacting fermions. Investigations illustrate a pure quantum current effect since the wave nature of quantum particles leads to nonlocal correlations. 
Wednesday, March 7, 2018 9:12AM  9:24AM 
K26.00005: Heralded Generation of Maximally Entangled State of Open Qubits Coupled to a 1D Environment Xin Zhang , Harold Baranger Maximally entangled states are of central importance in quantum information science and in modern quantum technologies. Here we show that heralded generation of a maximally entangled state of two intrinsically open qubits can be realized in a onedimensional (1d) system with strong coherent driving and continuous monitoring. In contrast to the common belief that systemenvironment interaction leads to decoherence and so destroys quantum effects, continuous measurement and strong interference in our 1d system generates a pure state with perfect quantum correlation between the two open qubits. Another surprise is that this maximally entangled state survives the strong coherent state input, which is a classical state that overwhelms the whole system. 
Wednesday, March 7, 2018 9:24AM  9:36AM 
K26.00006: Reservoir engineering of bosonic lattices using a single local dissipator and chiral symmetry Yariv Yanay , Aashish Clerk We show how a generalized kind of chiral symmetry can be used to construct highlyefficient reservoir engineering protocols for bosonic lattices, such as arrays of microwave cavities [arXiv:1710.10318]. These protocols exploit only a single squeezed reservoir coupled to a single lattice site; this is enough to stabilize the entire system in a pure, entangled steady state. Our approach is applicable to lattices in any dimension, and does not rely on translational invariance. We show how the relevant symmetry operation directly determines the real space correlation structure in the steady state. We demonstrate the power of the technique using a twodimensional Hofstadter lattice, which has recently been implemented with superconducting circuits [Owens et al, arXiv:1708.01651]. 
Wednesday, March 7, 2018 9:36AM  9:48AM 
K26.00007: Perturbation theory of Lindbladians with multiple steady states Victor Albert , Zlatko Minev , Florentin Reiter , Liang Jiang Lindbladians admitting multiple steady states are dissipative analogues of Hamiltonian systems with multiple ground states. As demonstrated with trappedion and circuitQED quantum engineering, such systems can be used to store, protect, and process quantum information. We develop a perturbation expansion of a general Lindbladian whose unperturbed part can have multiple steady states. After making a few minor assumptions and dividing the expansion into terms that keep the initial state within the steadystate subspace and terms that take the state out, the expansion is solved exactly [1] and admits a diagrammatic interpretation. The number of diagrams in the expansion that are the same order in the perturbation is shown to be a Catalan number. With appropriate additional assumptions, the formalism reduces to opensystem perturbation theory for a unique steady state [2], the effective operator formalism [3], and quantum Zeno dynamics [4]. 
Wednesday, March 7, 2018 9:48AM  10:00AM 
K26.00008: ReducedDensityMatrix Description of Decoherence and Relaxation Processes for ElectronSpin Systems Verne Jacobs Electronspin systems are investigated using a quantumopensystems description. Applications of interest include trapped atomic systems in optical lattices, semiconductor quantum dots, and vacancy defect centers in solids. Timedomain and frequencydomain formulations are developed. The general nonperturbative and nonMarkovian formulations provide a fundamental framework for systematic evaluations of corrections to the standard Born and Markov approximations. Particular attention is given to decoherence and relaxation processes, as well as spectralline broadening phenomena, that are induced by interactions with photons, phonons, nuclear spins, and external electric and magnetic fields. These processes are treated either as coherent interactions or as environmental interactions. The environmental interactions are incorporated by means of the general expressions derived for the timedomain and frequencydomain Liouvillespace selfenergy operators, for which the tetradicmatrix elements are explicitly evaluated in the diagonalresolvent, lowestorder, and Markov (shortmemory time) approximations. 
Wednesday, March 7, 2018 10:00AM  10:12AM 
K26.00009: Subdiffusion and nonequilibrium probes of phases in AubryAndreHarper Model Archak Purkayastha , Sambuddha Sanyal , Abhishek Dhar , Manas Kulkarni

Wednesday, March 7, 2018 10:12AM  10:24AM 
K26.00010: Timereversibility in Quantum Feedback Protocols Jonathan Monroe , Mahdi Naghiloo , Kater Murch , Sreenath Kizhakkumpurath Manikandan , Andrew Jordan In open quantum systems, measurements by a probe or the environment cause backaction. While these interactions do not necessarily break timereversal symmetry, a statistical arrow of time emerges due to the correlations between the quantum state and the measurement. Quantum feedback protocols utilize rotations to correct for quantum backaction, effectively closing the system evolution. We study time reversibility of state stabilization by examining the effectiveness of timereversed versus timeordered feedback protocols. We also investigate the effect of finite efficiency detection in these state stabilization protocols. 
Wednesday, March 7, 2018 10:24AM  10:36AM 
K26.00011: QuasiStationary States in Open Quantum Systems Ihor Vakulchyk , Mikhail Ivanchenko , Sergej Flach , Sergey Denisov Any open system will eventually end up in a steady state. Provided no symmetries are hidden in the evolution generator, the steady state is unique. The state of an open system is described with a positive element of some algebra: it is a probability vector or density (in classical physics) or a density matrix (in quantum physics). If the unique steady state is pure, evolution may take place is the bulk and might never rich the steady state during the physically meaningful time. Such evolution can be characterized with the asymptotic state of the bulk dynamics under the condition of nonadsorption to the steady state. 
Wednesday, March 7, 2018 10:36AM  10:48AM 
K26.00012: Bistability in the Nonequilibrium AndersonHolstein Model: Developing Tools to Study Fundamental Physics of Electron Transfer in Nonequilibrium Quantum Impurity Models Lyran Kidon , Haobin Wang , Michael Thoss , Eran Rabani Recent developments in the field of molecular electronics have exhibited multiple steadystate solutions in nonequilibrium impurity models with strong electronphonon (eph) interactions. These results have ignored the effect of electronelectron (ee) couplings, expected to play an important role in the steadystate characteristics. To understand the effect of ee interactions in transport and its effect on the uniqueness of the steadystate, we have developed tools to study the dynamics of a nonequilibrium quantum impurity system with eph and ee interactions described by the AndersonHolstein model. The first is a numerically exact scheme based on the generalized quantum master equation for the reduced density matrix combined with an impurity solver. We have extended the formalism to obtain the memory kernel for an arbitrary model. We have also developed an approximate scheme based on the nonequilibrium Green's function (NEGF) formalism and the equation of motion approach. Unlike previous approximations within the NEGF formalism, this approach is adequate for weak to strong ee couplings and reduces to the selfconsistent Born approximation for vanishing ee coupling. 
Wednesday, March 7, 2018 10:48AM  11:00AM 
K26.00013: Relaxation to Negative Temperatures in Double Domain Systems Yusuke Hama , William Munro , Kae Nemoto In recent years, quantum technologies have shown exceptional progress in their development that we can now engineer hybrid quantum systems using elements ranging from solids to atomicmolecule and optical systems. The investigation of hybrid quantum physics is an interesting and important focus as we can explore counterintuitive phenomena such as manybody and nonequilibrium physics inherent in the hybridization. 
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