Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session B50: Driven and Dissipative Atomic Systems 
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Sponsoring Units: DAMOP Chair: Ryan Wilson, Joint Quantum Institute, University of Maryland Room: Hilton Baltimore Holiday Ballroom 1 
Monday, March 14, 2016 11:15AM  11:27AM 
B50.00001: Selforganization of atoms coupled to a chiral reservoir Zachary Eldredge, Darrick Chang, Alexey Gorshkov Recently, there has been increasing interest in the properties of confined light in the vicinity of tapered optical nanofibers. Interesting avenues have been suggested concerning cold atoms trapped on the fiber by evanescent light fields. It has been shown that the interaction between atoms coupled to this onedimensional reservoir leads to equations of motion possessing selforganized stable solutions which exhibit striking manybody dynamics. Finally, it has also been observed that spinorbit coupling due to the extreme confinement of the light leads to a directionality in the coupling to the fiber. In this paper we explore the implications of a chiral interaction on selforganization and show that the overall configuration exhibits similar behavior to the symmetric case but undergoes dramatic changes in some regions of parameter space. We also present proposals for experimental realizations of our model as well as signatures of chiral behavior. [Preview Abstract] 
Monday, March 14, 2016 11:27AM  11:39AM 
B50.00002: Novel Infrared Dynamics of Cold Atoms on Hot Graphene Sanghita Sengupta, Valeri Kotov, Dennis Clougherty The lowenergy dynamics of cold atoms interacting with macroscopic graphene membranes exhibits severe infrared divergences when treated perturbatively. These infrared problems are even more pronounced at finite temperature due to the (infinitely) many flexural phonons excited in graphene. We have devised a technique to take account (resummation) of such processes in the spirit of the wellknown exact solution of the independent boson model. Remarkably, there is also similarity to the infrared problems and their treatment (via the BlochNordsieck scheme) in finite temperature ``hot'' quantum electrodynamics and chromodynamics due to the longrange, unscreened nature of gauge interactions. The method takes into account correctly the strong damping provided by the many emitted phonons at finite temperature. In our case, the inverse membrane size plays the role of an effective lowenergy scale, and, unlike the above mentioned field theories, there remains an unusual, highly nontrivial dependence on that scale due to the 2D nature of the problem. We present detailed results for the sticking (atomic damping rate) rate of cold atomic hydrogen as a function of the membrane temperature and size. We find that the rate is very strongly dependent on both quantities. [Preview Abstract] 
Monday, March 14, 2016 11:39AM  11:51AM 
B50.00003: Periodically driven system coupled to a fermionic bath: A Keldysh approach Dong E. Liu, Alex Levchenko, Roman M. Lutchyn We develop a Keldysh approach to study a timeperiodically driven system with dissipation. We apply this approach to a periodically driven metallic system coupled to a normal metal and a superconducting bath. After integrating out the fermionic bath degrees of freedom and incorporating its effects exactly through selfenergy, we find nonequilibrium Green functions for the driven system which take into account effect of the bath. Our formalism allows one to evaluate nonequilibrium distribution function for particles in the periodicallydriven system as well as other observable quantities (e.g. tunneling density of states). In the case of a superconducting bath, we study interplay of the proximityinduced superconducting pairing correlations and the dissipation due to lightexcited quasiparticles. [Preview Abstract] 

B50.00004: ABSTRACT WITHDRAWN 
Monday, March 14, 2016 12:03PM  12:15PM 
B50.00005: Nonequilibrium SteadyState Behavior in a ScaleFree Quantum Network Jianshi Zhao, Craig Price, Qi Liu, Nathan Gemelke We describe the nonequilibrium dynamics of a cold atomic gas held in a spatially random optical potential and gravity, subject to a controlled amount of dissipation in the form of an extremely slow darkstate laser cooling process. Reaching local kinetic temperatures below the $100$nK scale, such systems provide a novel context for observing the nonequilibrium steadystate (NESS) behavior of a disordered quantum system. For sufficiently deep potentials and strong dissipation, this system can be modeled by a selforganized version of directed percolation, and exhibits powerlaw decay of phasespace density with time due to the presence of absorbing clusters with a wide distribution of entropy and coupling rates. In the absence of dissipation, such a model cannot apply, and we observe the crossover to exponential loss of phasespace density. We provide measurements of the powerlaw decay constant by observing the nonequilibrium motion of atoms over a tenminute period, consistent with $\gamma=0.31\pm0.04$, and extract scaling of the absorbed number with dissipation rate, showing another powerlaw behavior, with exponent $0.5\pm0.2$ over two decades of optical excitation probability. [Preview Abstract] 
Monday, March 14, 2016 12:15PM  12:27PM 
B50.00006: Steady States in Fermionic Interacting Dissipative Floquet Systems Karthik Seetharam, Charles Bardyn, Netanel Lindner, Mark Rudner, Gil Refael The possibility to drive quantum systems periodically in time offers unique ways to deeply modify their fundamental properties, as exemplified by Floquet topological insulators. It also opens the door to a variety of nonequilibrium effects. Resonant driving fields, in particular, lead to excitations which can expose the system to heating. We previously demonstrated that the analog of thermal states can be achieved and controlled in a fermionic Floquet system in the presence of phonon scattering, spontaneous emission, and an energy filtered fermionic bath. However, interactions play an important role in thermalization and present additional sources of heating. We analyze the effects of weak interactions in the presence of dissipation and the role of coherences in determining the steady state of the driven system. Interactions generically create additional excitations and, in contrast to phonons, may sustain interFloquetband coherences at steady state. [Preview Abstract] 
Monday, March 14, 2016 12:27PM  12:39PM 
B50.00007: How should we understand nonequilibrium manybody steady states? Mohammad Maghrebi, Alexey Gorshkov : Manybody systems with both coherent dynamics and dissipation constitute a rich class of models which are nevertheless much less explored than their dissipationless counterparts. The advent of numerous experimental platforms that simulate such dynamics poses an immediate challenge to systematically understand and classify these models. In particular, nontrivial manybody states emerge as steady states under nonequilibrium dynamics. In this talk, I use a fieldtheoretic approach based on the Keldysh formalism to study nonequilibrium phases and phase transitions in such models. I show that an effective temperature generically emerges as a result of dissipation, and the universal behavior including the dynamics near the steady state is described by a thermodynamic universality class. In the end, I will also discuss possibilities that go beyond the paradigm of an effective thermodynamic behavior. [Preview Abstract] 
Monday, March 14, 2016 12:39PM  12:51PM 
B50.00008: Dissipation induced topological insulators: A recipe Moshe Goldstein It has recently been realized that drivendissipative dynamics, which usually tends to destroy subtle quantum interference and correlation effects, could actually be used as a resource. By proper engineering of the reservoirs and their couplings, one may drive a system towards a desired quantumcorrelated steady state, even in the absence of internal Hamiltonian dynamics. An intriguing class of quantum phases is characterized by topology, including the quantum Hall effect and topological insulators and superconductors. Which of these noninteracting topological states can be achieved as the result of purely dissipative Lindbladtype dynamics? Recent studies have only provided partial answers to this question. In this talk I will present a general recipe for the creation, classification, and detection of states of the integer quantum Hall and 2D topological insulator type as the outcomes of coupling a system to reservoirs, and show how the recipe can be realized with ultracold atoms and other quantum simulators. The mixed states so created can be made arbitrarily close to pure states, and the construction may be generalized to other topological phases. [Preview Abstract] 
Monday, March 14, 2016 12:51PM  1:03PM 
B50.00009: Engineering nonHermitian optical potentials for Polariton Condensation Saeed Khan, Li Ge, Hakan Tureci We present a theoretical study of incoherently pumped excitonpolariton condensates in general cavity geometries, based on an analysis of the linear nonHermitian modes of the (optical) pump induced potential. An analytical description is obtained for how the threshold pump power for condensation into a specific mode depends quantitatively on the relative spatial profiles of that mode and the pump. Specifically, we show that for a general pump profile, modes which best organize to balance the amplification from the pump against the repulsive pump potential achieve the lowest threshold power~[1]. Reversing this idea, choosing the spatial profile of the pump provides control over which spatial mode condenses at lowest power. Our work hence provides a scheme to engineer nonHermitian optical potentials for preferential polariton condensation into a specific mode, by an appropriate choice of pump profile. This approach has recently been used to achieve condensation in the flat band of a Lieb chain of micropillar cavities, where the flat band has energy above the ground state and hence cannot be studied in systems in thermal equilibrium~[2]. \\ \textbf{References:} \\ {[}1{]} L. Ge, \emph{et. al.}, arXiv: 1311.4847 (2013) \\ {[}2{]} F. Baboux \emph{et. al.}, arXiv: 1505.05652 (2015) [Preview Abstract] 
Monday, March 14, 2016 1:03PM  1:15PM 
B50.00010: Collective phases of strongly interacting cavity photons Ryan Wilson, Michael FossFeig, Khan Mahmud, Mohammad Hafezi We study the steady state phases of the BoseHubbard model in the presence of dissipation and coherent driving, which in the limit of strong interactions maps onto a drivendissipative XX spin$\frac{1}{2}$ model with transverse and longitudinal fields. Using a sitedecoupled meanfield approximation, we identify phases with antiferromagnetic and spin density wave order, in addition to limit cycle phases, where oscillatory dynamics persist indefinitely. We also identify collective bistable phases, where the system supports two steady states among spatially uniform, antiferromagnetic, and limit cycle phases. We compare these meanfield results to exact quantum trajectories for one dimensional cavity arrays. The quantum results exhibit shortrange antiferromagnetic and spin density wave order, in good qualitative agreement with the meanfield predictions. In the bistable regime, this system exhibits realtime collective switching between macroscopically distinguishable states. We present a clear physical picture for these dynamics, and establish a simple relationship between the switching times and properties of the quantum Liouvillian. [Preview Abstract] 
Monday, March 14, 2016 1:15PM  1:27PM 
B50.00011: Drivendissipative bosons in open boundary and inhomogeneous cavity arrays Khan W. Mahmud, Ryan M. Wilson, Michael FossFeig, Mohammad Hafezi We study the drivendissipative BoseHubbard model, which describes the physics of coherently pumped photonic cavity arrays as well as strongly interacting ultracold bosons in an optical lattice in a driven dissipative setting. We investigate manybody states and their quantum correlations on finite size lattices with open boundary conditions, a set up which is experimentally relevant. We show that the effects of hard boundaries on the steadystates are nontrivial, and explain the results in terms of finite system size excitations and the underlying phases of a thermodynamically large system. Furthermore, we explore the effects of trap inhomogeneity, such as an external harmonic trap, quantifying the breakdown of local density approximation for finite system size. We use a mixed state version of matrix product states algorithm for the numerical investigation. [Preview Abstract] 
Monday, March 14, 2016 1:27PM  1:39PM 
B50.00012: Flying over decades Judith Hoeller, Mena Issler, Atac Imamoglu Levy flights haven been extensively used in the past three decades to describe nonBrownian motion of particles. In this presentation I give an overview on how Levy flights have been used across several disciplines, ranging from biology to finance to physics. In our publication we describe how a single electron spin 'flies' when captured in quantum dot using the central spin model. At last I motivate the use of Levy flights for the description of anomalous diffusion in modern experiments, concretely to describe the lifetimes of quasiparticles in Josephson junctions. [Preview Abstract] 
Monday, March 14, 2016 1:39PM  1:51PM 
B50.00013: Multiple timescale analysis of dynamical evolution near two coalescing eigenvalues in open quantum systems Savannah Garmon, Gonzalo Ordonez Recently the physics of coalescing eigenvalues at an exceptional point (EP) has been studied in a wide range of physical contexts, including open quantum systems. At an EP$N$ at which $N$ eigenvalues coalesce the Hamiltonian can no longer be diagonalized but instead only reduced to a Jordan block of dimension $N$. In order to describe the survival probability $P(t)$ for an initially prepared state in the vicinity of two coalescing levels, we further subdivide the EP2 case into the EP2A and EP2B [1], where the EP2A involves the coalesce of two virtual bound states to form a resonance/antiresonance pair and the EP2B occurs when two resonances collide to form two new resonances. We show that in the vicinity of the EP2B the usual exponential decay appearing for resonances on intermediate timescales is modified as $P(t) \sim t e^{\Gamma t}$. However, the longtime evolution near the EP2B follows a $1/t^3$ power law decay. Meanwhile the evolution for the EP2A is nonexponential on all timescales, and may be strongly influenced by continuum threshold effects [2]. [1] S. Garmon, M. Gianfreda, and N. Hatano, Phys. Rev. A 92, 022125 (2015). [2] S. Garmon, T. Petrosky, L. Simine and D. Segal, Fortschr. Phys. 61, 261 (2013). [3] N. Hatano and G. Ordonez, J. Math. Phys. 55, 122106 (2014). [Preview Abstract] 
Monday, March 14, 2016 1:51PM  2:03PM 
B50.00014: Quantum Spontaneous Stochasticity Theodore Drivas, Gregory Eyink Classical Newtonian dynamics is expected to be deterministic, but recent fluid turbulence theory predicts that a particle advected at high Reynoldsnumbers by "nearly rough" flows moves nondeterministically. Small stochastic perturbations to the flow velocity or to the initial data lead to persistent randomness, even in the limit where the perturbations vanish! Such ``spontaneous stochasticity’’ has profound consequences for astrophysics, geophysics, and our daily lives. We show that a similar effect occurs with a quantum particle in a "nearly rough" force, for the semiclassical (largemass) limit, where spreading of the wavepacket is usually expected to be negligible and dynamics to be deterministic Newtonian. Instead, there are nonzero probabilities to observe multiple, nonunique solutions of the classical equations. Although the quantum wavefunction remains split, rapid phase oscillations prevent any coherent superposition of the branches. Classical spontaneous stochasticity has not yet been seen in controlled laboratory experiments of fluid turbulence, but the corresponding quantum effects may be observable by current techniques. We suggest possible experiments with neutral atomicmolecular systems in repulsive electric dipole potentials. [Preview Abstract] 
Monday, March 14, 2016 2:03PM  2:15PM 
B50.00015: Currentcarrying quasisteady states in a periodically driven manybody system Mark Rudner, Netanel Lindner, Erez Berg We investigate manybody dynamics in a onedimensional interacting periodically driven system, based on a partiallyfilled version of Thouless’s topologically quantized adiabatic pump. The corresponding single particle Floquet bands are chiral, with the Floquet spectrum realizing nontrivial cycles around the quasienergy Brillouin zone. For noninteger filling the system is gapless; here the driving cannot be adiabatic and the system is expected to rapidly absorb energy from the driving field. We identify parameter regimes where scattering between Floquet bands of opposite chirality is exponentially suppressed, opening a long time window where the manybody evolution separately conserves the occupations of the two chiral bands. Within this intermediate time regime we predict that the system reaches a quasisteady state with uniform crystal momentum occupation within each Floquet band. This state furthermore carries a nonvanishing current given directly by the difference of densities in the right and left moving chiral bands. This remarkable behavior, which holds for both bosons and fermions, may be readily studied experimentally in recently developed cold atom systems. [Preview Abstract] 
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