Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session X52: Many-Body Physics in Quantum Information Theory |
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Sponsoring Units: GQI GSNP Chair: Justin Elenewski, NIST Room: 399 |
Friday, March 17, 2017 8:00AM - 8:12AM |
X52.00001: Applying Discrete Wigner Functions to capture MBL in extended Phase Space with dTWA Jonathan Wurtz, Anatoli Polkovnikov, Shainen Davidson Phase-space descriptions of quantum systems are an equivalent way to describe dynamics of a quantum system. Here, we use discrete Wigner Functions in extended phase space to describe approximate evolution of a disordered spin lattice to capture MBL through phase-space sampling of a positive-definite quasiprobability distribution. Our approximation is controlled as a clustering of sites, which expands the phase space with extra “quantum” degrees of freedom from mean-field classical evolution, improving the accuracy of the expansion, known as the Truncated Discrete Wigner Approximation (dTWA). [Preview Abstract] |
Friday, March 17, 2017 8:12AM - 8:24AM |
X52.00002: Chiral Floquet Phases of Many-body Localized Bosons Hoi Chun Po, Lukasz Fidkowski, Takahiro Morimoto, Andrew C. Potter, Ashvin Vishwanath We construct and classify chiral topological phases in driven (Floquet) systems of strongly interacting bosons, with finite-dimensional site Hilbert spaces, in two spatial dimensions. The construction proceeds by introducing exactly soluble models with chiral edges, which in the presence of many-body localization (MBL) in the bulk are argued to lead to stable chiral phases. These chiral phases do not require any symmetry, and in fact owe their existence to the absence of energy conservation in driven systems. Surprisingly, we show that they are classified by a quantized many-body index, which is well defined for any MBL Floquet system. The value of this index, which is always the logarithm of a positive rational number, can be interpreted as the entropy per Floquet cycle pumped along the edge, formalizing the notion of quantum-information flow. We explicitly compute this index for specific models, and show that the nontrivial topology leads to edge thermalization, which provides an interesting link between bulk topology and chaos at the edge. We also discuss chiral Floquet phases in interacting fermionic systems and their relation to chiral bosonic phases. [Preview Abstract] |
Friday, March 17, 2017 8:24AM - 8:36AM |
X52.00003: Out-of-time-order correlations in many-body localized and thermal phases Xiao Chen, Tianci Zhou, David Huse, Eduardo Fradkin We use the out-of-time-order (OTO) correlators to study the slow dynamics in the many-body localized (MBL) phase. We investigate OTO correlators in the effective (``l-bit'') model of the MBL phase, and show that their amplitudes after disorder averaging approach their long-time limits as power-laws of time. This power-law dynamics is due to dephasing caused by interactions between the localized operators that fall off exponentially with distance. The long-time limits of the OTO correlators are determined by the overlaps of the local operators with the conserved l-bits. We demonstrate numerically our results in the effective model and three other more ``realistic'' spin chain models. Furthermore, we extend our calculations to the thermal phase and find that for a time-independent Hamiltonian, the OTO correlators also appear to vanish as a power law at long time, perhaps due to coupling to conserved densities. In contrast, we find that in the thermal phase of a Floquet spin model with no conserved densities the OTO correlator decays exponentially at long times. [Preview Abstract] |
Friday, March 17, 2017 8:36AM - 8:48AM |
X52.00004: Skyrmion Gas Manipulation for Unconventional Computing Daniele Pinna, Joo-Von Kim, Vincent Cros, Damien Querlioz, Paul Bessiere, Jacques Droulez, Julie Grollier In this talk we will discuss how the collective variable Thiele dynamics can be used to simplify the dynamical study of interacting skyrmions. We will show under what limits the many-particle interactions can be reduced to sums of two-particle interactions as well as how to characterize boundary repulsions. After justifying our results through micromagnetic simulations, we will demonstrate our approach by simulating a chamber of mutually interacting skyrmions by means of coupled Thiele equations. The parallelized simulation of the dynamics allows for a detailed study of ensemble repulsion and thermal diffusion over long timescales. Our toy model lays the groundwork for a novel device capable of reshuffling electrical signals as well as emulating neuromorphic behavior. By using input telegraph noise to inject a skyrmion gas into our toy chamber, we can successfully decorrelate the input signal while preserving statistical coherence through particle number conservation. Numerical simulations will justify the proposition and show how chamber size, temperature and spin-current intensities can be tuned to influence the correlations between input and output signals. Such a device, with no analogues as of date, allows for the cascading of multiple stochastic computing gates. [Preview Abstract] |
Friday, March 17, 2017 8:48AM - 9:00AM |
X52.00005: A numerical study of coarsening in the two-dimensional complex Ginzburg-Landau equation Weigang Liu, Uwe Tauber The complex Ginzburg-Landau equation with additive noise is a stochastic partial differential equation that describes a remarkably wide range of physical systems: coupled non-linear oscillators subject to external noise near a Hopf bifurcation instability; spontaneous structure formation in non-equilibrium systems, e.g., in cyclically competing populations; and driven-dissipative Bose-Einstein condensation, realized in open systems on the interface of quantum optics and many-body physics. We employ a finite-difference method to numerically solve the noisy complex Ginzburg-Landau equation on a two-dimensional domain with the goal to investigate the coarsening dynamics following a quench from a strongly fluctuating defect turbulence phase to a long-range ordered phase. We start from a simplified amplitude equation, solve it numerically, and then study the spatio-temporal behavior characterized by the spontaneous creation and annihilation of topological defects (spiral waves). We check our simulation results against the known dynamical phase diagram in this non-equilibrium system, tentatively analyze the coarsening kinetics following sudden quenches, and characterize the ensuing aging scaling behavior. In addition, we aim to use Voronoi triangulation to study the cellular structure in the phase turbulence and frozen states. [Preview Abstract] |
Friday, March 17, 2017 9:00AM - 9:12AM |
X52.00006: Temporal disorder in discontinuous absorbing phase transitions Marcelo M. de Oliveira, Carlos E. Fiore Recently it was shown that spatial (quenched) disorder can suppress discontinuous absorbing phase transitions (APTs) [1]. However, the scenario for temporal disorder is still unknown. Here, we investigate the effects of temporal disorder in models exhibiting discontinuous APTs. In contrast to spatial disorder, our results strongly suggest that uncorrelated temporal disorder does not forbid the existence of discontinuous APTs. We found they are characterized by behaviors similar to their pure (without disorder) counterparts, including bistability around the coexistence point and common finite size scaling behavior with the inverse of the system volume, as recently proposed in [2]. We also observe that temporal disorder does not induce temporal Griffiths phases around discontinuous APTs [3].\\ \\P. Villa Mart\'in, J. A. Bonachela and M. A. Mu\~noz, Phys. Rev. E {\bf 89}, 012145 (2014). \bibitem{oliv2015} M.M. de Oliveira, M.G.E. da Luz and C.E. Fiore, Phys. Rev. E. {\bf 92}, 062126 (2015). \bibitem{oliv2016} M. M de Oliveira and C. E. Fiore, to appear in Phys. Rev E, arXiv:1603.08742 (2016). [Preview Abstract] |
Friday, March 17, 2017 9:12AM - 9:24AM |
X52.00007: Renormalization Group Calculation of Anomalous Dimension in the Trapping Reaction Benjamin Vollmayr-Lee, Jack Hanson, Scott McIsaac, Joshua Hellerick We consider the trapping reaction $A+B\to B$, with diffusing particles ($A$) and traps ($B$), where the traps additionally undergo either an annihilation ($B+B\to\emptyset$) or coalescence ($B+B\to B$) reaction. This two-species reaction-diffusion system exhibits asymptotic power law decays in both the trap and particle densities, and simple scaling in the trap-trap and particle-trap correlation functions. However, simulations indicate the induced particle-particle correlations scale as $C_{AA}(x,t)=t^\phi f(x/t^{1/2})$ with an anomalous dimension $\phi$. We perform a one-loop renormalization group calculation of this exponent for $d<2$ and demonstrate that the anomalous dimension is universal and is due to a renormalization of the initial particle density. Our results are compared to the simulation data. [Preview Abstract] |
Friday, March 17, 2017 9:24AM - 9:36AM |
X52.00008: Flux line relaxation kinetics following current quenches in disordered type-II superconductors Harshwardhan Chaturvedi, Hiba Assi, Ulrich Dobramysl, Michel Pleimling, Uwe T\"auber We describe the disordered vortex system in type-II superconductors with an elastic line model, whose dynamics we investigate numerically by means of Langevin Molecular Dynamics. A system of driven interacting flux lines in a sample with randomly distributed point pinning centers is subjected to drive quench from a moving non-equilibrium steady state into one of three regimes viz. moving (steady state), pinned (glassy) or depinning (critical). The first yields fast exponential relaxation to the new non-equilibrium stationary state while the second displays algebraically slow relaxation and aging scaling with non-universal exponents. Our most recent work consists of aging and finite temperature scaling studies for drive quenches into the critical depinning regime. [Preview Abstract] |
Friday, March 17, 2017 9:36AM - 9:48AM |
X52.00009: Electronic Squeezing of Pumped Phonons: Negative U and Transient Superconductivity Dante Kennes, Eli Wilner, David Reichman, Andrew Millis Advances in light sources and time resolved spectroscopy have made it possible to excite specific atomic vibrations in solids and to observe the resulting changes in electronic properties but the mechanism by which phonon excitation causes qualitative changes in electronic properties has remained unclear. Here we show that the dominant symmetry-allowed coupling between electron density and dipole active modes implies an electron density-dependent squeezing of the phonon state which provides an attractive contribution to the electron-electron interaction, independent of the sign of the bare electron-phonon coupling and with a magnitude proportional to the degree of laser-induced phonon excitation. Reasonable excitation amplitudes lead to non-negligible attractive interactions that may cause significant transient changes in electronic properties including superconductivity. The mechanism is generically applicable to a wide range of systems, offering a promising route to manipulating and controlling electronic phase behavior in novel materials. [Preview Abstract] |
Friday, March 17, 2017 9:48AM - 10:00AM |
X52.00010: Variational Approaches to Quantify Self-organization in Complex Systems Atanu Chatterjee, Georgi Georgiev, Germano Iannacchione Complex systems are thermodynamically open, far from equilibrium and are composed of a large number of interacting elements. The local interactions in these systems are initially uncorrelated however, due to the self-organizing, order appears spontaneously accompanied by a reduction in the local entropy. Here, we propose a first-principles variational approach to quantify the appearance of order in a complex system due to self-organization. For complex systems modeled as flow networks, a new metric is introduced, the action efficiency, which is the ratio of a u nit action for an element to cross between two adjacent nodes to the total action of the system. Due to the fundamental and universal nature of this metric, it serves as the basis to study self-organization and quantify order for our system of interest, the evolution of CPUs. We also incorporate other metrics like, system size, flows and computations in the system, and observe the inter-dependencies between them due to the presence of feedback loops between them. In part, our results also provide insights about the underlying physical essence of the Moore's law and the multiple logistic growth observed in technological progress. [Preview Abstract] |
Friday, March 17, 2017 10:00AM - 10:12AM |
X52.00011: Is there a Tolman length in droplet solutions of the Allen-Cahn equation? Eric Horsley, Maxim Lavrentovich, Randall Kamien In the nucleation process of first-order transitions, the basic phenomenological description involves the competition between an interfacial and bulk energy. Generically, the surface tension associated with the interfacial energy is assumed to be constant; however, Tolman showed in 1949 that the surface tension could depend on the nucleated droplet radius. The matter remains unresolved. We investigate the droplet size dependence of the surface tension by means of the dynamics of a continuum scalar field given by the Allen-Cahn equation. Particular analytical solutions of the Allen-Cahn equation require an assumption that the interface width is much smaller than the droplet radius. If the surface tension is expected to change at small radii, as Tolman's result suggests, then this assumption becomes unsavory. Therefore, we choose to numerically solve the Allen-Cahn equation without this assumption. [Preview Abstract] |
Friday, March 17, 2017 10:12AM - 10:24AM |
X52.00012: Theory of deflagration in disordered media Mauro Schiulaz, Christopher R. Laumann, Alexander V. Balatsky, Boris Z. Spivak The conventional theory of burning works well in the case of uniform media where all system parameters are spatially independent. We develop a theory of burning in disordered media. In this case, rare regions (hot spots) where the burning process is more effective than on average may control the heat propagation in an explosive sample. We show that most predictions of the theory of burning are quite different from the conventional case. In particular, we show that a system of randomly distributed hot spots exhibits a dynamic phase transition, which is similar to the localization transition. Depending on parameters of the system the phase transition can be either first or second order. These two regimes are separated by a tricritical point. The above results may be applicable to dynamics of any over-heated disordered system with a first order phase transition. [Preview Abstract] |
Friday, March 17, 2017 10:24AM - 10:36AM |
X52.00013: Finite-Time and -Size Scalings in the Evaluation of Large Deviation Functions. Numerical Analysis in Continuous Time. Esteban Guevara Hidalgo, Takahiro Nemoto, Vivien Lecomte Rare trajectories of stochastic systems are important to understand because of their potential impact. However, their properties are by definition difficult to sample directly. Population dynamics provide a numerical tool allowing their study, by means of simulating a large number of copies of the system, which are subjected to a selection rule that favors the rare trajectories of interest. However, such algorithms are plagued by finite simulation time- and finite population size- effects that can render their use delicate. Using the continuous-time cloning algorithm, we analyze the finite-time and finite-size scalings of estimators of the large deviation functions associated to the distribution of the rare trajectories. We use these scalings in order to propose a numerical approach which allows to extract the infinite-time and infinite-size limit of these estimators. [Preview Abstract] |
Friday, March 17, 2017 10:36AM - 10:48AM |
X52.00014: Escape and Finite-Size Scaling in Diffusion-Controlled Annihilation Eli Ben-Naim, Paul Krapivsky |
Friday, March 17, 2017 10:48AM - 11:00AM |
X52.00015: Abstract Withdrawn We study out-of-time-order (OTO) correlation functions of the form $\langle\hat A(t)\hat B(0)\hat C(t) \hat D(0)\rangle$ in a system weakly coupled to a dissipative environment. We express the OTO correlator in terms of a generalised density matrix of two replicas of the original system coupled to an environment. We derive microscopically the equations of evolution of the generalised density matrix in terms of the correlation functions of the environment degrees of freedom, assuming the coupling is weak and the environment is short-time correlated, and, using these equations, analyse quantum information scrambling and quantum butterfly effects. |
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