Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session E25: Disorder and Localization in AMO Systems I: Time Crystals, Diffusion, Quantum ChaosFocus
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Sponsoring Units: DAMOP DCMP Chair: Anushya Chandran, Boston University Room: BCEC 160A |
Tuesday, March 5, 2019 8:00AM - 8:36AM |
E25.00001: Discrete time crystals in long-range interacting systems Invited Speaker: Norman Yao A generic periodically driven, isolated system will absorb energy until it looks, locally, like an infinite-temperature state. However, when the drive frequency is large compared with the local energy scales of the system, then the system can only absorb energy from the drive by spreading it out over many excitations. Consequently, heating occurs very slowly, and there is a long-lived quasi-steady state - a so-called "pre-thermal" state - in which ordered phases of matter can occur. In this context, I will describe how long-range interactions can stabilize pre-thermal time-translation symmetry breaking in one dimensional systems (even in the absence of disorder). I will begin by motivating a new definition for light-cones in power-law interacting quantum systems and using this definition, I will prove that long-range, pre-thermal time crystals naturally exhibit exponentially long lifetimes. Finally, an experimental realization of a one dimensional, pre-thermal time crystal in trapped atomic ions will be discussed. |
Tuesday, March 5, 2019 8:36AM - 8:48AM |
E25.00002: NMR observations of discrete time crystalline signatures in an ordered crystal of ammonium dihydrogen phosphate Robert Blum, Jared Rovny, Sean Barrett The discrete time crystal (DTC) is a recently-described phase of driven quantum systems that breaks the discrete time translation symmetry of its drive. If the Hamiltonian has period T, the signature of a DTC is a response of period nT that is robust to "error" in the drive. Two experiments recently reported this signature in trapped ions [1] and in diamond NV centers [2]. We present signatures of DTC order [3,4] in an NMR system of 31P spins in an oriented crystal of ammonium dihydrogen phosphate (ADP), a clean sample with multiple dipolar-coupled spin species (1H, 31P, 14N). By varying the pulse angle θ and delay time τ of the DTC pulse sequence, we observe robust DTC oscillations across a much greater range in (θ, τ) than has been observed in earlier experiments [1,2], both with and without 1H decoupling. |
Tuesday, March 5, 2019 8:48AM - 9:00AM |
E25.00003: Beyond discrete time crystal signatures: hidden coherence, causes of decay, and the first ‘discrete time crystal echo’ Jared Rovny, Robert Blum, Sean Barrett The phase structure of driven quantum systems can include exotic phenomena, including the recently-described discrete time crystal (DTC). The key signature of a DTC is a response with period nT (n=2,3,…) for drive period T, even when the drive is imperfect. Two experiments recently demonstrated this signature, one in trapped ions [1], and the other in diamond NV centers [2]. We have shown this signature in an NMR system of 31P spins on a crystal lattice, with little to no disorder [3,4]. We study the decay of the DTC oscillations, with two main results. First, we use a novel “DTC echo” sequence to demonstrate that the decay is caused in part by coherent evolution. Second, we demonstrate that the observed decay for perfect pi pulses (ε = 0) can be produced by the action of the internal Hamiltonian during the pulses. |
Tuesday, March 5, 2019 9:00AM - 9:12AM |
E25.00004: Spatial-Translation-Induced Discrete Time Crystals Kaoru Mizuta, Kazuaki Takasan, Masaya Nakagawa, Norio Kawakami Time crystals, where time translation symmetry is spontaneously broken, are novel phases of matters in that they are proved to exist only in nonequilibrium[1]. In particular, time crystals in Floquet systems, called discrete time crystals (DTCs), have attracted much interest because of theoretical developments[2] and recent experimental realization[3]. |
Tuesday, March 5, 2019 9:12AM - 9:24AM |
E25.00005: Quantum diffusion in the strong tunneling regime Nisarga Paul, Ariel Amir We discuss the dynamics of a quantum-mechanical wavepacket in a noisy environment (i.e., time-dependent disorder), modeled using a tight-binding Hamlitonian. It has been found that the fluctuating environment may give rise to diffusive behavior (rather than Anderson localization, which occurs for time-independent disorder). We develop a new approach to this problem by considering the dynamics as arising from multiple Landau-Zener crossing events. We find the conditions for the validity of the approach, and use it to calculate how the diffusion constant depends on the noise. The analytical results are corroborated numerically. The results may be applicable to exciton diffusion in photosynthesis and electronic transport in solid-state physics. |
Tuesday, March 5, 2019 9:24AM - 9:36AM |
E25.00006: Quantum-Spin Diffusion Driven by Ergodic and Non-Ergodic Finite Spin Baths Walter Hahn, Viatcheslav Dobrovitski We investigate spin diffusion driven by a finite quantum spin bath in a system accessible for solid-state NMR experiments; namely polycrystalline L-alanine. The direct spin transport within the subsystem consisting of dipolar coupled carbon spins is suppressed due to disorder given by different Larmor frequencies. Spin diffusion is, therefore, governed by the surrounding network of proton spins-1/2. This proton network consists of strongly coupled groups which are weakly interacting among each other. By means of numerical simulations, we model realistic solid-state NMR experiments. We show that nearby proton spins govern the local magnetic field for carbon spins, while the farther proton spins determine dynamics within the proton bath. In particular, the farther proton spins provide ergodicity in the proton bath and, thereby, drastically change the spin diffusion in the carbon subsystem. We also consider deuterated alanine with all proton spins-1/2 replaced by deuteron spins-1. For deuterated L-alanine, we show that the local magnetic fields created by deuterons is insufficient to allow for spin diffusion due to the small magnetic dipole moment of deuterons. Instead, spin diffusion is governed by the spin-lattice relaxation of deuteron spins on much larger time scales. |
Tuesday, March 5, 2019 9:36AM - 9:48AM |
E25.00007: Signatures of Quantum Chaos in Classically Non-Chaotic Systems Efim Rozenbaum, Leonid Bunimovich, Victor Galitski One of the original goals in the field of quantum chaos was to establish a correspondence between the dynamics of chaotic classical systems and their quantum counterparts. The general issue is that quantum-mechanical interference washes out classical chaos after a very short (logarithmic) time, and the correspondence breaks down. Recently, out-of-time-ordered correlator, a universal tool to study quantum chaos, has received a lot of attention due to its versatility and natural interpretation. We use this diagnostic to show that a new kind of drastic disagreement can occur between quantum and classical counterparts of the same model. Remarkably, quantum mechanics appears capable of playing the opposite to its usual role. In particular, it brings chaos to a family of classically non-chaotic systems, where on the quantum side, we demonstrate the Lyapunov-like exponential growth of OTOC. |
Tuesday, March 5, 2019 9:48AM - 10:00AM |
E25.00008: The longest-lived current in a quantum chaotic spin chain Arnold K. Mong, David Huse To explore issues of numerically capturing dissipative dynamics in closed many-body quantum systems, we have studied the relaxation of nonconserved current operators in a certain quantum chaotic spin chain. The Hamiltonian is a translationally-invariant spin-1/2 chain with nearest-neighbor XY interactions and a tilted field that breaks the conservation of total Z magnetization. We look at an infinite chain and examine the relaxation of all “current” operators that have total momentum zero and are odd under spatial inversion. The relaxation is via operator spreading: a unitary flow in operator space from simple short Pauli strings to long (and thus nonlocal) Pauli strings. To approximate this numerically, we limit the length of the Pauli strings and introduce an artificial nonunitary damping that acts only on the longest Pauli strings that we keep, and solve exactly for the longest-lived current operator in this approximation. We find that there is a regime of this artificial damping where we obtain a good approximation to the correct unitary dynamics, while in other regimes the artificial damping causes a blockage of the proper unitary flow in operator space and, as a result of this “bad plumbing”, gives incorrect results. |
Tuesday, March 5, 2019 10:00AM - 10:12AM |
E25.00009: Quantum Chaos for the Unitary Fermi Gas from the Generalized Boltzmann Equations Pengfei Zhang we study the chaotic behavior of the unitary Fermi gas in both high and low temperature limits by calculating the Quantum Lyapunov exponent defined in terms of the out-of-time-order correlator. We take the method of generalized Boltzmann equations derived from the augmented Keldysh approach. At high temperature, the system is described by weakly interacting fermions with two spin components and in the low temperature limit, the system is a superfluid and can be described by phonon modes. By comparing these to existing results of heat conductivity, we find that D\ll v^2 \tau_L. We argue that this is related to conservation laws for such systems with quasi-particles. |
Tuesday, March 5, 2019 10:12AM - 10:24AM |
E25.00010: Chaos and integrability in experimentally accessible all-to-all spin models Gregory Bentsen, Thomas Scaffidi, Vir Bulchandani, Ionut-Dragos Potirniche, Monika Schleier-Smith, Ehud Altman In recent years, models of disordered fermions with random, all-to-all couplings have emerged as prime candidates for studying the limit of strong chaos in quantum mechanical systems. However, such models are prohibitively difficult to realize experimentally. By contrast, spin models with random, all-to-all couplings can be engineered in the context of cavity QED and could provide an opportunity to probe strongly interacting, disordered physics in the laboratory. We show that the class of models most naturally realized in this system has the unusual property of possessing two integrable points in its phase diagram. We construct the integrals of motion explicitly and propose a method to directly measure their characteristics in the experiment. This scheme raises the possibility of tuning the system between classical and quantum physics on the one hand, by varying the effective spin per site, and between integrable and chaotic physics on the other, by varying the effective cavity interactions. |
Tuesday, March 5, 2019 10:24AM - 10:36AM |
E25.00011: Scrambling in the Dicke model Yahya Alavirad, Seyed Ali Hosseini Lavasani The scrambling rate $\lambda_L$ associated with the exponential growth of out-of-time-ordered correlators can be used to characterize quantum chaos. Here we use the Majorana Fermion representation of spin $1/2$ systems to study quantum chaos in the Dicke model. We take the system to be in thermal equilibrium and compute $\lambda_L$ throughout the phase diagram to leading order in $1/N$. We find that the chaotic behavior is strongest close to the critical point. At high temperatures $\lambda_L$ is nonzero over an extended region that includes both the normal and super-radiant phases. At low temperatures $\lambda_L$ is nonzero in (a) close vicinity of the critical point and (b) a region within the super-radiant phase. In the process we also derive a new effective theory for the super-radiant phase at finite temperatures. Our formalism does not rely on the assumption of total spin conservation. |
Tuesday, March 5, 2019 10:36AM - 10:48AM |
E25.00012: Quantum inverse freezing and mirror-glass order Thomas Iadecola, Michael Schecter It is well known that spontaneous symmetry breaking in one spatial dimension is thermodynamically forbidden at finite energy density. Here we show that mirror-symmetric disorder in an interacting quantum system can invert this paradigm, yielding spontaneous breaking of mirror symmetry only at finite energy density and giving rise to “mirror-glass” order. The mirror-glass transition, which occurs via the energetic activation of a finite density of emergent Ising degrees of freedom, is enabled by many-body localization and appears to occur simultaneously with the localization transition. This counterintuitive manifestation of localization-protected order can be viewed as a quantum analog of inverse freezing, a phenomenon that occurs, e.g., in certain models of classical spin glasses. |
Tuesday, March 5, 2019 10:48AM - 11:00AM |
E25.00013: Out of Time Ordered Correlators in the Random Field XX Spin Chain Jonathon Riddell, Erik Sorensen We study out of time order correlations, C(x,t) and entanglement growth in the random field XX model with open boundary conditions using the exact Jordan-Wigner transformation to a fermionic Hamiltonian. For any non-zero strength of the random field this model describes an Anderson insulator. Two scenarios are considered: A global quench with the initial state corresponding to a product state of the Néel form, and the behaviour in a typical thermal state at β=1. As a result of the presence of disorder the information spreading as described by the out of time correlations stops beyond a typical length scale, |
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