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 S21: Precision Many-Body Physics V: DynamicsFocus Session Live
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Sponsoring Units: DCOMP DCMP DAMOP Chair: Zubin Jacob, Purdue Univ |
Thursday, March 18, 2021 11:30AM - 11:42AM Live |
S21.00001: Real-time dynamics of string breaking in quantum spin chains Roberto Verdel, Fangli Liu, Seth P Whitsitt, Alexey V Gorshkov, Markus Heyl String breaking is a central dynamical process in theories featuring confinement, where a string connecting two charges decays at the expense of the creation of new particle-antiparticle pairs. Here, we show that this process can also be observed in quantum Ising chains where domain walls get confined either by a symmetry-breaking field or by long-range interactions [1]. We find that string breaking occurs, in general, as a two-stage process. First, the initial charges remain essentially static and stable. The connecting string, however, can become a dynamical object. We develop an effective description of this motion, which we find is strongly constrained. In the second stage, which can be severely delayed due to these dynamical constraints, the string finally breaks. We observe that the associated timescale can depend crucially on the initial separation between domain walls and can grow by orders of magnitude by changing the distance by just a few lattice sites. We discuss how our results generalize to one-dimensional confining gauge theories and how they can be made accessible in quantum simulator experiments such as Rydberg atoms or trapped ions. |
Thursday, March 18, 2021 11:42AM - 11:54AM Live |
S21.00002: Quantum many-body dynamics in two dimensions with artificial neural networks Markus Schmitt, Markus Heyl The efficient numerical simulation of nonequilibrium real-time evolution in isolated quantum matter constitutes a key challenge for current computational methods. This holds in particular in the regime of two spatial dimensions, whose experimental exploration is currently pursued with strong efforts in quantum simulators. We present a versatile and efficient machine learning inspired approach based on a recently introduced artificial neural network encoding of quantum many-body wave functions. We identify and resolve some key challenges for the simulation of time evolution, which previously imposed significant limitations on the accurate description of large systems and long-time dynamics. As a concrete example, we study the dynamics of the paradigmatic two-dimensional transverse field Ising model, as recently also realized in systems of Rydberg atoms. Calculating the nonequilibrium real-time evolution across a broad range of parameters, we demonstrate that the reached time scales are comparable to or exceed the capabilities of state-of-the-art tensor network methods. |
Thursday, March 18, 2021 11:54AM - 12:06PM Live |
S21.00003: Superdiffusion and the chaotic-integrable crossover in Heisenberg spin-chains Ewan McCulloch, Tibor Rakovszky, Curt von Keyserlingk, Frank Pollmann Motivated by lessons drawn from scrambling in random circuits, we implement a Heiseberg operator evolution algorithm that artificially dissipates non-local operators—we call the method dissipation-assisted operator evolution (DAOE). The algorithm can efficiently evolve hydrodynamical observables to late times using a matrix product state representation, and has already been used to accurately predict spin/energy diffusion constants in non-integrable spin-chain models. In this work we use DAOE to obtain well-converged predictions for the super-diffusion exponent and super-diffusion constant in the isotropic XXZ chain and discuss the crossover between integrable and chaotic dynamics in experimentally accessible quantum magnets. |
Thursday, March 18, 2021 12:06PM - 12:18PM Live |
S21.00004: Integrability breaking in cellular automata Javier Lopez-Piqueres, Sarang Gopalakrishnan, Romain Vasseur Cellular automata have recently attracted much attention as testbeds where to explore the interplay of quantum chaos, integrability, and hydrodynamics. In this work we study the Rule 54 model, perhaps the simplest non-trivial quantum integrable model, which features two species of solitons. On the first part of the talk I will show how the recently developed generalized hydrodynamics (GHD) framework allows us to analytically calculate transport coefficients which we verify by means of first-principle numerical simulations. In the second part of the talk I will show how the breaking of integrability in the model can lead to a crossover from the standard Boltzmann-type exponential relaxation at short times to anomalous transport at intermediate times. |
Thursday, March 18, 2021 12:18PM - 12:30PM Live |
S21.00005: Generalized hydrodynamics in strongly interacting 1D Bose gases (I) Yuan Le, Neel Malvania, Yicheng Zhang, Jerome Dubail, Marcos Rigol, David Scott Weiss The dynamics of strongly interacting many-body quantum systems are notoriously complex and difficult to simulate. One-dimensional (1D) Bose gases can be studied experimentally with great precision and so are ideal for testing new dynamical theories. One such theory, generalized hydrodynamics (GHD), promises to efficiently simulate nearly-integrable systems by focusing on the evolution of the distribution of rapidities, which are the momenta of emergent quasiparticles. We experimentally test GHD with 1D Bose gases by suddenly increasing the trap depth in both the strong and intermediate coupling regimes and directly observing the rapidity distributions as they evolve [1]. We find excellent agreement between the experiment and GHD theory for long times after the trap quench, with dimensionless coupling parameters that range from 0.3 to 9.3, demonstrating that the approximations that underlie GHD are appropriate for many cold atom experiments. We also measure the momentum distributions after the quench, which allows us to observe the way in which the quasiparticles themselves evolve. |
Thursday, March 18, 2021 12:30PM - 12:42PM Live |
S21.00006: Generalized hydrodynamics in strongly interacting 1D Bose gases (II) Yicheng Zhang, Neel Malvania, Yuan Le, Jerome Dubail, Marcos Rigol, David Scott Weiss The dynamics of strongly interacting many-body quantum systems are notoriously complex and difficult to simulate. A new theory, generalized hydrodynamics (GHD), promises to efficiently accomplish such simulations for nearly-integrable systems. We test GHD with bundles of ultracold one-dimensional (1D) Bose gases by performing large trap quenches in both the strong and intermediate coupling regimes [1]. We find that theory and experiment agree well over dozens of trap oscillations, for average dimensionless coupling strengths that range from 0.3 to 9.3. The accuracy of GHD demonstrated here shows its wide applicability to the simulation of dynamics in nearly-integrable quantum systems. In this second talk on our results [1], we discuss the theoretical calculations behind the theory-experiment comparison. In particular, we explain how we model the initial state created experimentally and its dynamics within the GHD approach. We also present a comparison between GHD and exact numerical results in the Tonks-Girardeau limit that supports our use of GHD to study dynamics when the number of atoms per 1D gas is ~10 (the case in one of our experimental trap quenches). |
Thursday, March 18, 2021 12:42PM - 12:54PM Live |
S21.00007: Dynamical properties of the spin-boson model using real-time quantum Monte Carlo Olga Goulko, Guy Cohen, Moshe Goldstein, Hsing-Ta Chen We present results for the real-time dynamics of the spin-boson model (a two-state system coupled to a bath of non-interacting harmonic modes) using the inchworm Monte Carlo algorithm. In particular, we study the population difference between the two states at strong system-bath coupling. We focus on sub-Ohmic spectral densities of the bosonic bath, where the system exhibits a second order quantum phase transition between localized and delocalized regimes. The inchworm algorithm is efficient over a wide range of temperatures (including low, intermediate, and high temperatures) and thus allows us to examine the changes in dynamics as the temperature is increased above zero and the emergence of a quantum critical fan. |
Thursday, March 18, 2021 12:54PM - 1:06PM Live |
S21.00008: Optimised counderdiabatic driving with additional terms Ieva Cepaite, Callum Duncan, Andrew Daley, Anatoli S Polkovnikov Counterdiabatic driving protocols provide a way to speed up adiabatic dynamics. In principle, they allow us to bypass typical adiabatic limitations like slow driving rates and suppress excitations into unwanted states. These protocols can be utilised to implement arbitrarily fast quantum annealing. However, the usual approaches to counterdiabatic driving require the knowledge of the eigenstates of the system, which limits their application. It has recently been shown that the need to know the systems solutions can be removed by using a variational approach [D Sels and A Polkovnikov, PNAS 114 (20), E3909 (2017)]. We couple such a protocol with new additional parametrised driving terms and optimise the enhanced driving. We apply this new approach to random graph problems encoded in spin Hamiltonians, such as the maximum independent set and find a substantial improvement over naive adiabatic methods when optimising the additional driving terms. This new protocol allows us to improve solutions to the MIS problem, while heralding promising applications in other near-term quantum technologies. |
Thursday, March 18, 2021 1:06PM - 1:18PM Live |
S21.00009: Space- and time-crystallization effects in multicomponent superfluids Boris Svistunov, Nikolai Prokof'ev We observe that space- and time-crystallization effects in multicomponent superfluids—while having the same physical origin and mathematical description as in the single-component case—are conceptually much more straightforward. Specifically, the values of the temporal and spatial periods are absolute rather than relative, and the broken translation symmetry in space and/or time can be revealed with experiments involving only one equilibrium sample. We discuss two realistic setups—one with cold atoms and another one with bilayer superconductors—for the observation of space and time crystallization in two-component counterflow superfluids. |
Thursday, March 18, 2021 1:18PM - 1:30PM Live |
S21.00010: Simulating open quantum many-body systems using matrix product state purifications Yikang Zhang, Xin Zhang, Thomas Barthel Although we usually try to isolate quantum systems in the lab as effectively as possible, some degree of external noise and interaction with the environment is inevitable. This generally leads to decoherence, dissipation, and can alter the critical behavior in many-body systems. One can also try to engineer the environment coupling to achieve new physical phenomena. We consider Markovian systems, evolving according to Lindblad master equations. We introduce a new algorithm based on matrix product state purifications to simulate open many-body systems. This resolves a fundamental problem in simulations using matrix product density operators (MPDO) which generally lose positivity in truncations, leading to nonphysical states. We test and demonstrate our algorithm for spin chains and fermionic systems, comparing to exact diagonalization, analytical solutions, and MPDO simulations. |
Thursday, March 18, 2021 1:30PM - 2:06PM Live |
S21.00011: Quantum Phase Transitions Go Dynamical Invited Speaker: Victor Gurarie Just like a thermal partition function can be a nonanalytic function of temperature, the trace of the evolution operator of a quantum system can be a nonanalytic function of time, the phenomenon somtimes referred to as dynamical quantum phase transitions. These singularities which occur at certain points in time in the evolution of a quantum system are the subject of this talk. Interestingly, they can even appear in systems which do not undergo conventional thermal or quantum phase transitions. While there might be no obvious way to measure the trace of the evolution operator directly, it is possible to observe the “return probability”, the probability that a system which underwent a quantum quench and subsequently evolved for some time finds itself back in its original state. Sharing some similarity with the trace of the evolution operator, this probability can also be singular at certain times. In the context of quantum quench dynamics these singularities were already observed experimentally. Interpreting the trace of the evolution operator in terms of spectral form factors allows to further narrow down the class of quantum systems which could feature these singularities. In particular, they can be seen in integrable and many body localized systems which can have appropriate spectral form factors, although they are not limited to these types of systems. In the absence of a comprehensive theory of these singularities, their numerical study is often the only tool at our disposal to identify relevant systems where they may be present. |
Thursday, March 18, 2021 2:06PM - 2:18PM Live |
S21.00012: Simulation of Finite Temperature Dynamics using Purification MPS Sajant Anand, Johannes Hauschild, Michael Zaletel Finite temperature dynamics are difficult to study using Matrix Product States due to the complexity of representing a thermal density matrix. One method for studying systems at finite temperature is to use a purification, in which a mixed density matrix is represented by a pure state MPS at the cost of enlarging the Hilbert space by the addition of ancilla degrees of freedom. In a purification, there exists a gauge freedom as the represented density matrix is invariant under unitary transformations on the ancilla degrees of freedom. In this work, we optimize the purification and exploit this gauge freedom to minimize the entanglement of the purification MPS. Using a global isometry generating procedure recently introduced (arXiv:1902.05100) for a class of 2D tensor network states, we investigate the feasibility of projecting out ancilla degrees of freedom suitably unentangled from the system, which yields a more compact purification representation that is more efficiently simulated. |
Thursday, March 18, 2021 2:18PM - 2:30PM Not Participating |
S21.00013: Prethermalization and relaxation rates of observables in isolated quantum systems Krishnanand Mallayya, Marcos Rigol We study the phenomenon of prethermalization following quantum quenches in generic isolated many body systems wherein the relaxation dynamics of observables involve two regimes: a fast prethermalization and a slow thermalization. We show that in order to observe prethermalization, perturbations should break at least one local conserved quantity of the unperturbed system. We calculate the relaxation rates of observables to thermal equilibrium and show that they are given by Fermi's golden rule in the thermodynamic limit. We use numerical linked cluster expansions and exact diagonalization of finite systems for our calculations. |
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