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arXiv:1802.07729 · Quantum Science and Technology

Facilitated quantum cellular automata as simple models with non-thermal eigenstates and dynamics

Sarang Gopalakrishnan, Bahti Zakirov

Published Aug 1, 2018• 93 citations
arXivHTMLPDF

Abstract

Abstract We introduce and describe a class of simple facilitated quantum spin models in which the dynamics is due to the repeated application of unitary gates. The gates are applied periodically in time, so their combined action constitutes a Floquet unitary. The dynamics of the models we discuss can be classically simulated, and their eigenstates classically constructed (despite being highly entangled). We consider a variety of models in one and two dimensions: we are primarily concerned with models involving Clifford gates, but we also briefly discuss a model involving Clifford and Toffoli gates. Two of these models are integrable free-particle models; we construct their conserved densities. For all five models, however, the eigenstate entanglement (in all cases) and operator dynamics (in all but one case) differ from those of generic chaotic systems. In addition, two of these models have exponentially many eigenstates in which one or more sites ‘disentangle’ from the rest of the system, as a consequence of reflection symmetry.

DOI
10.1088/2058-9565/aad759

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Abstract We introduce and describe a class of simple facilitated quantum spin models in which the dynamics is due to the repeated application of unitary gates. The gates are applied periodically in time, so their combined action constitutes a Floquet unitary. The dynamics of the models we discuss can be classically simulated, and their eigenstates classically constructed (despite being highly entangled). We consider a variety of models in one and two dimensions: we are primarily concerned with models involving Clifford gates, but we also briefly discuss a model involving Clifford and Toffoli gates. Two of these models are integrable free-particle models; we construct their conserved densities. For all five models, however, the eigenstate entanglement (in all cases) and operator dynamics (in all but one case) differ from those of generic chaotic systems. In addition, two of these models have exponentially many eigenstates in which one or more sites ‘disentangle’ from the rest of the system, as a consequence of reflection symmetry.
We study the ergodic properties of a unitary Floquet dynamics arising from the repeated application of a translationally-invariant Clifford Quantum Cellular Automata to an infinite system of qubits in d dimensions. One expects that if the QCA does not exhibit any periodicity, a generic initial state of qubits will thermalize, that is, approach the infinite-temperature state. We show that this is true for many classes of states, both pure and mixed. In particular, this is true for all initial states that are short-range entangled and close to the equilibrium state. We also point out a subtle distinction between weak and strong thermalization.
Classical reversible cellular automata (CAs), which describe the discrete-time dynamics of classical degrees of freedom in a finite state space, can exhibit exact, nonthermal quantum eigenstates despite being classically chaotic. We show that every classical CA defines a family of generically nonintegrable, periodically driven (Floquet) quantum dynamics with exact, nonthermal eigenstates. These Floquet dynamics are nonergodic in the sense that certain product states on a periodic classical orbit fail to thermalize, while generic initial states thermalize as expected in a quantum chaotic system. We demonstrate that some signatures of these effects can be probed in quantum simulators based on Rydberg atoms in the blockade regime. These results establish classical CAs as parent models for a class of quantum chaotic systems with rare nonthermal eigenstates.
Abstract Many-body quantum dynamics defined on a spatial lattice and in discrete time—either as stroboscopic Floquet systems or quantum circuits—has been an active area of research for several years. Being discrete in space and time, a natural question arises: when can such a model be viewed as evolving unitarily in space as well as in time? Models with this property, which sometimes goes by the name space-time duality, have been shown to have a number of interesting features related to entanglement growth and correlations. One natural way in which the property arises in the context of (brickwork) quantum circuits is by choosing dual unitary gates: two site operators that are unitary in both the space and time directions. We introduce a class of models with q states per site, defined on the square lattice by a complex partition function and paremeterized in terms of q × q Hadamard matrices, that have the property of space-time duality. These may interpreted as particular dual unitary circuits or stroboscopically evolving systems, and generalize the well studied self-dual kicked Ising model. We explore the operator dynamics in the case of Clifford circuits, making connections to Clifford cellular automata (Schlingemann et al 2008 J. Math. Phys. 49 112104) and in the q → ∞ limit to the classical spatiotemporal cat model of many body chaos (Gutkin et al 2021 Nonlinearity 34 2800). We establish integrability and the corresponding conserved charges for a large subfamily and show how the long-range entanglement protocol discussed in the recent paper (Lotkov et al 2022 Phys. Rev. B 105 144306) can be reinterpreted in purely graphical terms and directly applied here.
We define quantum cellular automata as infinite quantum lattice systems with discrete time dynamics, such that the time step commutes with lattice translations and has strictly finite propagation speed. In contrast to earlier definitions this allows us to give an explicit characterization of all local rules generating such automata. The same local rules also generate the global time step for automata with periodic boundary conditions. Our main structure theorem asserts that any quantum cellular automaton is structurally reversible, i.e., that it can be obtained by applying two blockwise unitary operations in a generalized Margolus partitioning scheme. This implies that, in contrast to the classical case, the inverse of a nearest neighbor quantum cellular automaton is again a nearest neighbor automaton. We present several construction methods for quantum cellular automata, based on unitaries commuting with their translates, on the quantization of (arbitrary) reversible classical cellular automata, on quantum circuits, and on Clifford transformations with respect to a description of the single cells by finite Weyl systems. Moreover, we indicate how quantum random walks can be considered as special cases of cellular automata, namely by restricting a quantum lattice gas automaton with local particle number conservation to the single particle sector.
Treating the time evolution of many-body systems in discrete time steps is very useful in both classical and quantum physics, simplifying exact and numerical calculations of different observables. In this thesis, we study different classical and quantum discrete-time models. In the first part, we focus on classical systems through the lens of cellular automata which, despite being simple in construction, exhibit a multitude of differing integrable and chaotic behaviors. We study a specific subset of momentum-conserving parity check automata (MCPCA), which are cellular automata defined on bipartite graphs that conserve momentum and parity along their interaction vertices. We show that these automata describe systems with non-ergodic behavior in the spectra of their autocorrelation functions for different types of lattices. Studying their conserved charges, we demonstrate that the information spreading in such systems undergoes a phase transition between a localized and a delocalized phase. Additionally, we investigate the transport properties of the conserved charges in a specific sector of charge configuration of the MCPCA, we observe that the transport is anomalous in this sector. Moreover, we provide a classification of all possible two-states 3-to-3 cellular automata rules defined on the honeycomb lattice based on their dynamics and spectral properties. In the final part of the thesis, we focus on the quantum dual-unitary gates, which are a special kind of quantum gates that preserve their unitarity condition under space-time exchange. We give exact results on the dynamical correlation functions of dual-unitary circuits defined on random geometries that we dubbed Mikado circuits. We show different random geometries realizations and the exact results for the first two moments of the averaged correlation functions.
Random quantum circuits are paradigmatic models of minimally structured and analytically tractable chaotic dynamics. We study a family of Floquet unitary circuits with Haar random U(1)U(1)U(1) charge conserving dynamics; the minimal such model has nearest-neighbor gates acting on spin-1/2 qubits, and a single layer of even/odd gates repeated periodically in time. We find that this minimal model is not robustly thermalizing at numerically accessible system sizes, and displays slow subdiffusive dynamics for long times. We map out the thermalization dynamics in a broader parameter space of charge conserving circuits, and understand the origin of the slow dynamics in terms of proximate localized and integrable regimes in parameter space. In contrast, we find that small extensions to the minimal model are sufficient to achieve robust thermalization; these include (i) increasing the interaction range to three-site gates, (ii) increasing the local Hilbert space dimension by appending an additional unconstrained qubit to the conserved charge on each site, or (iii) using a larger Floquet period comprised of two independent layers of gates. Our results should inform future studies of charge conserving circuits, which are relevant for a wide range of topical theoretical questions.