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arXiv:2106.07673

Unpredictability and entanglement in open quantum systems

Javad Kazemi, Hendrik Weimer

Published Jun 14, 2021• 1 citation
arXivHTMLPDF

Abstract

We investigate dynamical many-body systems capable of universal computation, which leads to their properties being unpredictable unless the dynamics is simulated from the beginning to the end. Unpredictable behavior can be quantitatively assessed in terms of a data compression of the states occurring during the time evolution, which is closely related to their Kolmogorov complexity. We analyze a master equation embedding of classical cellular automata and demonstrate the existence of a phase transition between predictable and unpredictable behavior as a function of the random noise introduced by the embedding. We then turn to have this dynamics competing with a second process inducing quantum fluctuations and dissipatively driving the system to a highly entangled steady state. Strikingly, for intermediate strength of the quantum fluctuations, we find that both unpredictability and quantum entanglement can coexist even in the long time limit. Finally, we show that the required many-body interactions for the cellular automaton embedding can be efficiently realized within a variational quantum simulator platform based on ultracold Rydberg atoms with high fidelity.

DOI
10.15488/16102

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We investigate dynamical many-body systems capable of universal computation, which leads to their properties being unpredictable unless the dynamics is simulated from the beginning to the end. Unpredictable behavior can be quantitatively assessed in terms of a data compression of the states occurring during the time evolution, which is closely related to their Kolmogorov complexity. We analyze a master equation embedding of classical cellular automata and demonstrate the existence of a phase transition between predictable and unpredictable behavior as a function of the random noise introduced by the embedding. We then turn to have this dynamics competing with a second process inducing quantum fluctuations and dissipatively driving the system to a highly entangled steady state. Strikingly, for intermediate strength of the quantum fluctuations, we find that both unpredictability and quantum entanglement can coexist even in the long time limit. Finally, we show that the required many-body interactions for the cellular automaton embedding can be efficiently realized within a variational quantum simulator platform based on ultracold Rydberg atoms with high fidelity.
Abstract We investigate dynamical many-body systems capable of universal computation, which leads to their properties being unpredictable unless the dynamics is simulated from the beginning to the end. Unpredictable behavior can be quantitatively assessed in terms of a data compression of the states occurring during the time evolution, which is closely related to their Kolmogorov complexity. We analyze a master equation embedding of classical cellular automata and demonstrate the existence of a phase transition between predictable and unpredictable behavior as a function of the random error introduced by the probabilistic character of the embedding. We then turn to have this dynamics competing with a second process inducing quantum fluctuations and dissipatively driving the system to a highly entangled steady state. Strikingly, for intermediate strength of the quantum fluctuations, we find that both unpredictability and quantum entanglement can coexist even in the long time limit. Finally, we show that the required many-body interactions for the cellular automaton embedding can be efficiently realized within a variational quantum simulator platform based on ultracold Rydberg atoms with high fidelity.
We propose a physical realization of quantum cellular automata (QCA) using arrays of ultracold atoms excited to Rydberg states. The key ingredient is the use of programmable multifrequency couplings which generalize the Rydberg blockade and facilitation effects to a broader set of nonadditive, unitary and nonunitary (dissipative) conditional interactions. Focusing on a 1D array we define a set of elementary QCA rules that generate complex and varied quantum dynamical behavior. Finally, we demonstrate theoretically that Rydberg QCA is ideally suited for variational quantum optimization protocols and quantum state engineering by finding parameters that generate highly entangled states as the steady state of the quantum dynamics.
Motivated by recent progress in the experimental development of quantum simulators based on Rydberg atoms, we introduce and investigate the dynamics of a class of (1+1)-dimensional quantum cellular automata. These nonequilibrium many-body models, which are quantum generalizations of the Domany-Kinzel cellular automaton, possess two key features: they display stationary behavior and nonequilibrium phase transitions despite being isolated systems. Moreover, they permit the controlled introduction of local quantum correlations, which allows for the impact of quantumness on the dynamics and phase transition to be assessed. We show that projected entangled pair state tensor networks permit a natural and efficient representation of the cellular automaton. Here, the degree of quantumness and complexity of the dynamics is reflected in the difficulty of contracting the tensor network.
This paper investigates the potential for emergent computation within cellular automata (CA) systems incorporating quantum mechanical principles. Traditional CAs, while capable of simulating complex patterns, are fundamentally deterministic. We propose an extension of the CA framework where individual cells possess quantum properties, specifically superposition and entanglement, influencing their interactions. This introduces a layer of probabilistic behavior and non-classical correlations, fundamentally altering the system's dynamics. The core claim is that these quantum interactions can lead to the emergence of computational processes previously unattainable within purely classical CAs. We detail a model where cells exist in a superposition of states, and interactions are mediated through entangled states, allowing for correlations that defy classical explanation. We explore the theoretical implications of such a system, outlining potential computational capabilities and challenges associated with simulating quantum effects within the discrete framework of a CA. The work highlights a novel approach to computation, suggesting a pathway toward harnessing quantum phenomena for emergent complex behavior in simulated systems.
We employ (1+11+11+1)-dimensional quantum cellular automata to study the evolution of entanglement and coherence near criticality in quantum systems that display nonequilibrium steady-state phase transitions. This construction permits direct access to the entire space-time structure of the underlying nonequilibrium dynamics, and allows for the analysis of unconventional correlations, such as entanglement in the time direction between the ``present'' and the ``past.'' We show how the uniquely quantum part of these correlations---the coherence---can be isolated and that, close to criticality, its dynamics displays a universal power-law behavior on approach to stationarity. Focusing on quantum generalizations of classical nonequilibrium systems: the Domany-Kinzel cellular automaton and the Bagnoli-Boccara-Rechtman model, we estimate the universal critical exponents for both the entanglement and coherence. As these models belong to the one-dimensional directed percolation universality class, the latter provides a key new critical exponent, one that is unique to quantum systems.
Quantum devices featuring mid-circuit measurement and reset capabilities, such as quantum computers and dual-species Rydberg quantum simulators, enable the realization of quantum cellular automata. These systems evolve in discrete time following local updates implemented by unitary gates, and allow for the realization of both closed and synthetic dissipative dynamics. Here, we focus on quantum cellular automata that implement minimal models of classical and quantum transport. To illustrate our ideas, we focus on a discrete-time totally asymmetric simple exclusion process and investigate how coherent dynamical contributions allow for the emergence of quantum effects and correlations. We find that bipartite entanglement dominates the transient evolution, while stationary states can retain quantum correlations beyond entanglement. Our results suggest viable routes for realizing transport models on quantum devices and characterizing collective quantum correlations in strongly driven systems.