Local memory - the ability to extract information from a subsystem about its initial state - is a central feature of many-body localization. We introduce, investigate, and compare several information-theoretic quantifications of memory and discover a hierarchical relationship among them. We also find that while the Holevo quantity is the most complete quantifier of memory, vastly outperforming the imbalance, its decohered counterpart is significantly better at capturing the critical properties of the many-body localization transition at small system sizes. This motivates our suggestion that one can emulate the thermodynamic limit by artifically decohering otherwise quantum quantities. Applying this method to the von Neumann entropy results in critical exponents consistent with analytic predictions, a feature missing from similar small finite-size system treatments. In addition, the decohering process makes experiments significantly simpler by avoiding quantum state tomography.
Local memory - the ability to extract information from a subsystem about its initial state - is a central feature of many-body localization. We introduce, investigate, and compare several information-theoretic quantifications of memory and discover a hierarchical relationship among them. We also find that while the Holevo quantity is the most complete quantifier of memory, vastly outperforming the imbalance, its decohered counterpart is significantly better at capturing the critical properties of the many-body localization transition at small system sizes. This motivates our suggestion that one can emulate the thermodynamic limit by artifically decohering otherwise quantum quantities. Applying this method to the von Neumann entropy results in critical exponents consistent with analytic predictions, a feature missing from similar small finite-size system treatments. In addition, the decohering process makes experiments significantly simpler by avoiding quantum state tomography.
Local memory---the ability to extract information from a subsystem about its initial state---is a central feature of many-body localization. We introduce, investigate, and compare several information-theoretic quantifications of memory and discover a hierarchical relationship among them. We also find that while the Holevo quantity is the most complete quantifier of memory, vastly outperforming the imbalance, its decohered counterpart is significantly better at capturing the critical properties of the many-body localization transition at small system sizes. This motivates our suggestion that one can emulate the thermodynamic limit by artificially decohering otherwise quantum quantities. Applying this method to the von Neumann entropy results in critical exponents consistent with analytic predictions, a feature missing from similar small finite-size system treatments. In addition, the decohering process makes experiments significantly simpler by avoiding quantum state tomography.
A key feature of many-body localization (MBL) is the breaking of ergodicity and consequently the emergence of local memory, revealed as the local preservation of information over time. As memory is necessarily a time-dependent concept, it has been partially captured by a few extant studies of dynamical quantities. However, these quantities suffer from a variety of issues which limit their value as true quantifiers of memory; thus, a fundamental and complete information-theoretic understanding of local memory in the context of MBL remains elusive. We outline these issues in detail and introduce the dynamical Holevo quantity to address them. We find that it shows clear scaling behavior across the MBL transition, and we determine a family of two-parameter scaling Ans\"atze which capture this behavior. We perform a comprehensive finite-sized scaling analysis to extract the transition point and scaling exponents.
We study the entropy of small subsystems in thermalizing quantum many-body systems governed by local Hamiltonians. Assuming the eigenstate thermalization hypothesis, we derive an analytical formula for the von Neumann entropy of equilibrated subsystems. This formula reveals how subsystem entropy depends on the microscopic parameters of the Hamiltonian and the macroscopic properties of the initial state. Furthermore, our results provide a theoretical explanation for recent numerical findings by Maceira and Läuchli, obtained via exact diagonalization.
Thermodynamics is operational in the sense that the very concept of thermal equilibrium depends crucially on the choice of observables, while the amount of extractable work is defined relative to the allowed operations. Here we show that isolated quantum many-body states admit a thermodynamic hierarchical structure of observables and allowed operations, where the same state can be thermal at one level of the hierarchy and athermal at another. Enlarging the set of allowed operations therefore renders such hidden athermality a potential resource for work extraction; the resulting work gain is bounded in terms of the difference between the entropy densities of the two levels. This entropy difference takes the form of the mutual-information density. We establish the bounds for three extensions of operational access: increased spatial resolution, increased duration of control, and nonlocal connectivity, which provide access to position-state Holevo information, correlations between neighboring regions and spatially nonlocal correlations, respectively. Thus the difference between entropies at different levels of the thermodynamic hierarchy governs the bound on the work gain associated with moving to a less restricted level.
We present a mathematically controlled, sharply falsifiable framework that tests whether many-body localization (MBL) can protect quantum information against local Markovian dephasing. Using the standard phenomenology of quasi-local l-bits, a Schrieffer-Wolff transformation yields an effective classical rate equation with transition rates ~γ (J/W)² e^{-d_H/ξ}. This implies a central bound on the von Neumann entropy production: Φ ≤ C γ N (J/W)² (W ≫ J, γ ≪ J). A variational argument provides an upper bound for the Liouvillian gap. The hypothesis is subjected to a five-point falsification matrix; any single violation disproves it. We provide an open-source QuTiP implementation and numerical results for N=6, confirming the predicted scaling exponent ≈ −2 and persistent Néel imbalance. The framework is compatible with a long-lived prethermal regime and offers a concrete realization of the VES (information degradation minima) hypothesis in open many-body quantum systems. The repository contains the manuscript, executable code (QuTiP, ITensor skeleton), and all figures.
As our demand for computational power grows, we encounter the question: "What are the physical limits to computation?" An answer is necessarily incomplete unless it can incorporate physics at the smallest scales, where we expect our near-term high-performance computing to occur. Microscopic physics -- namely, quantum mechanics -- behaves counterintuitively to our everyday experience, however. Quantum matter can occupy superpositions of states and build stronger correlations than are possible classically. This affects how quantum computers and quantum thermodynamic engines will behave. Though these properties may seem to overwhelmingly defeat our attempts to build a quantum computer at-first-glance, what is remarkable is that they can also be immensely helpful to computation. Quantum mechanics hinders and helps computation, and the nuanced details of how we perform computations are important. In this dissertation, we examine the transition between these two behaviors and connect it to a well-studied behavior in condensed matter physics, known as the many-body-localization transition. Our idea utilizes the fact that quantum many-body systems have an intrinsic fastest speed at which signals can travel. When this speed is maximal, we expect arbitrary universal quantum computation to be achievable, since strong quantum correlations, or entanglement, can be built quickly. When it is limited, however, the difficulty of the computation is classically simulatable. We demonstrate a similar transition in the amount of thermodynamic work that can be performed by a quantum system when entanglement is present. We first consider computations consisting of the evolution of a single particle or many noninteracting particles. When the number of such noninteracting particles is comparable to the total size of the system, we do not know of any way to simulate such computations classically. However, we find that we can still determine the fastest signal speed in such systems. We extend our result to interacting particles, which are universal for quantum computation, and observe a many-body-localization transition in a simple computational model using our algorithm. Finally, we apply ideas from quantum information to simulate the thermodynamic performance of a simple quantum system, showing that quantum effects can enable it to outperform its classical counterpart.