23 research outputs found
Many-body localization dynamics from gauge invariance
We show how lattice gauge theories can display many-body localization
dynamics in the absence of disorder. Our starting point is the observation
that, for some generic translationally invariant states, Gauss law effectively
induces a dynamics which can be described as a disorder average over gauge
super-selection sectors. We carry out extensive exact simulations on the
real-time dynamics of a lattice Schwinger model, describing the coupling
between U(1) gauge fields and staggered fermions. Our results show how memory
effects and slow entanglement growth are present in a broad regime of
parameters - in particular, for sufficiently large interactions. These findings
are immediately relevant to cold atoms and trapped ions experiments realizing
dynamical gauge fields, and suggest a new and universal link between
confinement and entanglement dynamics in the many-body localized phase of
lattice models.Comment: 5Pages + appendices; V2: updated discussion in page 2, more numerical
results, added reference
Multi-spin probes for thermometry in the strong-coupling regime
We study the sensitivity of thermometric probes that are composed of
spins coupled to a sample prepared at temperature . Our analysis extends
beyond the weak-coupling limit into the strong sample-probe coupling regime. In
particular, sample-induced interactions between each of the spins are generated
via strong coupling effects and are not fine-tuned amongst each body composing
the probe. By employing the reaction-coordinate mapping to evaluate the
non-canonical equilibrium state of the probe at finite coupling, we compute the
thermometric sensitivity via the quantum Fisher information through the
equilibrium state itself. We find that for single-spin probes ,
temperature sensitivity decreases in the regime of weak-to-intermediate
coupling strength, however, as the coupling increases we observe much higher
sensitivity of the probe in the low-temperature regime. Furthermore, as long as
, there exist optimal values of the sample-probe interaction energy that
allow one to attain enhanced thermometric sensitivity when compared to the
maximum achieved precision obtained from thermal Gibbs states at weak coupling,
particularly in the regime of low temperature. Finally, we show that this
enhanced sensitivity may be observed from suboptimal measurements.Comment: 9 pages. Comments welcome
Multipartite Entanglement Structure in the Eigenstate Thermalization Hypothesis
We study the quantum Fisher information (QFI) and, thus, the multipartite entanglement structure of thermal pure states in the context of the eigenstate thermalization hypothesis (ETH). In both the canonical ensemble and the ETH, the quantum Fisher information may be explicitly calculated from the response functions. In the case of the ETH, we find that the expression of the QFI bounds the corresponding canonical expression from above. This implies that although average values and fluctuations of local observables are indistinguishable from their canonical counterpart, the entanglement structure of the state is starkly different; with the difference amplified, e.g., in the proximity of a thermal phase transition. We also provide a state-of-the-art numerical example of a situation where the quantum Fisher information in a quantum many-body system is extensive while the corresponding quantity in the canonical ensemble vanishes. Our findings have direct relevance for the entanglement structure in the asymptotic states of quenched many-body dynamics. \ua9 2020 American Physical Society
Effective-Hamiltonian theory: An approximation to the equilibrium state of open quantum systems
We extend and benchmark the recently-developed Effective-Hamiltonian (EFFH)
method [PRX Quantum , 020307 (2023)] as an approximation to the
equilibrium state ("mean-force Gibbs state") of a quantum system at strong
coupling to a thermal bath. The EFFH method is an approximate framework.
Through a combination of the reaction-coordinate mapping, a polaron
transformation and a controlled truncation, it imprints the system-bath
coupling parameters into the system's Hamiltonian. First, we develop a
EFFH technique. In this method, system's parameters are
renormalized by both the system-bath coupling parameters (as in the original
EFFH approach) and the bath's temperature. Second, adopting the generalized
spin-boson model, we benchmark the equilibrium state from the EFFH treatment
against numerically-exact simulations and demonstrate a good agreement for both
polarization and coherences using the Brownian spectral function. Third, we
contrast the (normal and variational) EFFH approach with the familiar (normal
and variational) polaron treatment. We show that the two methods predict a
similar structure for the equilibrium state, albeit the EFFH approach offers
the advantage of simpler calculations and closed-form analytical results.
Altogether, we argue that for temperatures comparable to the system's
frequencies, the EFFH methodology provides a good approximation for the
mean-force Gibbs state in the full range of system-bath coupling, from
ultraweak to ultrastrong.Comment: 21 pages, 5 figure
Out-of-time-order correlations and the fine structure of eigenstate thermalisation
Out-of-time-order correlators (OTOCs) have become established as a tool to
characterise quantum information dynamics and thermalisation in interacting
quantum many-body systems. It was recently argued that the expected exponential
growth of the OTOC is connected to the existence of correlations beyond those
encoded in the standard Eigenstate Thermalisation Hypothesis (ETH). We show
explicitly, by an extensive numerical analysis of the statistics of operator
matrix elements in conjunction with a detailed study of OTOC dynamics, that the
OTOC is indeed a precise tool to explore the fine details of the ETH. In
particular, while short-time dynamics is dominated by correlations, the
long-time saturation behaviour gives clear indications of an operator-dependent
energy scale associated to the emergence of an effective Gaussian random matrix
theory.Comment: 5 pages main, 10 pages supplemental. 13 figures. V2: updated format
for readabilit
Taking the temperature of a pure quantum state
Temperature is a deceptively simple concept that still raises deep questions
at the forefront of quantum physics research. The observation of thermalisation
in completely isolated quantum systems, such as cold-atom quantum simulators,
implies that a temperature can be assigned even to individual, pure quantum
states. Here, we propose a scheme to measure the temperature of such pure
states through quantum interference. Our proposal involves interferometry of an
auxiliary qubit probe, which is prepared in a superposition state and
subsequently undergoes decoherence due to weak coupling with a closed,
thermalised many-body system. Using only a few basic assumptions about chaotic
quantum systems -- namely, the eigenstate thermalisation hypothesis and the
emergence of hydrodynamics at long times -- we show that the qubit undergoes
pure exponential decoherence at a rate that depends on the temperature of its
surroundings. We verify our predictions by numerical experiments on a quantum
spin chain that thermalises after absorbing energy from a periodic drive. Our
work provides a general method to measure the temperature of isolated, strongly
interacting systems under minimal assumptions.Comment: 5+6 pages, 4+3 figures. Comments welcome. v2: Improved text and
figures for clarit