arXiv subjects

Gabriel T. Landi

Publications and source records attributed to Gabriel T. Landi.

At least 73 records · Page 4Linked to original sources

arXiv2021

Bayesian estimation for collisional thermometry

Quantum thermometry exploits the high level of control in coherent devices to offer enhanced precision for temperature estimation. This highlights the need for constructing concrete estimation strategies. Of particular importance is collisional thermometry, where a series of ancillas are sent sequentially to probe the system's temperature. In this paper we put forth a complete framework for analyzing collisional thermometry using Bayesian inference. The approach is easily implementable and experimentally friendly. Moreover, it is guaranteed to always saturate the Cramer-Rao bound in the long-time limit. Subtleties concerning the prior information about the system's temperature are also discussed, and analyzed in terms of a modified Cramer-Rao bound associated to Van Trees and Schützenberger.

quant-ph↗

arXiv2021

Dephasing enhanced transport in boundary-driven quasiperiodic chains

We study dephasing-enhanced transport in boundary-driven quasi-periodic systems. Specifically we consider dephasing modelled by current preserving Lindblad dissipators acting on the non-interacting Aubry-André-Harper (AAH) and Fibonacci bulk systems. The former is known to undergo a critical localization transition with a suppression of ballistic transport above a critical value of the potential. At the critical point, the presence of non-ergodic extended states yields anomalous sub-diffusion. The Fibonacci model, on the other hand, yields anomalous transport with a continuously varying exponent depending on the potential strength. By computing the covariance matrix in the non-equilibrium steady-state, we show that sufficiently strong dephasing always renders the transport diffusive. The interplay between dephasing and quasi-periodicity gives rise to a maximum of the diffusion coefficient for finite dephasing, which suggests the combination of quasi-periodic geometries and dephasing can be used to control noise-enhanced transport.

quant-ph↗

arXiv2021

Waiting-times statistics in boundary driven free fermion chains

We study the waiting-time distributions (WTDs) of quantum chains coupled to two Lindblad baths at each end. Our focus is on free fermion chains, where we derive closed-form expressions in terms of single-particle matrices, allowing one to study arbitrarily large chain sizes. In doing so, we also derive formulas for 2-point correlation functions involving non-Hermitian propagators.

quant-ph↗

arXiv2021

Battery charging in collision models with Bayesian risk strategies

We construct a collision model where measurements in the system, together with a Bayesian decision rule, are used to classify the incoming ancillas as having either high or low ergotropy (maximum extractable work). The former are allowed to leave, while the latter are redirected for further processing, aimed at increasing their ergotropy further. The ancillas play the role of a quantum battery, and the collision model therefore implements a Maxwell demon. To make the process autonomous, and with a well defined limit cycle, the information collected by the demon is reset after each collision by means of a cold heat bath.

quant-ph↗

arXiv2021

Machine classification for probe based quantum thermometry

We consider probe-based quantum thermometry and show that machine classification can provide model-independent estimation with quantifiable error assessment. Our approach is based on the k-nearest-neighbor algorithm. The machine is trained using data from either computer simulations or a calibration experiment. This yields a predictor which can be used to estimate the temperature from new observations. The algorithm is highly flexible and works with any kind of probe observable. It also allows to incorporate experimental errors, as well as uncertainties about experimental parameters. We illustrate our method with an impurity thermometer in a Bose-gas, as well as in the estimation of the thermal phonon number in the Rabi model.

quant-ph↗

arXiv2021

Correlations break homogenization

The standard collisional model paradigm consists of a system that interacts sequentially with identically prepared ancillas. After infinitely many collisions, and under appropriate conditions, the system may converge to the same state as the ancillas. This process, known as homogenization, is independent of the ancilla initial state, being a property only of the underlying dynamics. In this paper we extend this idea to locally identical, but globally correlated, ancillas, and show that correlations break homogenization. This is done numerically using a minimal qubit model, and analytically using an exactly soluble Gaussian model. In both cases, we use Hamiltonian graph states with cyclic graphs as the prototypical method for building scalable many-body entangled ancillary states.

quant-ph↗

arXiv2021

Quantum quench thermodynamics at high temperatures

The entropy produced when a system undergoes an infinitesimal quench is directly linked to the work parameter susceptibility, making it sensitive to the existence of a quantum critical point. Its singular behavior at $T=0$, however, disappears as the temperature is raised, hindering its use as a tool for spotting quantum phase transitions. Notwithstanding the entropy production can be split into classical and quantum components, related with changes in populations and coherences. In this paper we show that these individual contributions continue to exhibit signatures of the quantum phase transition, even at arbitrarily high temperatures. This is a consequence of their intrinsic connection to the derivatives of the energy eigenvalues and eigenbasis. We illustrate our results in two prototypical quantum critical systems, the Landau-Zener and $XY$ models.

quant-ph↗

arXiv2021

Current fluctuations in nonequilibrium discontinuous phase transitions

Discontinuous phase transitions out of equilibrium can be characterized by the behavior of macroscopic stochastic currents. But while much is known about the the average current, the situation is much less understood for higher statistics. In this paper, we address the consequences of the diverging metastability lifetime -- a hallmark of discontinuous transitions -- in the fluctuations of arbitrary thermodynamic currents, including the entropy production. In particular, we center our discussion on the \emph{conditional} statistics, given which phase the system is in. We highlight the interplay between integration window and metastability lifetime, which is not manifested in the average current, but strongly influences the fluctuations. We introduce conditional currents and find, among other predictions, their connection to average and scaled variance through a finite-time version of Large Deviation Theory and a minimal model. Our results are then further verified in two paradigmatic models of discontinuous transitions: Schlögl's model of chemical reactions, and a $12$-states Potts model subject to two baths at different temperatures.

cond-mat.stat-mech↗

arXiv2021

Hyperaccurate thermoelectric currents

Thermodynamic currents can fluctuate significantly at the nanoscale. But some currents fluctuate less than others. Hyperaccurate currents are those which fluctuate the least, in the sense that they maximize the signal-to-noise ratio (precision). In this letter we analytically determine the hyperaccurate current in the case of a quantum thermoelectric, modeled by the Landauer-Büttiker formalism.

quant-ph↗

arXiv2021

Two-point measurement of entropy production from the outcomes of a single experiment with correlated photon pairs

Fluctuation theorems are one of the pillars of non-equilibrium thermodynamics. Broadly speaking, they concern the statistical distribution of quantities such as heat, work or entropy production. Quantum experiments, however, usually can only assess these distributions indirectly. In this letter we provide an experimental demonstration of a quantum fluctuation theorem where the distribution of entropy production is obtained directly from the outcomes (clicks) of an optical experiment. The setup consists of entangled photon pairs, one of which is sent an interferometer emulating a finite temperature amplitude damping device. Blocking specific paths of the interferometer is tantamount to restricting the possible configurations of the reservoir. And by measuring its entangled pair, we can directly implement the two-point measurement scheme, thus avoiding the destructive nature of photo-detection.

quant-ph↗

arXiv2021

Universal many-body diffusion from momentum dephasing

The open dynamics of quantum many-body systems involve not only the exchange of energy, but also of other conserved quantities, such as momentum. This leads to additional decoherence, which may have a profound impact in the dynamics. Motivated by this, we consider a many-body system subject to total momentum dephasing and show that under very general conditions this leads to a diffusive component in the dynamics of any local density, even far from equilibrium. Such component will usually have an intricate interplay with the unitary dynamics. To illustrate this, we consider the case of a superfluid and show that momentum dephasing introduces a damping in the sound-wave dispersion relation, similar to that predicted by the Navier-Stokes equation for ordinary fluids. Finally, we also study the effects of dephasing in linear response, and show that it leads to a universal additive contribution to the diffusion constant, which can be obtained from a Kubo formula.

cond-mat.quant-gas↗

arXiv2021

Optimal Quantum Thermometry with Coarse-grained Measurements

Precise thermometry for quantum systems is important to the development of new technology, and understanding the ultimate limits to precision presents a fundamental challenge. It is well known that optimal thermometry requires projective measurements of the total energy of the sample. However, this is infeasible in even moderately-sized systems, where realistic energy measurements will necessarily involve some coarse graining. Here, we explore the precision limits for temperature estimation when only coarse-grained measurements are available. Utilizing tools from signal processing, we derive the structure of optimal coarse-grained measurements and find that good temperature estimates can generally be attained even with a small number of outcomes. We apply our results to many-body systems and nonequilibrium thermometry. For the former, we focus on interacting spin lattices, both at and away from criticality, and find that the Fisher-information scaling with system size is unchanged after coarse-graining. For the latter, we consider a probe of given dimension interacting with the sample, followed by a measurement of the probe. We derive an upper bound on arbitrary, nonequilibrium strategies for such probe-based thermometry and illustrate it for thermometry on a Bose-Einstein condensate using an atomic quantum-dot probe.

quant-ph↗

arXiv2021

Contributions from populations and coherences in non-equilibrium entropy production

The entropy produced when a quantum system is driven away from equilibrium can be decomposed in two parts, one related with populations and the other with quantum coherences. The latter is usually based on the so-called relative entropy of coherence, a widely used quantifier in quantum resource theories. In this paper we argue that, despite satisfying fluctuation theorems and having a clear resource-theoretic interpretation, this splitting has shortcomings. First, it predicts that at low temperatures the entropy production will always be dominated by the classical term, irrespective of the quantum nature of the process. Second, for infinitesimal quenches, the radius of convergence diverges exponentially as the temperature decreases, rendering the functions non-analytic. Motivated by this, we provide here a complementary approach, where the entropy production is split in a way such that the contributions from populations and coherences are written in terms of a thermal state of a specially dephased Hamiltonian. The physical interpretation of our proposal is discussed in detail. We also contrast the two approaches by studying work protocols in a transverse field Ising chain, and a macrospin of varying dimension.

quant-ph↗

arXiv2021

Extracting Bayesian networks from multiple copies of a quantum system

Despite their theoretical importance, dynamic Bayesian networks associated with quantum processes are currently not accessible experimentally. We here describe a general scheme to determine the multi-time path probability of a Bayesian network based on local measurements on independent copies of a composite quantum system combined with postselection. We further show that this protocol corresponds to a non-projective measurement. It thus allows the investigation of the multi-time properties of a given local observable while fully preserving all its quantum features.

quant-ph↗

arXiv2020

Entropy production dynamics in quench protocols of a driven-dissipative critical system

Driven-dissipative phase transitions are currently a topic of intense research due to the prospect of experimental realizations in quantum optical setups. The most paradigmatic model presenting such a transition is the Kerr model, which predicts the phenomenon of optical bistability, where the system may relax to two different steady-states for the same driving condition. These states, however, are inherently out-of-equilibrium and are thus characterized by the continuous production of irreversible entropy, a key quantifier in thermodynamics. In this paper we study the dynamics of the entropy production rate in a quench scenario of the Kerr model, where the external pump is abruptly changed. This is accomplished using a recently developed formalism, based on the Husimi $Q$-function, which is particularly tailored for driven-dissipative and non-Gaussian bosonic systems [Phys. Rev. Res. 2, 013136 (2020)]. Within this framework the entropy production can be split into two contributions, one being extensive with the drive and describing classical irreversibility, and the other being intensive and directly related to quantum fluctuations. The latter, in particular, is found to reveal the high degree of non-adiabaticity, for quenches between different metastable states.

quant-ph↗

arXiv2020

Entropy Production in Continuously Measured Gaussian Quantum Systems

The entropy production rate is a key quantity in non-equilibrium thermodynamics of both classical and quantum processes. No universal theory of entropy production is available to date, which hinders progress towards its full grasping. By using a phase space-based approach, here we take the current framework for the assessment of thermodynamic irreversibility all the way to quantum regimes by characterizing entropy production -- and its rate -- resulting from the continuous monitoring of a Gaussian system. This allows us to formulate a sharpened second law of thermodynamics that accounts for the measurement back-action and information gain from a continuously monitored system. We illustrate our framework in a series of physically relevant examples.

quant-ph↗

arXiv2020

Heat rectification on the XX chain

In order to better understand the minimal ingredients for thermal rectification, we perform a detailed investigation of a simple spin chain, namely, the open XX model with a Lindblad dynamics involving global dissipators. We use a Jordan-Wigner transformation to derive a mathematical formalism to compute the heat currents and other properties of the steady state. We have rigorous results to prove the occurrence of thermal rectification even for slightly asymmetrical chains. Interestingly, we describe cases where the rectification does not decay to zero as we increase the system size, that is, the rectification remains finite in the thermodynamic limit. We also describe some numerical results for more asymmetrical chains. The presence of thermal rectification in this simple model indicates that the phenomenon is of general occurrence in quantum spin systems.

quant-ph↗

arXiv2020

Rectification induced by geometry in two-dimensional quantum spin lattices

We address the role of geometrical asymmetry in the occurrence of spin rectification in two-dimensional quantum spin chains subject to two reservoirs at the boundaries, modeled by quantum master equations. We discuss the differences in the rectification for some one-dimensional cases, and present numerical results of the rectification coefficient R for different values of the anisotropy parameter of the XXZ model, and different configurations of boundary drives, including both local and non-local dissipators. Our results also show that geometrical asymmetry, along with inhomogeneous magnetic fields, can induce spin current rectification even in the XX model, indicating that the phenomenon of rectification due to geometry may be of general occurrence in quantum spin systems.

quant-ph↗