1,721,032 research outputs found
Solution of the BEC to BCS Quench in One Dimension
Full text linkA gas of interacting fermions confined in a quasi one-dimensional geometry
shows a BEC to BCS crossover upon slowly driving its coupling constant through
a confinement-induced resonance. On one side of the crossover the fermions form
tightly-bound bosonic molecules behaving as a repulsive Bose gas, while on the
other they form Cooper pairs, whose size is much larger than the average
inter-particle distance. Here we consider the situation arising when the
coupling constant is varied suddenly from the BEC to the BCS value. Namely, we
study a BEC-to-BCS quench. By exploiting a suitable continuum limit of recently
discovered solvable quenches in the Hubbard model, we show that the local
stationary state reached at large times after the quench can be determined
exactly by means of the Quench Action approach. We provide an
experimentally-accessible characterisation of the stationary state by computing
local pair correlation function as well as the quasi-particle distribution
functions. We find that the steady state is increasingly dominated by two
particle spin singlet bound states for stronger interaction strength but that
bound state formation is inhibited at larger BEC density. The bound state
rapidity distribution displays quartic power law decay suggesting a violation
of Tan's contact relations.Comment: 5 pages, 2 figure
Quantum quench in the infinitely repulsive Hubbard model: The stationary state
No full textWe use the quench action approach to study the non-equilibrium dynamics after a quantum quench in the Hubbard model in the limit of infinite interaction. We identify a variety of low-entangled initial states for which we can directly compute the overlaps with the Hamiltonian's eigenstates. For these initial states, we analytically find the rapidity distributions of the stationary state characterising the expectation values of all local observables. Some of the initial states considered are not reflection symmetric and lead to non-symmetric rapidity distributions. To study such cases, we have to introduce a generalised form for the reduced entropy which measures the entropy restricted to states with non-zero overlap. The initial states considered are of direct experimental realisability and also represent ideal candidates for studying non-equilibrium dynamics in the Hubbard model for finite interactions
Entanglement and diagonal entropies after a quench with no pair structure
Full text linkA typical working condition in the study of quantum quenches is that the initial state produces a distribution of quasiparticle excitations with an opposite-momentum-pair structure. In this work we investigate the dynamical and stationary properties of the entanglement entropy after a quench from initial states which do not have such structure: instead of pairs of excitations they generate -plets of correlated excitations with > 2. Our study is carried out focusing on a system of non-interacting fermions on the lattice. We study the time evolution of the entanglement entropy showing that the standard semiclassical formula is not applicable. We propose a suitable generalisation which correctly describes the entanglement entropy evolution and perfectly matches numerical data. We finally consider the relation between the thermodynamic entropy of the stationary state and the diagonal entropy, showing that when there is no pair structure their ratio depends on the details of the initial state and lies generically between 1/2 and 1
Low-temperature transport in out-of-equilibrium XXZ chains
Full text linkWe study the low-temperature transport properties of out-of equilibrium XXZ spin-1/2 chains. We consider the protocol where two semiinfinite chains are prepared in two thermal states at small but different temperatures and suddenly joined together. We focus on the qualitative and quantitative features of the profiles of local observables, which at large times t and distances x from the junction become functions of the ratio ζ = x/t. By means of the generalized hydrodynamic equations, we analyse the rich phenomenology arising by considering different regimes of the phase diagram. In the gapped phases, variations of the profiles are found to be exponentially small in the temperatures, but described by non-trivial functions of ζ. We provide analytical formulae for the latter, which give accurate results also for small but finite temperatures. In the gapless regime, we show how the three-step conformal predictions for the profiles of energy density and energy current are naturally recovered from the hydrodynamic equations. Moreover, we also recover the recent non-linear Luttinger liquid predictions for low-temperature transport: universal peaks of width ∆ζ ∝ T emerge at the edges of the light cone in the profiles of generic observables. Such peaks are described by the same function of ζ for all local observables
Quantum quenches in the sinh-Gordon model: steady state and one-point correlation functions
No full textWe consider quantum quenches to the sinh-Gordon integrable quantum field theory from a particular class of initial states. Our analysis includes the case of mass and interaction quenches starting from a non-interacting theory. By means of the recently developed quench action method, we fully characterize the stationary state reached at long times after the quench in terms of the corresponding rapidity distribution. We also provide exact results for the expectation values of arbitrary vertex operators in the post-quench stationary state by proposing a formula based on the analogy with the standard thermodynamic Bethe ansatz. Finally, we comment on the behavior of the post-quench stationary state under the mapping between the sinh-Gordon field theory and the one-dimensional Lieb-Liniger model
Universal broadening of the light cone in low-temperature transport
Full text linkWe consider the low-temperature transport properties of critical one-dimensional systems that can be described, at equilibrium, by a Luttinger liquid. We focus on the prototypical setting where two semi-infinite chains are prepared in two thermal states at small but different temperatures and suddenly joined together. At large distances x and times t, conformal field theory characterizes the energy transport in terms of a single light cone spreading at the sound velocity v. Energy density and current take different constant values inside the light cone, on its left, and on its right, resulting in a three-step form of the corresponding profiles as a function of ζ=x/t. Here, using a nonlinear Luttinger liquid description, we show that for generic observables this picture is spoiled as soon as a nonlinearity in the spectrum is present. In correspondence of the transition points x/t=±v, a novel universal region emerges at infinite times, whose width is proportional to the temperatures on the two sides. In this region, expectation values have a different temperature dependence and show smooth peaks as a function of ζ. We explicitly compute the universal function describing such peaks. In the specific case of interacting integrable models, our predictions are analytically recovered by the generalized hydrodynamic approach
Acquisition source identification through a blind image classification
No full textImage forensics, besides understanding if a digital image has been forged, often aims at determining information about image origin. In particular, it could be worthy to individuate which is the kind of source (digital camera, scanner or computer graphics software) that has generated a certain photo. Such an issue has already been studied in literature, but the problem of doing that in a blind manner has not been faced so far. It is easy to understand that in many application scenarios information at disposal is usually very limited; this is the case when, given a set of L images, the authors want to establish if they belong to K different classes of acquisition sources, without having any previous knowledge about the number of specific types of generation processes. The proposed system is able, in an unsupervised and fast manner, to blindly classify a group of photos without neither any initial information about their membership nor by resorting at a trained classifier. Experimental results have been carried out to verify actual performances of the proposed methodology and a comparative analysis with two SVM-based clustering techniques has been performed too
Nonequilibrium Full Counting Statistics and Symmetry-Resolved Entanglement from Space-Time Duality
Full text linkDue to its probabilistic nature, a measurement process in quantum mechanics
produces a distribution of possible outcomes. This distribution - or its
Fourier transform known as full counting statistics (FCS) - contains much more
information than say the mean value of the measured observable and accessing it
is sometimes the only way to obtain relevant information about the system. In
fact, the FCS is the limit of an even more general family of observables - the
charged moments - that characterise how quantum entanglement is split in
different symmetry sectors in the presence of a global symmetry. Here we
consider the evolution of the FCS and of the charged moments of a U(1) charge
truncated to a finite region after a global quantum quench. For large scales
these quantities take a simple large-deviation form, showing two different
regimes as functions of time: while for times much larger than the size of the
region they approach a stationary value set by the local equilibrium state, for
times shorter than region size they show a non-trivial dependence on time. We
show that, whenever the initial state is also U(1) symmetric, the leading order
in time of FCS and charged moments in the out-of-equilibrium regime can be
determined by means of a space-time duality. Namely, it coincides with the
stationary value in the system where the roles of time and space are exchanged.
We use this observation to find some general properties of FCS and charged
moments out-of-equilibrium, and to derive an exact expression for these
quantities in interacting integrable models. We test this expression against
exact results in the Rule 54 quantum cellular automaton and exact numerics in
the XXZ spin-1/2 chain.Comment: 7+12 pages, 3+5 figures; v2 references added; v3 presentation
improved, as appears in PR
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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