7,698 research outputs found
Out-of-equilibrium Correlated Systems : Bipartite Entanglement as a Probe of Thermalization
Thermalization play a central role in out-of-equilibrium physics of ultracold
atoms or electronic transport phenomena. On the other hand, entanglement
concepts have proven to be extremely useful to investigate quantum phases of
matter. Here, it is argued that **bipartite** entanglement measures provide key
information on out-of-equilibrium states and might therefore offer stringent
thermalization criteria. This is illustrated by considering a global quench in
an (extended) XXZ spin-1/2 chain across its (zero-temperature) quantum critical
point. A non-local **bipartition** of the chain **preserving translation
symmetry** is proposed. The time-evolution after the quench of the **reduced**
density matrix of the half-system is computed and its associated
(time-dependent) entanglement spectrum is analyzed. Generically, the
corresponding entanglement entropy quickly reaches a "plateau" after a short
transient regime. However, in the case of the integrable XXZ chain, the
low-energy entanglement spectrum still reveals strong time-fluctuations. In
addition, its infinite-time average shows strong deviations from the spectrum
of a Boltzmann thermal density matrix. In contrast, when the integrability of
the model is broken (by small next-nearest neighbor couplings), the
entanglement spectra of the time-average and thermal density matrices become
remarkably similar.Comment: extended version: 15 pages, 9 figure
Electronic Transport in Graphene: Quantum Effects and Role of Local Defects
In this paper we present generic properties of quantum transport in
mono-layer graphene. In the scheme of the Kubo-Geenwood formula, we compute the
square spreading of wave packets of a given energy with is directly related to
conductivity. As a first result, we compute analytically the time dependent
diffusion for pure graphene. In addition to the semi-classical term a second
term exists that is due to matrix elements of the velocity operator between
electron and hole bands. This term is related to velocity fluctuations i.e.
Zitterbewegung effect. Secondly, we study numerically the quantum diffusion in
graphene with simple vacancies and pair of neighboring vacancies (divacancies),
that simulate schematically oxidation, hydrogenation and other
functionalisations of graphene. We analyze in particular the time dependence of
the diffusion and its dependence on energy in relation with the electronic
structure. We compute also the mean free path and the semi-classical value of
the conductivity as a function of energy in the limit of small concentration of
defects.Comment: 10 pages, 5 figure
The Gap Function Phi(k,w) for a Two-leg t-J Ladder and the Pairing Interaction
The gap function phi(k,omega), determined from a Lanczos calculation for a
doped 2-leg t-J ladder, is used to provide insight into the spatial and
temporal structure of the pairing interaction. It implies that this interaction
is a local near-neighbor coupling which is retarded. The onset frequency of the
interaction is set by the energy of an S=1 magnon-hole-pair and it is spread
out over a frequency region of order the bandwith
Confidence regions for the multinomial parameter with small sample size
Consider the observation of n iid realizations of an experiment with d>1
possible outcomes, which corresponds to a single observation of a multinomial
distribution M(n,p) where p is an unknown discrete distribution on {1,...,d}.
In many applications, the construction of a confidence region for p when n is
small is crucial. This concrete challenging problem has a long history. It is
well known that the confidence regions built from asymptotic statistics do not
have good coverage when n is small. On the other hand, most available methods
providing non-asymptotic regions with controlled coverage are limited to the
binomial case d=2. In the present work, we propose a new method valid for any
d>1. This method provides confidence regions with controlled coverage and small
volume, and consists of the inversion of the "covering collection"' associated
with level-sets of the likelihood. The behavior when d/n tends to infinity
remains an interesting open problem beyond the scope of this work.Comment: Accepted for publication in Journal of the American Statistical
Association (JASA
Heisenberg-limited qubit readout with two-mode squeezed light
We show how to use two-mode squeezed light to exponentially enhance
cavity-based dispersive qubit measurement. Our scheme enables true
Heisenberg-limited scaling of the measurement, and crucially, is not restricted
to small dispersive couplings or unrealistically long measurement times. It
involves coupling a qubit dispersively to two cavities, and making use of a
symmetry in the dynamics of joint cavity quadratures (a so-called
quantum-mechanics-free subsystem). We discuss the basic scaling of the scheme
and its robustness against imperfections, as well as a realistic implementation
in circuit quantum electrodynamics.Comment: 5 pages, 4 figures, Supplemental Materia
Automatic configuration of routing control platforms in OpenFlow networks
RouteFlow provides a way to run routing control platforms (e. g. Quagga) in OpenFlow networks. One of the issues of RouteFlow is that an administrator needs to devote a lot of time (typically 7 hours for 28 switches) in manual configurations. We propose and demonstrate a framework that can automatically configure RouteFlow. For this demonstration, we use an emulated pan-European topology of 28 switches. In the demonstration, we stream a video clip from a server to a remote client, and show that the video clip reaches at the remote client within 4 minutes (including the configuration time). In addition, we show automatic configuration of RouteFlow using a GUI (Graphical User Interface)
The various manifestations of collisionless dissipation in wave propagation
The propagation of an electrostatic wave packet inside a collisionless and
initially Maxwellian plasma is always dissipative because of the irreversible
acceleration of the electrons by the wave. Then, in the linear regime, the wave
packet is Landau damped, so that in the reference frame moving at the group
velocity, the wave amplitude decays exponentially with time. In the nonlinear
regime, once phase mixing has occurred and when the electron motion is nearly
adiabatic, the damping rate is strongly reduced compared to the Landau one, so
that the wave amplitude remains nearly constant along the characteristics. Yet,
we show here that the electrons are still globally accelerated by the wave
packet, and, in one dimension, this leads to a non local amplitude dependence
of the group velocity. As a result, a freely propagating wave packet would
shrink, and, therefore, so would its total energy. In more than one dimension,
not only does the magnitude of the group velocity nonlinearly vary, but also
its direction. In the weakly nonlinear regime, when the collisionless damping
rate is still significant compared to its linear value, this leads to an
effective defocussing effect which we quantify, and which we compare to the
self-focussing induced by wave front bowing.Comment: 23 pages, 6 figure
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