1,007 research outputs found
Entanglement protection via periodic environment resetting in continuous-time quantum-dynamical processes
Degradation of Entanglement in Markovian Noise
The entanglement survival time is defined as the maximum time a system which
is evolving under the action of local Markovian, homogenous in time noise, is
capable to preserve the entanglement it had at the beginning of the temporal
evolution. In this paper we study how this quantity is affected by the
interplay between the coherent preserving and dissipative contributions of the
corresponding dynamical generator. We report the presence of a
counterintuitive, non-monotonic behaviour in such functional, capable of
inducing sudden death of entanglement in models which, in the absence of
unitary driving are capable to sustain entanglement for arbitrarily long times.Comment: 15 pages, 5 figure
Building versatile bipartite probes for quantum metrology
We consider bipartite systems as versatile probes for the estimation of
transformations acting locally on one of the subsystems. We investigate what
resources are required for the probes to offer a guaranteed level of
metrological performance, when the latter is averaged over specific sets of
local transformations. We quantify such a performance via the average skew
information, a convex quantity which we compute in closed form for bipartite
states of arbitrary dimensions, and which is shown to be strongly dependent on
the degree of local purity of the probes. Our analysis contrasts and
complements the recent series of studies focused on the minimum, rather than
the average, performance of bipartite probes in local estimation tasks, which
was instead determined by quantum correlations other than entanglement. We
provide explicit prescriptions to characterize the most reliable states
maximizing the average skew information, and elucidate the role of state
purity, separability and correlations in the classification of optimal probes.
Our results can help in the identification of useful resources for sensing,
estimation and discrimination applications when complete knowledge of the
interaction mechanism realizing the local transformation is unavailable, and
access to pure entangled probes is technologically limited.Comment: 13+5 pages, 2 figures (added new section
A tomographic approach to non-Markovian master equations
We propose a procedure based on symplectic tomography for reconstructing the
unknown parameters of a convolutionless non-Markovian Gaussian noisy evolution.
Whenever the time-dependent master equation coefficients are given as a
function of some unknown time-independent parameters, we show that these
parameters can be reconstructed by means of a finite number of tomograms. Two
different approaches towards reconstruction, integral and differential, are
presented and applied to a benchmark model made of a harmonic oscillator
coupled to a bosonic bath. For this model the number of tomograms needed to
retrieve the unknown parameters is explicitly computed.Comment: 15 pages, 2 figure
Reconstruction of Markovian Master Equation parameters through symplectic tomography
In open quantum systems, phenomenological master equations with unknown
parameters are often introduced. Here we propose a time-independent procedure
based on quantum tomography to reconstruct the potentially unknown parameters
of a wide class of Markovian master equations. According to our scheme, the
system under investigation is initially prepared in a Gaussian state. At an
arbitrary time t, in order to retrieve the unknown coefficients one needs to
measure only a finite number (ten at maximum) of points along three
time-independent tomograms. Due to the limited amount of measurements required,
we expect our proposal to be especially suitable for experimental
implementations.Comment: 7 pages, 3 figure
Gaussian Discriminating Strength
We present a quantifier of non-classical correlations for bipartite,
multi-mode Gaussian states. It is derived from the Discriminating Strength
measure, introduced for finite dimensional systems in A. Farace et al., New. J.
Phys. 16, 073010 (2014). As the latter the new measure exploits the Quantum
Chernoff Bound to gauge the susceptibility of the composite system with respect
to local perturbations induced by unitary gates extracted from a suitable set
of allowed transformations (the latter being identified by posing some general
requirements). Closed expressions are provided for the case of two-mode
Gaussian states obtained by squeezing or by linearly mixing via a beam-splitter
a factorized two-mode thermal state. For these density matrices, we study how
non-classical correlations are related with the entanglement present in the
system and with its total photon number.Comment: 11+6 pages, 4 figure
Local quantum thermal susceptibility
Thermodynamics relies on the possibility to describe systems composed of a
large number of constituents in terms of few macroscopic variables. Its
foundations are rooted into the paradigm of statistical mechanics, where
thermal properties originate from averaging procedures which smoothen out local
details. While undoubtedly successful, elegant and formally correct, this
approach carries over an operational problem: what is the precision at which
such variables are inferred, when technical/practical limitations restrict our
capabilities to local probing? Here we introduce the local quantum thermal
susceptibility, a quantifier for the best achievable accuracy for temperature
estimation via local measurements. Our method relies on basic concepts of
quantum estimation theory, providing an operative strategy to address the local
thermal response of arbitrary quantum systems at equilibrium. At low
temperatures it highlights the local distinguishability of the ground state
from the excited sub-manifolds, thus providing a method to locate quantum phase
transitions.Comment: 9 pages, 9 figures; supplemental material (2 pages). Substantial
change
Entropy production and asymptotic factorization via thermalization: a collisional model approach
The Markovian evolution of an open quantum system is characterized by a
positive entropy production, while the global entropy gets redistributed
between the system and the environment degrees of freedom. Starting from these
premises, we analyze the entropy variation of an open quantum system in terms
of two distinct relations: the Clausius inequality, that provides an intrinsic
bound for the entropy variation in terms of the heat absorbed by the system,
and an extrinsic inequality, which instead relates the former to the
corresponding entropy increment of the environment. By modeling the
thermalization process with a Markovian collisional model, we compare and
discuss the two bounds, showing that the latter is asymptotically saturated in
the limit of large interaction time. In this regime not only the reduced
density matrix of the system reaches an equilibrium configuration, but it also
factorizes from the environment degrees of freedom. This last result is proven
analytically when the system-bath coupling is sufficiently strong and through
numerical analysis in the weak-coupling regime.Comment: 10 pages, 2 figure
Predator and detritivore niche width helps to explain biocomplexity of experimental detritus-based food webs in four aquatic and terrestrial ecosystems
In the study of food webs, the existence and explanation of recurring patterns, such as the scale
invariance of linkage density, predator–prey ratios and mean chain length, constitute long-standing
issues. Our study focused on litter-associated food webs and explored the influence of detritivore and
predator niche width (as d13C range) on web topological structure. To compare patterns within and
between aquatic and terrestrial ecosystems and take account of intra-habitat variability, we constructed
42 macroinvertebrate patch-scale webs in four different habitats (lake, lagoon, beech forest and
cornfield), using an experimental approach with litterbags. The results suggest that although web
differences exist between ecosystems, patterns are more similar within than between aquatic and
terrestrial web types. In accordance with optimal foraging theory, we found that the niche width of
predators and prey increased with the number of predators and prey taxa as a proportion of total taxa in
the community. The tendency was more marked in terrestrial ecosystems and can be explained by a
lower per capita food level than in aquatic ecosystems, particularly evident for predators. In accordance
with these results, the number of links increased with the number of species but with a significantly
sharper regression slope for terrestrial ecosystems. As a consequence, linkage density, which was found
to be directly correlated to niche width, increased with the total number of species in terrestrial webs,
whereas it did not change significantly in aquatic ones, where connectance scaled negatively with the
total number of species. In both types of ecosystem, web robustness to rare species removal increased
with connectance and the niche width of predators. In conclusion, although limited to litter-associated
macroinvertebrate assemblages, this study highlights structural differences and similarities between
aquatic and terrestrial detrital webs, providing field evidence of the central role of niche width in
determining the structure of detritus-based food webs and posing foraging optimisation constraints as a
general mechanistic explanation of food web complexity differences within and between ecosystem
types
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