241 research outputs found
The role of initial entanglement and nonGaussianity in the decoherence of photon number entangled states evolving in a noisy channel
We address the degradation of continuous variable (CV) entanglement in a
noisy channel focusing on the set of photon-number entangled states. We exploit
several separability criteria and compare the resulting separation times with
the value of non-Gaussianity at any time, thus showing that in the
low-temperature regime: i) non-Gaussianity is a bound for the relative entropy
of entanglement and ii) Simon' criterion provides a reliable estimate of the
separation time also for nonGaussian states. We provide several evidences
supporting the conjecture that Gaussian entanglement is the most robust against
noise, i.e. it survives longer than nonGaussian one, and that this may be a
general feature for CV systems in Markovian channels.Comment: revised version, title and figures change
Momentum-space analysis of multipartite entanglement at quantum phase transitions
We investigate entanglement properties at quantum phase transitions of an
integrable extended Hubbard model in the momentum space representation. Two
elementary subsystems are recognized: the single mode of an electron, and the
pair of modes (electrons coupled through the eta-pairing mechanism). We first
detect the two/multi-partite nature of each quantum phase transition by a
comparative study of the singularities of Von Neumann entropy and quantum
mutual information. We establish the existing relations between the
correlations in the momentum representation and those exhibited in the
complementary picture: the direct lattice representation. The presence of
multipartite entanglement is then investigated in detail through the Q-measure,
namely a generalization of the Meyer-Wallach measure of entanglement. Such a
measure becomes increasingly sensitive to correlations of a multipartite nature
increasing the size of the reduced density matrix. In momentum space, we
succeed in obtaining the latter for our system at arbitrary size and we relate
its behaviour to the nature of the various QPTs.Comment: 8 pages, 4 figure
Topology and energy transport in networks of interacting photosynthetic complexes
We address the role of topology in the energy transport process that occurs
in networks of photosynthetic complexes. We take inspiration from light
harvesting networks present in purple bacteria and simulate an incoherent
dissipative energy transport process on more general and abstract networks,
considering both regular structures (Cayley trees and hyperbranched fractals)
and randomly-generated ones. We focus on the the two primary light harvesting
complexes of purple bacteria, i.e., the LH1 and LH2, and we use
network-theoretical centrality measures in order to select different LH1
arrangements. We show that different choices cause significant differences in
the transport efficiencies, and that for regular networks centrality measures
allow to identify arrangements that ensure transport efficiencies which are
better than those obtained with a random disposition of the complexes. The
optimal arrangements strongly depend on the dissipative nature of the dynamics
and on the topological properties of the networks considered, and depending on
the latter they are achieved by using global vs. local centrality measures. For
randomly-generated networks a random arrangement of the complexes already
provides efficient transport, and this suggests the process is strong with
respect to limited amount of control in the structure design and to the
disorder inherent in the construction of randomly-assembled structures.
Finally, we compare the networks considered with the real biological networks
and find that the latter have in general better performances, due to their
higher connectivity, but the former with optimal arrangements can mimic the
real networks' behaviour for a specific range of transport parameters. These
results show that the use of network-theoretical concepts can be crucial for
the characterization and design of efficient artificial energy transport
networks.Comment: 14 pages, 16 figures, revised versio
Structure of quantum correlations in momentum space and off diagonal long range order in eta pairing and BCS states
The quantum states built with the eta paring mechanism i.e., eta pairing
states, were first introduced in the context of high temperature
superconductivity where they were recognized as important example of states
allowing for off-diagonal long-range order (ODLRO). In this paper we describe
the structure of the correlations present in these states when considered in
their momentum representation and we explore the relations between the quantum
bipartite/multipartite correlations exhibited in k space and the direct lattice
superconducting correlations. In particular, we show how the negativity between
paired momentum modes is directly related to the ODLRO. Moreover, we
investigate the dependence of the block entanglement on the choice of the modes
forming the block and on the ODLRO; consequently we determine the multipartite
content of the entanglement through the evaluation of the generalized "Meyer
Wallach" measure in the direct and reciprocal lattice. The determination of the
persistency of entanglement shows how the network of correlations depicted
exhibits a self-similar structure which is robust with respect to "local"
measurements. Finally, we recognize how a relation between the momentum-space
quantum correlations and the ODLRO can be established even in the case of BCS
states.Comment: 11 pages, 3 figure
Optimal estimation of entanglement
Entanglement does not correspond to any observable and its evaluation always
corresponds to an estimation procedure where the amount of entanglement is
inferred from the measurements of one or more proper observables. Here we
address optimal estimation of entanglement in the framework of local quantum
estimation theory and derive the optimal observable in terms of the symmetric
logarithmic derivative. We evaluate the quantum Fisher information and, in
turn, the ultimate bound to precision for several families of bipartite states,
either for qubits or continuous variable systems, and for different measures of
entanglement. We found that for discrete variables, entanglement may be
efficiently estimated when it is large, whereas the estimation of weakly
entangled states is an inherently inefficient procedure. For continuous
variable Gaussian systems the effectiveness of entanglement estimation strongly
depends on the chosen entanglement measure. Our analysis makes an important
point of principle and may be relevant in the design of quantum information
protocols based on the entanglement content of quantum states.Comment: 9 pages, 2 figures, v2: minor correction
Quantum Entanglement in Second-quantized Condensed Matter Systems
The entanglement between occupation-numbers of different single particle
basis states depends on coupling between different single particle basis states
in the second-quantized Hamiltonian. Thus in principle, interaction is not
necessary for occupation-number entanglement to appear. However, in order to
characterize quantum correlation caused by interaction, we use the eigenstates
of the single-particle Hamiltonian as the single particle basis upon which the
occupation-number entanglement is defined. Using the proper single particle
basis, we discuss occupation-number entanglement in important eigenstates,
especially ground states, of systems of many identical particles. The
discussions on Fermi systems start with Fermi gas, Hatree-Fock approximation,
and the electron-hole entanglement in excitations. The entanglement in a
quantum Hall state is quantified as -fln f-(1-f)ln(1-f), where f is the proper
fractional part of the filling factor. For BCS superconductivity, the
entanglement is a function of the relative momentum wavefunction of the Cooper
pair, and is thus directly related to the superconducting energy gap. For a
spinless Bose system, entanglement does not appear in the
Hatree-Gross-Pitaevskii approximation, but becomes important in the Bogoliubov
theory.Comment: 11 pages. Journal versio
Quantum Correlation in One-dimensional Extend Quantum Compass Model
We study the correlations in the one-dimensional extended quantum compass
model in a transverse magnetic field. By exactly solving the Hamiltonian, we
find that the quantum correlation of the ground state of one-dimensional
quantum compass model is vanishing. We show that quantum discord can not only
locate the quantum critical points, but also discern the orders of phase
transitions. Furthermore, entanglement quantified by concurrence is also
compared.Comment: 8 pages, 14 figures, to appear in Eur. Phys. J.
Geometric measure of quantum discord and the geometry of a class of two-qubit states
We investigate the geometric picture of the level surfaces of quantum
entanglement and geometric measure of quantum discord (GMQD) of a class of
X-states, respectively. This pictorial approach provides us a direct
understanding of the structure of entanglement and GMQD. The dynamic evolution
of GMQD under two typical kinds of quantum decoherence channels is also
investigated. It is shown that there exists a class of initial states for which
the GMQD is not destroyed by decoherence in a finite time interval.
Furthermore, we establish a factorization law between the initial and final
GMQD, which allows us to infer the evolution of entanglement under the
influences of the environment.Comment: 10 pages, 4 figures, comments are welcom
Morganella morganii septicemia and concurrent renal crassicaudiasis in a Cuvierâs beaked whale (Ziphius cavirostris) stranded in Italy
Information regarding bacterial diseases in Cuvier's beaked whale (CBW, Ziphius cavirostris) is scattered and mostly incomplete. This report describes a case of septicemia by Morganella morganii in a juvenile male CBW with concurrent renal crassicaudiasis. The animal stranded along the Ligurian coastline (Italy) and underwent a systematic post-mortem examination to determine the cause of death. Histopathology showed lesions consistent with a septicemic infection, severe meningoencephalitis, and renal crassicaudiasis. An M. morganii alpha-hemolytic strain was isolated in pure culture from liver, lung, prescapular lymph node, spleen, hepatic and renal abscesses, and central nervous system (CNS). The antimicrobial susceptibility profile of the strain was evaluated with the minimum inhibitory concentrations (MICs) method and reduced susceptibility to Trimethoprim-Sulfamethoxazole is reported. Crassicauda sp. nematodes were retrieved from both kidneys. No other pathogens were detected by immunohistochemistry, serology, or biomolecular analyses. Toxicological investigations detected high concentrations of immunosuppressant pollutants in the blubber. The chronic parasitic infestation and the toxic effects of xenobiotics likely compromised the animal's health, predisposing it to an opportunistic bacterial infection. To our knowledge, this is the first description of M. morganii septicemia with CNS involvement in a wild cetacean
Quantum Correlations in NMR systems
In conventional NMR experiments, the Zeeman energy gaps of the nuclear spin
ensembles are much lower than their thermal energies, and accordingly exhibit
tiny polarizations. Generally such low-purity quantum states are devoid of
quantum entanglement. However, there exist certain nonclassical correlations
which can be observed even in such systems. In this chapter, we discuss three
such quantum correlations, namely, quantum contextuality, Leggett-Garg temporal
correlations, and quantum discord. In each case, we provide a brief theoretical
background and then describe some results from NMR experiments.Comment: 21 pages, 7 figure
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