584 research outputs found
Non-Gaussian quantum discord for Gaussian states
In recent years the paradigm based on entanglement as the unique measure of
quantum correlations has been challenged by the rise of new correlation
concepts, such as quantum discord, able to reveal quantum correlations that are
present in separable states. It is in general difficult to compute quantum
discord, because it involves a minimization over all possible local
measurements in a bipartition. In the realm of continuous variable (CV)
systems, a Gaussian version of quantum discord has been put forward upon
restricting to Gaussian measurements. It is natural to ask whether non-Gaussian
measurements can lead to a stronger minimization than Gaussian ones. Here we
focus on two relevant classes of two-mode Gaussian states: squeezed thermal
states (STS) and mixed thermal states (MTS), and allow for a range of
experimentally feasible non-Gaussian measurements, comparing the results with
the case of Gaussian measurements. We provide evidence that Gaussian
measurements are optimal for Gaussian states.Comment: 12 pages, 9 figures (3 appendices
Coherent cavity networks with complete connectivity
When cavity photons couple to an optical fiber with a continuum of modes,
they usually leak out within a finite amount of time. However, if the fiber is
about one meter long and linked to a mirror, photons bounce back and forth
within the fiber on a much faster time scale. As a result, {\em dynamical
decoupling} prevents the cavity photons from entering the fiber. In this paper
we use the simultaneous dynamical decoupling of a large number of distant
cavities from the fiber modes of linear optics networks to mediate effective
cavity-cavity interactions in a huge variety of configurations. Coherent cavity
networks with complete connectivity can be created with potential applications
in quantum computing and simulation of the complex interaction Hamiltonians of
biological systems.Comment: revised version, improved analysis, 4 pages and 4 figure
Oxidative protein folding in the mitochondrial intermembrane space
Disulfide bond formation is a crucial step for oxidative folding and necessary for the acquisition of a protein's native conformation. Introduction of disulfide bonds is catalyzed in specialized subcellular compartments and requires the coordinated action of specific enzymes. The intermembrane space of mitochondria has recently been found to harbor a dedicated machinery that promotes the oxidative folding of substrate proteins by shuttling disulfide bonds. The newly identified oxidative pathway consists of the redox-regulated receptor Mia40 and the sulfhydryl oxidase Erv1. Proteins destined to the intermembrane space are trapped by a disulfide relay mechanism that involves an electron cascade from the incoming substrate to Mia40, then on to Erv1, and finally to molecular oxygen via cytochrome c. This thiol–disulfide exchange mechanism is essential for the import and for maintaining the structural stability of the incoming precursors. In this review we describe the mechanistic parameters that define the interaction and oxidation of the substrate proteins in light of the recent publications in the mitochondrial oxidative folding field
Bipartite quantum states and random complex networks
We introduce a mapping between graphs and pure quantum bipartite states and
show that the associated entanglement entropy conveys non-trivial information
about the structure of the graph. Our primary goal is to investigate the family
of random graphs known as complex networks. In the case of classical random
graphs we derive an analytic expression for the averaged entanglement entropy
while for general complex networks we rely on numerics. For large
number of nodes we find a scaling where both
the prefactor and the sub-leading O(1) term are a characteristic of
the different classes of complex networks. In particular, encodes
topological features of the graphs and is named network topological entropy.
Our results suggest that quantum entanglement may provide a powerful tool in
the analysis of large complex networks with non-trivial topological properties.Comment: 4 pages, 3 figure
Bures metric over thermal state manifolds and quantum criticality
We analyze the Bures metric over the manifold of thermal density matrices for
systems featuring a zero temperature quantum phase transition. We show that the
quantum critical region can be characterized in terms of the temperature
scaling behavior of the metric tensor itself. Furthermore, the analysis of the
metric tensor when both temperature and an external field are varied, allows to
complement the understanding of the phase diagram including cross-over regions
which are not characterized by any singular behavior. These results provide a
further extension of the scope of the metric approach to quantum criticality.Comment: 9 pages, 4 figures, LaTeX problems fixed, references adde
Ground-State Entanglement in Interacting Bosonic Graphs
We consider a collection of bosonic modes corresponding to the vertices of a
graph Quantum tunneling can occur only along the edges of
and a local self-interaction term is present. Quantum entanglement of one
vertex with respect the rest of the graph is analyzed in the ground-state of
the system as a function of the tunneling amplitude The topology of
plays a major role in determining the tunneling amplitude
which leads to the maximum ground-state entanglement. Whereas in most of the
cases one finds the intuitively expected result we show that it
there exists a family of graphs for which the optimal value of is pushed
down to a finite value. We also show that, for complete graphs, our bi-partite
entanglement provides useful insights in the analysis of the cross-over between
insulating and superfluid ground statesComment: 5 pages (LaTeX) 5 eps figures include
Two-Point Versus Multipartite Entanglement in Quantum Phase Transitions
We analyze correlations between subsystems for an extended Hubbard model
exactly solvable in one dimension, which exhibits a rich structure of quantum
phase transitions (QPTs). The T=0 phase diagram is exactly reproduced by
studying singularities of single-site entanglement. It is shown how comparison
of the latter quantity and quantum mutual information allows one to recognize
whether two-point or shared quantum correlations are responsible for each of
the occurring QPTs. The method works in principle for any number D of degrees
of freedom per site. As a by-product, we are providing a benchmark for direct
measures of bipartite entanglement; in particular, here we discuss the role of
negativity at the transition.Comment: 4 pages, 2 figures, 1 tabl
Linear amplification and quantum cloning for non-Gaussian continuous variables
We investigate phase-insensitive linear amplification at the quantum limit
for single- and two-mode states and show that there exists a broad class of
non-Gaussian states whose nonclassicality survives even at an arbitrarily large
gain. We identify the corresponding observable nonclassical effects and find
that they include, remarkably, two-mode entanglement. The implications of our
results for quantum cloning outside the Gaussian regime are also addressed.Comment: published version with reference updat
Geometry shapes propagation: Assessing the presence and absence of cortical symmetries through a computational model of cortical spreading depression
Cortical spreading depression (CSD), a depolarization wave which originates in the visual cortex and travels toward the frontal lobe, has been suggested to be one neural correlate of aura migraine. To the date, little is known about the mechanisms which can trigger or stop aura migraine. Here, to shed some light on this problem and, under the hypothesis that CSD might mediate aura migraine, we aim to study different aspects favoring or disfavoring the propagation of CSD. In particular, by using a computational neuronal model distributed throughout a realistic cortical mesh, we study the role that the geometry has in shaping CSD. Our results are two-fold: first, we found significant differences in the propagation traveling patterns of CSD, both intra and inter-hemispherically, revealing important asymmetries in the propagation profile. Second, we developed methods able to identify brain regions featuring a peculiar behavior during CSD propagation. Our study reveals dynamical aspects of CSD, which, if applied to subject-specific cortical geometry, might shed some light on how to differentiate between healthy subjects and those suffering migraine
Key aspects for effective mathematical modelling of fractional-diffusion in cardiac electrophysiology: A quantitative study
Microscopic structural features of cardiac tissue play a fundamental role in determining complex spatio-temporal excitation dynamics at the macroscopic level. Recent efforts have been devoted to the development of mathematical models accounting for non-local spatio-temporal coupling able to capture these complex dynamics without the need of resolving tissue heterogeneities down to the micro-scale. In this work, we analyse in detail several important aspects affecting the overall predictive power of these modelling tools and provide some guidelines for an effective use of space-fractional models of cardiac electrophysiology in practical applications. Through an extensive computational study in simplified computational domains, we highlight the robustness of models belonging to different categories, i.e., physiological and phenomenological descriptions, against the introduction of non-locality, and lay down the foundations for future research and model validation against experimental data. A modern genetic algorithm framework is used to investigate proper parameterisations of the considered models, and the crucial role played by the boundary assumptions in the considered settings is discussed. Several numerical results are provided to support our claims.Italian National Group of Mathematical Physics (GNFM-INdAM);
NSF grant No. 1762553;
NIH grant No. 1R01HL143450-0
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