205 research outputs found
Methods for Estimating Capacities and Rates of Gaussian Quantum Channels
Optimization methods aimed at estimating the capacities of a general Gaussian
channel are developed. Specifically evaluation of classical capacity as maximum
of the Holevo information is pursued over all possible Gaussian encodings for
the lossy bosonic channel, but extension to other capacities and other Gaussian
channels seems feasible. Solutions for both memoryless and memory channels are
presented. It is first dealt with single use (single-mode) channel where the
capacity dependence from channel's parameters is analyzed providing a full
classification of the possible cases. Then it is dealt with multiple uses
(multi-mode) channel where the capacity dependence from the (multi-mode)
environment state is analyzed when both total environment energy and
environment purity are fixed. This allows a fair comparison among different
environments, thus understanding the role of memory (inter-mode correlations)
and phenomenon like superadditivity of the capacity. The developed methods are
also used for deriving transmission rates with heterodyne and homodyne
measurements at the channel output. Classical capacity and transmission rates
are presented within a unique framework where the rates can be treated as
logarithmic approximations of the capacity.Comment: 39 pages, 30 figures. New results and graphs were added. Errors and
misprints were corrected. To appear in IEEE Trans. Inf. T
Stationary and uniform entanglement distribution in qubit networks with quasi-local dissipation
We consider qubit networks where adjacent qubits besides interacting via
XY-coupling, also dissipate into the same environment. The steady states are
computed exactly for all network sizes and topologies, showing that they are
always symmetric under permutation of network sites, leading to a uniform
distribution of the stationary entanglement across the network. The maximum
entanglement between two arbitrary qubits is shown to depend only on the total
number of qubits in the network, and scales linearly with it. A possible
physical realization by means of an array of doped cavities is discussed for
the case of a linear chain.Comment: 6 pages, comments welcome. v2: published version with typos corrected
and updated bibliograph
Super-additivity and entanglement assistance in quantum reading
Quantum information theory determines the maximum rates at which information can be transmitted through physical systems described by quantum mechanics. Here we consider the communication protocol known as quantum reading. Quantum reading is a protocol for retrieving the information stored in a digital memory by using a quantum probe, e.g., shining quantum states of light to read an optical memory. In a variety of situations using a quantum probe enhances the performance of the reading protocol in terms of fidelity, data density and energy efficiency. Here we review and characterize the quantum reading capacity of a memory model, defined as the maximum rate of reliable reading. We show that, like other quantities in quantum information theory, the quantum reading capacity is super-additive. Moreover, we determine conditions under which the use of an entangled ancilla improves the performance of quantum reading
Entanglement Typicality
We provide a summary of both seminal and recent results on typical
entanglement. By typical values of entanglement, we refer here to values of
entanglement quantifiers that (given a reasonable measure on the manifold of
states) appear with arbitrarily high probability for quantum systems of
sufficiently high dimensionality. We work within the Haar measure framework for
discrete quantum variables, where we report on results concerning the average
von Neumann and linear entropies as well as arguments implying the typicality
of such values in the asymptotic limit. We then proceed to discuss the
generation of typical quantum states with random circuitry. Different phases of
entanglement, and the connection between typical entanglement and
thermodynamics are discussed. We also cover approaches to measures on the
non-compact set of Gaussian states of continuous variable quantum systems.Comment: Review paper with two quotes and minimalist figure
Gaussian maximally multipartite entangled states
We study maximally multipartite entangled states in the context of Gaussian
continuous variable quantum systems. By considering multimode Gaussian states
with constrained energy, we show that perfect maximally multipartite entangled
states, which exhibit the maximum amount of bipartite entanglement for all
bipartitions, only exist for systems containing n=2 or 3 modes. We further
numerically investigate the structure of these states and their frustration for
n<=7.Comment: 6 pages, 2 figures, comments are welcom
Forgetfulness of continuous Markovian quantum channels
The notion of forgetfulness, used in discrete quantum memory channels, is
slightly weakened in order to be applied to the case of continuous channels.
This is done in the context of quantum memory channels with Markovian noise. As
a case study, we apply the notion of weak-forgetfulness to a bosonic memory
channel with additive noise. A suitable encoding and decoding unitary
transformation allows us to unravel the effects of the memory, hence the
channel capacities can be computed using known results from the memoryless
setting.Comment: 6 pages, 2 figures, comments are welcome. Minor corrections and
acknoledgment adde
Molecular mechanisms of neurogenic aging in the adult mouse subventricular zone
In the adult rodent brain, the continuous production of new neurons by neural stem/progenitor cells (NSPCs) residing in specialized neurogenic niches and their subsequent integration into pre-existing cerebral circuitries supports odour discrimination, spatial learning, and contextual memory capabilities. Aging is recognized as the most potent negative regulator of adult neurogenesis. The neurogenic process markedly declines in the aged brain, due to the reduction of the NSPC pool and the functional impairment of the remaining NSPCs. This decline has been linked to the progressive cognitive deficits of elderly individuals and it may also be involved in the onset/progression of neurological disorders. Since the human lifespan has been dramatically extended, the incidence of age-associated neuropsychiatric conditions in the human population has increased. This has prompted efforts to shed light on the mechanisms underpinning the age-related decline of adult neurogenesis, whose knowledge may foster therapeutic approaches to prevent or delay cognitive alterations in elderly patients. In this review, we summarize recent progress in elucidating the molecular causes of neurogenic aging in the most abundant NSPC niche of the adult mouse brain: the subventricular zone (SVZ). We discuss the age-associated changes occurring both in the intrinsic NSPC molecular networks and in the extrinsic signalling pathways acting in the complex environment of the SVZ niche, and how all these changes may steer young NSPCs towards an aged phenotype
Entanglement frustration in multimode Gaussian states
Bipartite entanglement between two parties of a composite quantum system can
be quantified in terms of the purity of one party and there always exists a
pure state of the total system that maximizes it (and minimizes purity). When
many different bipartitions are considered, the requirement that purity be
minimal for all bipartitions gives rise to the phenomenon of entanglement
frustration. This feature, observed in quantum systems with both discrete and
continuous variables, can be studied by means of a suitable cost function whose
minimizers are the maximally multipartite-entangled states (MMES). In this
paper we extend the analysis of multipartite entanglement frustration of
Gaussian states in multimode bosonic systems. We derive bounds on the
frustration, under the constraint of finite mean energy, in the low and high
energy limit.Comment: 4 pages, 2 figures. Contribution to "Folding and Unfolding:
Interactions from Geometry. Workshop in honour of Giuseppe Marmo's 65th
birthday", 8-12 June 2011, Ischia (NA) Ital
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