2,910 research outputs found
The general structure of quantum resource theories
In recent years it was recognized that properties of physical systems such as
entanglement, athermality, and asymmetry, can be viewed as resources for
important tasks in quantum information, thermodynamics, and other areas of
physics. This recognition followed by the development of specific quantum
resource theories (QRTs), such as entanglement theory, determining how quantum
states that cannot be prepared under certain restrictions may be manipulated
and used to circumvent the restrictions. Here we discuss the general structure
of QRTs, and show that under a few assumptions (such as convexity of the set of
free states), a QRT is asymptotically reversible if its set of allowed
operations is maximal; that is, if the allowed operations are the set of all
operations that do not generate (asymptotically) a resource. In this case, the
asymptotic conversion rate is given in terms of the regularized relative
entropy of a resource which is the unique measure/quantifier of the resource in
the asymptotic limit of many copies of the state. This measure also equals the
smoothed version of the logarithmic robustness of the resource.Comment: 5 pages, no figures, few references added, published versio
Detection of Multiparticle Entanglement: Quantifying the Search for Symmetric Extensions
We provide quantitative bounds on the characterisation of multiparticle
separable states by states that have locally symmetric extensions. The bounds
are derived from two-particle bounds and relate to recent studies on quantum
versions of de Finetti's theorem. We discuss algorithmic applications of our
results, in particular a quasipolynomial-time algorithm to decide whether a
multiparticle quantum state is separable or entangled (for constant number of
particles and constant error in the LOCC or Frobenius norm). Our results
provide a theoretical justification for the use of the Search for Symmetric
Extensions as a practical test for multiparticle entanglement.Comment: 5 pages, 1 figur
When does noise increase the quantum capacity?
Superactivation is the property that two channels with zero quantum capacity
can be used together to yield positive capacity. Here we demonstrate that this
effect exists for a wide class of inequivalent channels, none of which can
simulate each other. We also consider the case where one of two zero capacity
channels are applied, but the sender is ignorant of which one is applied. We
find examples where the greater the entropy of mixing of the channels, the
greater the lower bound for the capacity. Finally, we show that the effect of
superactivation is rather generic by providing example of superactivation using
the depolarizing channel.Comment: Corrected minor typo
Macroscopic Entanglement and Phase Transitions
This paper summarises the results of our research on macroscopic entanglement
in spin systems and free Bosonic gases. We explain how entanglement can be
observed using entanglement witnesses which are themselves constructed within
the framework of thermodynamics and thus macroscopic observables. These
thermodynamical entanglement witnesses result in bounds on macroscopic
parameters of the system, such as the temperature, the energy or the
susceptibility, below which entanglement must be present. The derived bounds
indicate a relationship between the occurrence of entanglement and the
establishment of order, possibly resulting in phase transition phenomena. We
give a short overview over the concepts developed in condensed matter physics
to capture the characteristics of phase transitions in particular in terms of
order and correlation functions. Finally we want to ask and speculate whether
entanglement could be a generalised order concept by itself, relevant in
(quantum induced) phase transitions such as BEC, and that taking this view may
help us to understand the underlying process of high-T superconductivity.Comment: 9 pages, 7 figures (color), Submitted to special OSID issue,
Proceedings of the 38th Symposium on Mathematical Physics - Quantum
Entanglement & Geometry, Torun (Poland), June 200
Witnessed Entanglement
We present a new measure of entanglement for mixed states. It can be
approximately computable for every state and can be used to quantify all
different types of multipartite entanglement. We show that it satisfies the
usual properties of a good entanglement quantifier and derive relations between
it and other entanglement measures.Comment: Revised version. 7 pages and one figur
Macroscopic thermodynamic reversibility in quantum many-body systems
The resource theory of thermal operations, an established model for small-scale thermodynamics, provides an extension of equilibrium thermodynamics to nonequilibrium situations. On a lattice of any dimension with any translation-invariant local Hamiltonian, we identify a large set of translation-invariant states that can be reversibly converted to and from the thermal state with thermal operations and a small amount of coherence. These are the spatially ergodic states, i.e., states that have sharp statistics for any translation-invariant observable, and mixtures of such states with the same thermodynamic potential. As an intermediate result, we show for a general state that if the gap between the min- and the max-relative entropies to the thermal state is small, then the state can be approximately reversibly converted to and from the thermal state with thermal operations and a small source of coherence. Our proof provides a quantum version of the Shannon-McMillan-Breiman theorem for the relative entropy and a quantum Stein’s lemma for ergodic states and local Gibbs states. Our results provide a strong link between the abstract resource theory of thermodynamics and more realistic physical systems as we achieve a robust and operational characterization of the emergence of a thermodynamic potential in translation-invariant lattice systems
MÃstica e Paidéia : O Pseudo-DionÃsio Areopagita
O autor que se apresenta como DionÃsio Areopagita, o ateniense convertido por São Paulo no Areópago (Atos XVII, 16-34) foi provavelmente um monge sÃrio que escreveu no fim do século V. Este artigo, ao discutir as tentativas contemporâneos de descobrir sua identidade e a influência que exerceu sobre o pensamento posterior, interpreta sua obra como uma tentativa de assimilação da filosofia e paidéia grega pelo cristianismo, representando um momento importante do processo simbolizado pela viagem de São Paulo à Atenas.The author who presents himself as Dionysius the Areopagite, the converted by Saint Paul on the Areopagus was probably a syrian monk who wrote in the late V century. This article, dicussing the contemporany attempts to find his identity and the influence on the posterior thought, interprets his writings as an attempt to assimilate the greek philosophy and paideia by christianity, representing an important moment in the process symbolized by the Saint Paul's journey to Athens
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