246 research outputs found
McCarty v. McCarty: The Moving Target of Federal Pre-Emption Threatening All Non-Employee Spouses Symposium - Texas Community Property Law in Transition.
Abstract Forthcoming
Zeno Dynamics in Quantum Statistical Mechanics
We study the quantum Zeno effect in quantum statistical mechanics within the
operator algebraic framework. We formulate a condition for the appearance of
the effect in W*-dynamical systems, in terms of the short-time behaviour of the
dynamics. Examples of quantum spin systems show that this condition can be
effectively applied to quantum statistical mechanical models. Further, we
derive an explicit form of the Zeno generator, and use it to construct Gibbs
equilibrium states for the Zeno dynamics. As a concrete example, we consider
the X-Y model, for which we show that a frequent measurement at a microscopic
level, e.g. a single lattice site, can produce a macroscopic effect in changing
the global equilibrium.Comment: 15 pages, AMSLaTeX; typos corrected, references updated and added,
acknowledgements added, style polished; revised version contains corrections
from published corrigend
The power of symmetric extensions for entanglement detection
In this paper, we present new progress on the study of the symmetric
extension criterion for separability. First, we show that a perturbation of
order O(1/N) is sufficient and, in general, necessary to destroy the
entanglement of any state admitting an N Bose symmetric extension. On the other
hand, the minimum amount of local noise necessary to induce separability on
states arising from N Bose symmetric extensions with Positive Partial Transpose
(PPT) decreases at least as fast as O(1/N^2). From these results, we derive
upper bounds on the time and space complexity of the weak membership problem of
separability when attacked via algorithms that search for PPT symmetric
extensions. Finally, we show how to estimate the error we incur when we
approximate the set of separable states by the set of (PPT) N -extendable
quantum states in order to compute the maximum average fidelity in pure state
estimation problems, the maximal output purity of quantum channels, and the
geometric measure of entanglement.Comment: see Video Abstract at
http://www.quantiki.org/video_abstracts/0906273
Free energy density for mean field perturbation of states of a one-dimensional spin chain
Motivated by recent developments on large deviations in states of the spin
chain, we reconsider the work of Petz, Raggio and Verbeure in 1989 on the
variational expression of free energy density in the presence of a mean field
type perturbation. We extend their results from the product state case to the
Gibbs state case in the setting of translation-invariant interactions of finite
range. In the special case of a locally faithful quantum Markov state, we
clarify the relation between two different kinds of free energy densities (or
pressure functions).Comment: 29 pages, Section 5 added, to appear in Rev. Math. Phy
One-and-a-half quantum de Finetti theorems
We prove a new kind of quantum de Finetti theorem for representations of the
unitary group U(d). Consider a pure state that lies in the irreducible
representation U_{mu+nu} for Young diagrams mu and nu. U_{mu+nu} is contained
in the tensor product of U_mu and U_nu; let xi be the state obtained by tracing
out U_nu. We show that xi is close to a convex combination of states Uv, where
U is in U(d) and v is the highest weight vector in U_mu. When U_{mu+nu} is the
symmetric representation, this yields the conventional quantum de Finetti
theorem for symmetric states, and our method of proof gives near-optimal bounds
for the approximation of xi by a convex combination of product states. For the
class of symmetric Werner states, we give a second de Finetti-style theorem
(our 'half' theorem); the de Finetti-approximation in this case takes a
particularly simple form, involving only product states with a fixed spectrum.
Our proof uses purely group theoretic methods, and makes a link with the
shifted Schur functions. It also provides some useful examples, and gives some
insight into the structure of the set of convex combinations of product states.Comment: 14 pages, 3 figures, v4: minor additions (including figures),
published versio
Continuity and Stability of Partial Entropic Sums
Extensions of Fannes' inequality with partial sums of the Tsallis entropy are
obtained for both the classical and quantum cases. The definition of kth
partial sum under the prescribed order of terms is given. Basic properties of
introduced entropic measures and some applications are discussed. The derived
estimates provide a complete characterization of the continuity and stability
properties in the refined scale. The results are also reformulated in terms of
Uhlmann's partial fidelities.Comment: 9 pages, no figures. Some explanatory and technical improvements are
made. The bibliography is extended. Detected errors and typos are correcte
Burnout in the ICU : potential consequences for staff and patient well-being
Peer reviewedAuthor versio
Symmetry implies independence
Given a quantum system consisting of many parts, we show that symmetry of the
system's state, i.e., invariance under swappings of the subsystems, implies
that almost all of its parts are virtually identical and independent of each
other. This result generalises de Finetti's classical representation theorem
for infinitely exchangeable sequences of random variables as well as its
quantum-mechanical analogue. It has applications in various areas of physics as
well as information theory and cryptography. For example, in experimental
physics, one typically collects data by running a certain experiment many
times, assuming that the individual runs are mutually independent. Our result
can be used to justify this assumption.Comment: LaTeX, contains 4 figure
Monogamy of Correlations vs. Monogamy of Entanglement
A fruitful way of studying physical theories is via the question whether the
possible physical states and different kinds of correlations in each theory can
be shared to different parties. Over the past few years it has become clear
that both quantum entanglement and non-locality (i.e., correlations that
violate Bell-type inequalities) have limited shareability properties and can
sometimes even be monogamous. We give a self-contained review of these results
as well as present new results on the shareability of different kinds of
correlations, including local, quantum and no-signalling correlations. This
includes an alternative simpler proof of the Toner-Verstraete monogamy
inequality for quantum correlations, as well as a strengthening thereof.
Further, the relationship between sharing non-local quantum correlations and
sharing mixed entangled states is investigated, and already for the simplest
case of bi-partite correlations and qubits this is shown to be non-trivial.
Also, a recently proposed new interpretation of Bell's theorem by Schumacher in
terms of shareability of correlations is critically assessed. Finally, the
relevance of monogamy of non-local correlations for secure quantum key
distribution is pointed out, although, and importantly, it is stressed that not
all non-local correlations are monogamous.Comment: 12 pages, 2 figures. Invited submission to a special issue of Quantum
Information Processing. v2: Published version. Open acces
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