13,755 research outputs found
Squeezing as an irreducible resource
We show that squeezing is an irreducible resource which remains invariant
under transformations by linear optical elements. In particular, we give a
decomposition of any optical circuit with linear input-output relations into a
linear multiport interferometer followed by a unique set of single mode
squeezers and then another multiport interferometer. Using this decomposition
we derive a no-go theorem for superpositions of macroscopically distinct states
from single-photon detection. Further, we demonstrate the equivalence between
several schemes for randomly creating polarization-entangled states. Finally,
we derive minimal quantum optical circuits for ideal quantum non-demolition
coupling of quadrature-phase amplitudes.Comment: 4 pages, 3 figures, new title, removed the fat
Superfluid to normal phase transition in strongly correlated bosons in two and three dimensions
Using quantum Monte Carlo simulations, we investigate the finite-temperature
phase diagram of hard-core bosons (XY model) in two- and three-dimensional
lattices. To determine the phase boundaries, we perform a finite-size-scaling
analysis of the condensate fraction and/or the superfluid stiffness. We then
discuss how these phase diagrams can be measured in experiments with trapped
ultracold gases, where the systems are inhomogeneous. For that, we introduce a
method based on the measurement of the zero-momentum occupation, which is
adequate for experiments dealing with both homogeneous and trapped systems, and
compare it with previously proposed approaches.Comment: 13 pages, 11 figures.
http://link.aps.org/doi/10.1103/PhysRevA.86.04362
Entanglement-Saving Channels
The set of Entanglement Saving (ES) quantum channels is introduced and
characterized. These are completely positive, trace preserving transformations
which when acting locally on a bipartite quantum system initially prepared into
a maximally entangled configuration, preserve its entanglement even when
applied an arbitrary number of times. In other words, a quantum channel
is said to be ES if its powers are not entanglement-breaking for all
integers . We also characterize the properties of the Asymptotic
Entanglement Saving (AES) maps. These form a proper subset of the ES channels
that is constituted by those maps which, not only preserve entanglement for all
finite , but which also sustain an explicitly not null level of entanglement
in the asymptotic limit~. Structure theorems are provided
for ES and for AES maps which yield an almost complete characterization of the
former and a full characterization of the latter.Comment: 26 page
Discrimination between evolution operators
Under broad conditions, evolutions due to two different Hamiltonians are
shown to lead at some moment to orthogonal states. For two spin-1/2 systems
subject to precession by different magnetic fields the achievement of
orthogonalization is demonstrated for every scenario but a special one. This
discrimination between evolutions is experimentally much simpler than
procedures proposed earlier based on either sequential or parallel application
of the unknown unitaries. A lower bound for the orthogonalization time is
proposed in terms of the properties of the two Hamiltonians.Comment: 7 pages, 2 figures, REVTe
Spectral Conditions on the State of a Composite Quantum System Implying its Separability
For any unitarily invariant convex function F on the states of a composite
quantum system which isolates the trace there is a critical constant C such
that F(w)<= C for a state w implies that w is not entangled; and for any
possible D > C there are entangled states v with F(v)=D. Upper- and lower
bounds on C are given. The critical values of some F's for qubit/qubit and
qubit/qutrit bipartite systems are computed. Simple conditions on the spectrum
of a state guaranteeing separability are obtained. It is shown that the thermal
equilbrium states specified by any Hamiltonian of an arbitrary compositum are
separable if the temperature is high enough.Comment: Corrects 1. of Lemma 2, and the (under)statement of Proposition 7 of
the earlier version
General criterion for the entanglement of two indistinguishable particles
We relate the notion of entanglement for quantum systems composed of two
identical constituents to the impossibility of attributing a complete set of
properties to both particles. This implies definite constraints on the
mathematical form of the state vector associated with the whole system. We then
analyze separately the cases of fermion and boson systems, and we show how the
consideration of both the Slater-Schmidt number of the fermionic and bosonic
analog of the Schmidt decomposition of the global state vector and the von
Neumann entropy of the one-particle reduced density operators can supply us
with a consistent criterion for detecting entanglement. In particular, the
consideration of the von Neumann entropy is particularly useful in deciding
whether the correlations of the considered states are simply due to the
indistinguishability of the particles involved or are a genuine manifestation
of the entanglement. The treatment leads to a full clarification of the subtle
aspects of entanglement of two identical constituents which have been a source
of embarrassment and of serious misunderstandings in the recent literature.Comment: 18 pages, Latex; revised version: Section 3.2 rewritten, new Theorems
added, reference [1] corrected. To appear on Phys.Rev.A 70, (2004
Singular value decomposition and matrix reorderings in quantum information theory
We review Schmidt and Kraus decompositions in the form of singular value
decomposition using operations of reshaping, vectorization and reshuffling. We
use the introduced notation to analyse the correspondence between quantum
states and operations with the help of Jamiolkowski isomorphism. The presented
matrix reorderings allow us to obtain simple formulae for the composition of
quantum channels and partial operations used in quantum information theory. To
provide examples of the discussed operations we utilize a package for the
Mathematica computing system implementing basic functions used in the
calculations related to quantum information theory.Comment: 11 pages, no figures, see
http://zksi.iitis.pl/wiki/projects:mathematica-qi for related softwar
Local Distinguishability of Multipartite Unitary Operations
We show that any two different unitary operations acting on an arbitrary
multipartite quantum system can be perfectly distinguishable by local
operations and classical communication when a finite number of runs is allowed.
We then directly extend this result into the case when the number of unitary
operations to be discriminated is more than two. Intuitively, our result means
that the lost identity of a nonlocal (entangled) unitary operation can be
recovered locally, without any use of entanglement or joint quantum operations.Comment: 5 pages (in Revtex 4), 1 eps. A preliminary version. Comments are
welcom
Irreducible multiparty correlation can be created by local operations
Generalizing Amari's work titled "Information geometry on hierarchy of
probability distributions", we define the degrees of irreducible multiparty
correlations in a multiparty quantum state based on quantum relative entropy.
We prove that these definitions are equivalent to those derived from the
maximal von Neaumann entropy principle. Based on these definitions, we find a
counterintuitive result on irreducible multiparty correlations: although the
degree of the total correlation in a three-party quantum state does not
increase under local operations, the irreducible three-party correlation can be
created by local operations from a three-party state with only irreducible
two-party correlations. In other words, even if a three-party state is
initially completely determined by measuring two-party Hermitian operators, the
determination of the state after local operations have to resort to the
measurements of three-party Hermitian operators.Comment: 4 pages, comments are welcom
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