21,294 research outputs found
Strong monotonicity in mixed-state entanglement manipulation
A strong entanglement monotone, which never increases under local operations
and classical communications (LOCC), restricts quantum entanglement
manipulation more strongly than the usual monotone since the usual one does not
increase on average under LOCC. We propose new strong monotones in mixed-state
entanglement manipulation under LOCC. These are related to the decomposability
and 1-positivity of an operator constructed from a quantum state, and reveal
geometrical characteristics of entangled states. These are lower bounded by the
negativity or generalized robustness of entanglement.Comment: 6 pages and 1 figure. A brief discussion about the connection to
asymptotic distillability was adde
The joys of permutation symmetry: direct measurements of entanglement
So-called direct measurements of entanglement are collective measurements on
multiple copies of a (bipartite or multipartite) quantum system that directly
provide one a value for some entanglement measure, such as the concurrence for
bipartite states. Multiple copies are needed since the entanglement of a mixed
state is not a linear function of the density matrix. Unfortunately, so far all
experimental implementations of direct measurements made unverified assumptions
about the form of the states, and, therefore, do not qualify as entanglement
verification tests. I discuss how a direct measurement can be turned into a
quantitative entanglement verification test by exploiting a recent theorem by
Renner (R. Renner, Nature Physics 3, 645 (2007)).Comment: 4 pages, 3 figure
Entanglement-assisted local operations and classical communications conversion in the quantum critical systems
Conversions between the ground states in quantum critical systems via
entanglement-assisted local operations and classical communications (eLOCC) are
studied. We propose a new method to reveal the different convertibility by
local operations when a quantum phase transition occurs. We have studied the
ground state local convertibility in the one dimensional transverse field Ising
model, XY model and XXZ model. It is found that the eLOCC convertibility sudden
changes at the phase transition points. In transverse field Ising model the
eLOCC convertibility between the first excited state and the ground state are
also distinct for different phases. The relation between the order of quantum
phase transitions and the local convertibility is discussed.Comment: 7 pages, 5 figures, 5 table
Entanglement entropy of multipartite pure states
Consider a system consisting of -dimensional quantum particles and
arbitrary pure state of the whole system. Suppose we simultaneously
perform complete von Neumann measurements on each particle. One can ask: what
is the minimal possible value of the entropy of outcomes joint
probability distribution? We show that coincides with entanglement
entropy for bipartite states. We compute for two sample multipartite
states: the hexacode state () and determinant states (). The
generalization of determinant states to the case is considered.Comment: 7 pages, REVTeX, corrected some typo
Optimal quantum teleportation with an arbitrary pure state
We derive the maximum fidelity attainable for teleportation using a shared
pair of d-level systems in an arbitrary pure state. This derivation provides a
complete set of necessary and sufficient conditions for optimal teleportation
protocols. We also discuss the information on the teleported particle which is
revealed in course of the protocol using a non-maximally entangled state.Comment: 10 pages, REVTe
Modelling ultraviolet-line diagnostics of stars, the ionized and the neutral interstellar medium in star-forming galaxies
We combine state-of-the-art models for the production of stellar radiation
and its transfer through the interstellar medium (ISM) to investigate
ultraviolet-line diagnostics of stars, the ionized and the neutral ISM in
star-forming galaxies. We start by assessing the reliability of our stellar
population synthesis modelling by fitting absorption-line indices in the
ISM-free ultraviolet spectra of 10 Large-Magellanic-Cloud clusters. In doing
so, we find that neglecting stochastic sampling of the stellar initial mass
function in these young (-100 Myr), low-mass clusters affects
negligibly ultraviolet-based age and metallicity estimates but can lead to
significant overestimates of stellar mass. Then, we proceed and develop a
simple approach, based on an idealized description of the main features of the
ISM, to compute in a physically consistent way the combined influence of
nebular emission and interstellar absorption on ultraviolet spectra of
star-forming galaxies. Our model accounts for the transfer of radiation through
the ionized interiors and outer neutral envelopes of short-lived stellar birth
clouds, as well as for radiative transfer through a diffuse intercloud medium.
We use this approach to explore the entangled signatures of stars, the ionized
and the neutral ISM in ultraviolet spectra of star-forming galaxies. We find
that, aside from a few notable exceptions, most standard ultraviolet indices
defined in the spectra of ISM-free stellar populations are prone to significant
contamination by the ISM, which increases with metallicity. We also identify
several nebular-emission and interstellar-absorption features, which stand out
as particularly clean tracers of the different phases of the ISM.Comment: 27 pages, 21 figures. Accepted for publication in MNRA
Mixture of multiple copies of maximally entangled states is quasi-pure
Employing the general BXOR operation and local state discrimination, the
mixed state of the form
\rho^{(k)}_{d}=\frac{1}{d^{2}}\sum_{m,n=0}^{d-1}(|\phi_{mn}><\phi_{mn}|)^{\otim
es k} is proved to be quasi-pure, where is the canonical set
of mutually orthogonal maximally entangled states in . Therefore
irreversibility does not occur in the process of distillation for this family
of states. Also, the distillable entanglement is calculated explicitly.Comment: 6 pages, 1 figure. The paper is subtantially revised and the general
proof is give
Quantum Critical Point and Entanglement in a Matrix Product Ground State
In this paper, we study the entanglement properties of a spin-1 model the
exact ground state of which is given by a Matrix Product state. The model
exhibits a critical point transition at a parameter value a=0. The longitudinal
and transverse correlation lengths are known to diverge as a tends to zero. We
use three different entanglement measures S(i) (the one-site von Neumann
entropy), S(i,j) (the two-body entanglement) and G(2,n) (the generalized global
entanglement) to determine the entanglement content of the MP ground state as
the parameter a is varied. The entanglement length, associated with S(i,j), is
found to diverge in the vicinity of the quantum critical point a=0. The first
derivative of the entanglement measure E (=S(i), S(i,j)) w.r.t. the parameter a
also diverges. The first derivative of G(2,n) w.r.t. a does not diverge as a
tends to zero but attains a maximum value at a=0. At the QCP itself all the
three entanglement measures become zero. We further show that multipartite
correlations are involved in the QPT at a=0.Comment: 14 pages, 6 figure
Catalytic Conversion Probabilities for Bipartite Pure States
For two given bipartite-entangled pure states, an expression is obtained for
the least upper bound of conversion probabilities using catalysis. The
attainability of the upper bound can also be decided if that bound is less than
one.Comment: 4 pages; comments appreciated; the article is a modified version of
this preprint combined with arXiv:0707.044
Finite-Size Scaling Exponents in the Dicke Model
We consider the finite-size corrections in the Dicke model and determine the
scaling exponents at the critical point for several quantities such as the
ground state energy or the gap. Therefore, we use the Holstein-Primakoff
representation of the angular momentum and introduce a nonlinear transformation
to diagonalize the Hamiltonian in the normal phase. As already observed in
several systems, these corrections turn out to be singular at the transition
point and thus lead to nontrivial exponents. We show that for the atomic
observables, these exponents are the same as in the Lipkin-Meshkov-Glick model,
in agreement with numerical results. We also investigate the behavior of the
order parameter related to the radiation mode and show that it is driven by the
same scaling variable as the atomic one.Comment: 4 pages, published versio
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