139 research outputs found

### Connections and Metrics Respecting Standard Purification

Standard purification interlaces Hermitian and Riemannian metrics on the
space of density operators with metrics and connections on the purifying
Hilbert-Schmidt space. We discuss connections and metrics which are well
adopted to purification, and present a selected set of relations between them.
A connection, as well as a metric on state space, can be obtained from a metric
on the purification space. We include a condition, with which this
correspondence becomes one-to-one. Our methods are borrowed from elementary
*-representation and fibre space theory. We lift, as an example, solutions of a
von Neumann equation, write down holonomy invariants for cyclic ones, and ``add
noise'' to a curve of pure states.Comment: Latex, 27 page

### Alice falls into a black hole: Entanglement in non-inertial frames

Two observers determine the entanglement between two free bosonic modes by
each detecting one of the modes and observing the correlations between their
measurements. We show that a state which is maximally entangled in an inertial
frame becomes less entangled if the observers are relatively accelerated. This
phenomenon, which is a consequence of the Unruh effect, shows that entanglement
is an observer-dependent quantity in non-inertial frames. In the high
acceleration limit, our results can be applied to a non-accelerated observer
falling into a black hole while the accelerated one barely escapes. If the
observer escapes with infinite acceleration, the state's distillable
entanglement vanishes.Comment: I.F-S published before with maiden name Fuentes-Guridi Replaced with
published version. Phys. Rev. Lett. in pres

### Degradation of non-maximal entanglement of scalar and Dirac fields in non-inertial frames

The entanglement between two modes of the free scalar and Dirac fields as
seen by two relatively accelerated observers has been investigated. It is found
that the same initial entanglement for an initial state parameter $\alpha$ and
its "normalized partner" $\sqrt{1-\alpha^{2}}$ will be degraded by the Unruh
effect along two different trajectories except for the maximally entangled
state, which just shows the inequivalence of the quantization for a free field
in the Minkowski and Rindler coordinates. In the infinite acceleration limit
the state doesn't have the distillable entanglement for any $\alpha$ for the
scalar field but always remains entangled to a degree which is dependent of
$\alpha$ for the Dirac field. It is also interesting to note that in this limit
the mutual information equals to just half of the initially mutual information,
which is independent of $\alpha$ and the type of field.Comment: 9 pages, 4 figure

### Entanglement of Dirac fields in non-inertial frames

We analyze the entanglement between two modes of a free Dirac field as seen
by two relatively accelerated parties. The entanglement is degraded by the
Unruh effect and asymptotically reaches a non-vanishing minimum value in the
infinite acceleration limit. This means that the state always remains entangled
to a degree and can be used in quantum information tasks, such as
teleportation, between parties in relative uniform acceleration. We analyze our
results from the point of view afforded by the phenomenon of entanglement
sharing and in terms of recent results in the area of multi-qubit
complementarity.Comment: 15 pages, with 8 figures (Mar 2006); accepted to Physical Review A,
July 2006 - slightly revise

### Speeding up Entanglement Degradation

Entanglement between two free bosonic modes can be determined via detection
of each mode by different observers and then observing the correlations between
their measurements. We show that such entanglement is degraded as a function of
time if one observer begins in a state of inertial motion but ends in a state
of uniform acceleration while the other remains inertial. At late times we
recover previously established results for observers in relative uniform
acceleration.Comment: 5 pages, 2 figure

### Quantum discord dynamical behaviors due to initial system-cavity correlations

We analyze the roles of initial correlations between the two-qubit system and
a dissipative cavity on quantum discord dynamics of two qubits. Considering two
initial system-cavity states, we show that the initial system-cavity
correlations not only can initially increase the two-qubit quantum discord but
also would lead to a larger long-time quantum discord asymptotic value.
Moreover, quantum discord due to initial correlations is more robust than the
case of the initial factorized state. Finally, we show the initial
correlations' importance for dynamics behaviors of mutual information and
classical correlation

### Bures distance between two displaced thermal states

The Bures distance between two displaced thermal states and the corresponding
geometric quantities (statistical metric, volume element, scalar curvature) are
computed. Under nonunitary (dissipative) dynamics, the statistical distance
shows the same general features previously reported in the literature by
Braunstein and Milburn for two--state systems. The scalar curvature turns out
to have new interesting properties when compared to the curvature associated
with squeezed thermal states.Comment: 3 pages, RevTeX, no figure

### Entanglement Creation in Low-Energy Scattering

We study the entanglement creation in the low-energy scattering of two
particles in three dimensions, for a general class of interaction potentials
that are not required to be spherically symmetric. The incoming asymptotic
state, before the collision, is a product of two normalized Gaussian states.
After the scattering the particles are entangled. We take as a measure of the
entanglement the purity of one of them. We provide a rigorous explicit
computation, with error bound, of the leading order of the purity at
low-energy. The entanglement depends strongly in the difference of the masses.
It takes its minimum when the masses are equal, and it increases rapidly with
the difference of the masses. It is quite remarkable that the anisotropy of the
potential gives no contribution to the leading order of the purity, on spite of
the fact that entanglement is a second order effect.Comment: The paper has been edited and some comments have been adde

### Quantum Generalized Subsystems

We propose a new formalism of quantum subsystems which allows to unify the
existing and new methods of reduced description of quantum systems. The main
mathematical ingredients are completely positive maps and correlation
functions. In this formalism generalized quantum systems can be composed and
there is a notion of generalized entanglement. Models of fermionic and bosonic
systems and also quantum systems described by the SU(2) symmetry are studied.Comment: 21 page

### Hawking radiation, Entanglement and Teleportation in background of an asymptotically flat static black hole

The effect of the Hawking temperature on the entanglement and teleportation
for the scalar field in a most general, static and asymptotically flat black
hole with spherical symmetry has been investigated. It is shown that the same
"initial entanglement" for the state parameter $\alpha$ and its "normalized
partners" $\sqrt{1-\alpha^{2}}$ will be degraded by the Hawking effect with
increasing Hawking temperature along two different trajectories except for the
maximally entangled state. In the infinite Hawking temperature limit,
corresponding to the case of the black hole evaporating completely, the state
has no longer distillable entanglement for any $\alpha$. It is interesting to
note that the mutual information in this limit equals to just half of the
"initially mutual information". It has also been demonstrated that the fidelity
of teleportation decreases as the Hawking temperature increases, which just
indicates the degradation of entanglement.Comment: 17 pages, 3 figures, to be published in Physical Review

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