106 research outputs found
Entanglement degradation of a two-mode squeezed vacuum in absorbing and amplifying optical fibers
Applying the recently developed formalism of quantum-state transformation at
absorbing dielectric four-port devices [L.~Kn\"oll, S.~Scheel, E.~Schmidt,
D.-G.~Welsch, and A.V.~Chizhov, Phys. Rev. A {\bf 59}, 4716 (1999)], we
calculate the quantum state of the outgoing modes of a two-mode squeezed vacuum
transmitted through optical fibers of given extinction coefficients. Using the
Peres--Horodecki separability criterion for continuous variable systems
[R.~Simon, Phys. Rev. Lett. {\bf 84}, 2726 (2000)], we compute the maximal
length of transmission of a two-mode squeezed vacuum through an absorbing
system for which the transmitted state is still inseparable. Further, we
calculate the maximal gain for which inseparability can be observed in an
amplifying setup. Finally, we estimate an upper bound of the entanglement
preserved after transmission through an absorbing system. The results show that
the characteristic length of entanglement degradation drastically decreases
with increasing strength of squeezing.Comment: Paper presented at the International Conference on Quantum Optics and
VIII Seminar on Quantum Optics, Raubichi, Belarus, May 28-31, 2000, 11 pages,
LaTeX2e, 4 eps figure
Asymmetric quantum channel for quantum teleportation
There are a few obstacles, which bring about imperfect quantum teleportation
of a continuous variable state, such as unavailability of maximally entangled
two-mode squeezed states, inefficient detection and imperfect unitary
transformation at the receiving station. We show that all those obstacles can
be understood by a combination of an {\it asymmetrically-decohered} quantum
channel and perfect apparatuses for other operations. For the
asymmetrically-decohered quantum channel, we find some counter-intuitive
results; one is that teleportation does not necessarily get better as the
channel is initially squeezed more and another is when one branch of the
quantum channel is unavoidably subject to some imperfect operations, blindly
making the other branch as clean as possible may not result in the best
teleportation result. We find the optimum strategy to teleport an unknown field
for a given environment or for a given initial squeezing of the channel.Comment: 4pages, 1figur
Long-range adiabatic quantum state transfer through a linear array of quantum dots
We introduce an adiabatic long-range quantum communication proposal based on
a quantum dot array. By adiabatically varying the external gate voltage applied
on the system, the quantum information encoded in the electron can be
transported from one end dot to another. We numerically solve the Schr\"odinger
equation for a system with a given number of quantum dots. It is shown that
this scheme is a simple and efficient protocol to coherently manipulate the
population transfer under suitable gate pulses. The dependence of the energy
gap and the transfer time on system parameters is analyzed and shown
numerically. We also investigate the adiabatic passage in a more realistic
system in the presence of inevitable fabrication imperfections. This method
provides guidance for future realizations of adiabatic quantum state transfer
in experiments.Comment: 7 pages, 7 figure
Causality in quantum teleportation: information extraction and noise effects in entanglement distribution
Quantum teleportation is possible because entanglement allows a definition of
precise correlations between the non-commuting properties of a local system and
corresponding non-commuting properties of a remote system. In this paper, the
exact causality achieved by maximal entanglement is analyzed and the results
are applied to the transfer of effects acting on the entanglement distribution
channels to the teleported output state. In particular, it is shown how
measurements performed on the entangled system distributed to the sender
provide information on the teleported state while transferring the
corresponding back-action to the teleported quantum state.Comment: 14 pages, including three figures, discussion of fidelity adde
Self-homodyne tomography of a twin-beam state
A self-homodyne detection scheme is proposed to perform two-mode tomography
on a twin-beam state at the output of a nondegenerate optical parametric
amplifier. This scheme has been devised to improve the matching between the
local oscillator and the signal modes, which is the main limitation to the
overall quantum efficiency in conventional homodyning. The feasibility of the
measurement is analyzed on the basis of Monte-Carlo simulations, studying the
effect of non-unit quantum efficiency on detection of the correlation and the
total photon-number oscillations of the twin-beam state.Comment: 13 pages (two-column ReVTeX) including 21 postscript figures; to
appear on Phys. Rev.
Route discovery with constant memory in oriented planar geometric networks
We address the problem of discovering routes in strongly connected planar geometric networks with directed links. Motivated by the necessity for establishing communication in wireless ad hoc networks in which the only information available to a vertex is its immediate neighborhood, we are considering routing algorithms that use the neighborhood information of a vertex for routing with constant memory only. We solve the problem for three types of directed planar geometric networks: Eulerian (in which every vertex has the same number of incoming and outgoing edges), Outerplanar (in which a single face contains all vertices of the network), and Strongly Face Connected, a new class of geometric networks that we define in the article, consisting of several faces, each face being a strongly connected outerplanar graph
Entanglement transfer from dissociated molecules to photons
We introduce and study the concept of a reversible transfer of the quantum
state of two internally-translationally entangled fragments, formed by
molecular dissociation, to a photon pair. The transfer is based on intracavity
stimulated Raman adiabatic passage and it requires a combination of processes
whose principles are well established.Comment: 5 pages, 3 figure
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