16,845 research outputs found
Decoherence of number states in phase-sensitive reservoirs
The non-unitary evolution of initial number states in general Gaussian
environments is solved analytically. Decoherence in the channels is quantified
by determining explicitly the purity of the state at any time. The influence of
the squeezing of the bath on decoherence is discussed. The behavior of coherent
superpositions of number states is addressed as well.Comment: 5 pages, 2 figures, minor changes, references adde
Security of coherent state quantum cryptography in the presence of Gaussian noise
We investigate the security against collective attacks of a continuous
variable quantum key distribution scheme in the asymptotic key limit for a
realistic setting. The quantum channel connecting the two honest parties is
assumed to be lossy and imposes Gaussian noise on the observed quadrature
distributions. Secret key rates are given for direct and reverse reconciliation
schemes including postselection in the collective attack scenario. The effect
of a non-ideal error correction and two-way communication in the classical
post-processing step is also taken into account.Comment: 12 pages, 5 figures updated version including two-way communication;
changed the definition of the excess noise to match the definition given
earlier (Phys. Rev. Lett. 92, 117901); submitted to PRA; presented at the 8th
International Conference on Quantum Communication, Measurement and Computing,
Tsukub
Eliminating the low-mass axigluon window
Using recent collider data, especially on the hadronic width the Z0, we
exclude axigluons in the currently allowed low-mass window, namely axigluons in
the mass range 50 GeV < M_A < 120 GeV. Combined with hadron collider data from
di-jet production, axigluons with masses below roughly 1 TeV are now completely
excluded.Comment: 8 pages, no figures, LaTe
Passing quantum correlations to qubits using any two-mode state
We draw an explicit connection between the statistical properties of an
entangled two-mode continuous variable (CV) resource and the amount of
entanglement that can be dynamically transferred to a pair of non-interacting
two-level systems. More specifically, we rigorously reformulate entanglement
transfer process by making use of covariance matrix formalism. When the
resource state is Gaussian, our method makes the approach to the transfer of
quantum correlations much more flexible than in previously considered schemes
and allows the straightforward inclusion of the effects of noise affecting the
CV system. Moreover, the proposed method reveals that the use of de-Gaussified
two-mode states is almost never advantageous for transferring entanglement with
respect to the full Gaussian picture, despite the entanglement in the
non-Gaussian resource can be much larger than in its Gaussian counterpart. We
can thus conclude that the entanglement-transfer map overthrows the "ordering"
relations valid at the level of CV resource states.Comment: 10 pages, 6 figures, RevTeX4, Accepted for publication in Physical
Review
Large-uncertainty intelligent states for angular momentum and angle
The equality in the uncertainty principle for linear momentum and position is
obtained for states which also minimize the uncertainty product. However, in
the uncertainty relation for angular momentum and angular position both sides
of the inequality are state dependent and therefore the intelligent states,
which satisfy the equality, do not necessarily give a minimum for the
uncertainty product. In this paper, we highlight the difference between
intelligent states and minimum uncertainty states by investigating a class of
intelligent states which obey the equality in the angular uncertainty relation
while having an arbitrarily large uncertainty product. To develop an
understanding for the uncertainties of angle and angular momentum for the
large-uncertainty intelligent states we compare exact solutions with analytical
approximations in two limiting cases.Comment: 20 pages, 9 figures, submitted to J. Opt. B special issue in
connection with ICSSUR 2005 conferenc
Maximum Entanglement in Squeezed Boson and Fermion States
A class of squeezed boson and fermion states is studied with particular
emphasis on the nature of entanglement. We first investigate the case of
bosons, considering two-mode squeezed states. Then we construct the fermion
version to show that such states are maximum entangled, for both bosons and
fermions. To achieve these results, we demonstrate some relations involving
squeezed boson states. The generalization to the case of fermions is made by
using Grassmann variables.Comment: 4 page
Bipartite quantum channels using multipartite cluster-type entangled coherent states
We propose a particular encoding for bipartite entangled states derived from
multipartite cluster-type entangled coherent states (CTECSs). We investigate
the effects of amplitude damping on the entanglement content of this bipartite
state, as well as its usefulness as a quantum channel for teleportation. We
find interesting relationships among the amplitude of the coherent states
constituting the CTECSs, the number of subsystems forming the logical qubits
(redundancy), and the extent to which amplitude damping affects the
entanglement of the channel. For instance, in the sense of sudden death of
entanglement, given a fixed value of the initial coherent state amplitude, the
entanglement life span is shortened if redundancy is increased.Comment: 6 pages, 3 figures, REVTeX 4.1, BibTe
Spontaneous emission and teleportation in cavity QED
In this work, we consider atomic spontaneous emission in a system consisting
of two identical two-level atoms interacting dispersively with the quantized
electromagnetic field in a high-Q cavity. We investigate the destructive effect
of the atomic decay on the generation of maximally entangled states, following
the proposal by Zheng S B and Guo G C (2000 Phys. Rev. Lett. 85 2392). In
particular, we analyze the fidelity of teleportation performed using such a
noisy channel and calculatethe maximum spontaneous decay rate we may have in
order to realize teleportation.Comment: 11 pages, 6 figures, LaTe
Generalized Hermite-Gauss decomposition of the two-photon state produced by spontaneous parametric down-conversion
We provide a general decomposition of the two-photon state produced by
spontaneous parametric down-conversion in Hermite-Gaussian modes, in the case
that the pump beam is described by a Hermite-Gaussian beam of any order. We
show that the spatial correlations depend explicitly on the order of the pump
beam, as well as other experimental parameters. We use the decomposition to
demonstrate a few interesting cases. Our results are applicable to the
engineering of two-photon spatial entanglement, in particular for non-Gaussian
states.Comment: 14 page draft, 5 figure
Optimal discrimination of quantum operations
We address the problem of discriminating with minimal error probability two
given quantum operations. We show that the use of entangled input states
generally improves the discrimination. For Pauli channels we provide a complete
comparison of the optimal strategies where either entangled or unentangled
input states are used.Comment: 4 pages, no figure
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