1,896 research outputs found
Multipartite entanglement for continuous variables: A quantum teleportation network
We show that {\it one} single-mode squeezed state distributed among
parties using linear optics suffices to produce a truly -partite entangled
state for any nonzero squeezing and arbitrarily many parties. From this
-partite entangled state, via quadrature measurements of modes,
bipartite entanglement between any two of the parties can be `distilled',
which enables quantum teleportation with an experimentally determinable
fidelity better than could be achieved in any classical scheme.Comment: 4 pages, 2 figures, published version, paper shorter, title longe
Quantum teleportation with squeezed vacuum states
We show how the partial entanglement inherent in a two mode squeezed vacuum
state admits two different teleportation protocols. These two protocols refer
to the different kinds of joint measurements that may be made by the sender.
One protocol is the recently implemented quadrature phase approach of
Braunstein and Kimble[Phys. Rev. Lett.{\bf 80}, 869 (1998)]. The other is based
on recognising that a two mode squeezed vacuum state is also entangled with
respect to photon number difference and phase sum. We show that this protocol
can also realise teleportation, however limitations can arise due to the fact
that the photon number spectrum is bounded from below by zero. Our examples
show that a given entanglement resource may admit more than a single
teleportation protocol and the question then arises as to what is the optimum
protocol in the general case
Optical implementation of continuous-variable quantum cloning machines
We propose an optical implementation of the Gaussian continuous-variable
quantum cloning machines. We construct a symmetric N -> M cloner which
optimally clones coherent states and we also provide an explicit design of an
asymmetric 1 -> 2 cloning machine. All proposed cloning devices can be built
from just a single non-degenerate optical parametric amplifier and several beam
splitters.Comment: 4 pages, 3 figures, REVTe
Universal teleportation with a twist
We give a transfer theorem for teleportation based on twisting the
entanglement measurement. This allows one to say what local unitary operation
must be performed to complete the teleportation in any situation, generalizing
the scheme to include overcomplete measurements, non-abelian groups of local
unitary operations (e.g., angular momentum teleportation), and the effect of
non-maximally entangled resources.Comment: 4 pages, 1 figur
Compiling gate networks on an Ising quantum computer
Here we describe a simple mechanical procedure for compiling a quantum gate
network into the natural gates (pulses and delays) for an Ising quantum
computer. The aim is not necessarily to generate the most efficient pulse
sequence, but rather to develop an efficient compilation algorithm that can be
easily implemented in large spin systems. The key observation is that it is not
always necessary to refocus all the undesired couplings in a spin system.
Instead the coupling evolution can simply be tracked and then corrected at some
later time. Although described within the language of NMR the algorithm is
applicable to any design of quantum computer based on Ising couplings.Comment: 5 pages RevTeX4 including 4 figures. Will submit to PR
Quantum entanglement and teleportation in pulsed cavity-optomechanics
Entangling a mechanical oscillator with an optical mode is an enticing and
yet a very challenging goal in cavity optomechanics. Here we consider a pulsed
scheme to create Einstein-Podolsky-Rosen-type entanglement between a
traveling-wave light pulse and a mechanical oscillator. The entanglement can be
verified unambiguously by a pump-probe sequence of pulses. In contrast to
schemes that work in a steady-state regime under a continuous-wave drive, this
protocol is not subject to stability requirements that normally limit the
strength of achievable entanglement. We investigate the protocol's performance
under realistic conditions, including mechanical decoherence, in full detail.
We discuss the relevance of a high mechanical Qf product for entanglement
creation and provide a quantitative statement on which magnitude of the Qf
product is necessary for a successful realization of the scheme. We determine
the optimal parameter regime for its operation and show it to work in current
state-of-the-art systems.Comment: 10 pages, 2 figure
Discrete teleportation protocol of continuum spectra field states
A discrete protocol for teleportation of superpositions of coherent states of
optical cavity fields is presented. Displacement and parity operators are
unconventionally used in Bell-like measurement for field states.Comment: 12 pages, 1 figur
Survey Propagation as local equilibrium equations
It has been shown experimentally that a decimation algorithm based on Survey
Propagation (SP) equations allows to solve efficiently some combinatorial
problems over random graphs. We show that these equations can be derived as
sum-product equations for the computation of marginals in an extended space
where the variables are allowed to take an additional value -- -- when they
are not forced by the combinatorial constraints. An appropriate ``local
equilibrium condition'' cost/energy function is introduced and its entropy is
shown to coincide with the expected logarithm of the number of clusters of
solutions as computed by SP. These results may help to clarify the geometrical
notion of clusters assumed by SP for the random K-SAT or random graph coloring
(where it is conjectured to be exact) and helps to explain which kind of
clustering operation or approximation is enforced in general/small sized models
in which it is known to be inexact.Comment: 13 pages, 3 figure
Pulsed squeezed vacuum characterization without homodyning
Direct photon detection is experimentally implemented to measure the
squeezing and purity of a single-mode squeezed vacuum state without an
interferometric homodyne detection. Following a recent theoretical proposal
[arXiv quant-ph/0311119], the setup only requires a tunable beamsplitter and a
single-photon detector to fully characterize the generated Gaussian states. The
experimental implementation of this procedure is discussed and compared with
other reference methods.Comment: 8 pages, 7 figure
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