377 research outputs found
Preparation of distilled and purified continuous variable entangled states
The distribution of entangled states of light over long distances is a major
challenge in the field of quantum information. Optical losses, phase diffusion
and mixing with thermal states lead to decoherence and destroy the
non-classical states after some finite transmission-line length. Quantum
repeater protocols, which combine quantum memory, entanglement distillation and
entanglement swapping, were proposed to overcome this problem. Here we report
on the experimental demonstration of entanglement distillation in the
continuous-variable regime. Entangled states were first disturbed by random
phase fluctuations and then distilled and purified using interference on beam
splitters and homodyne detection. Measurements of covariance matrices clearly
indicate a regained strength of entanglement and purity of the distilled
states. In contrast to previous demonstrations of entanglement distillation in
the complementary discrete-variable regime, our scheme achieved the actual
preparation of the distilled states, which might therefore be used to improve
the quality of downstream applications such as quantum teleportation
An experimental investigation of criteria for continuous variable entanglement
We generate a pair of entangled beams from the interference of two amplitude
squeezed beams. The entanglement is quantified in terms of EPR-paradox [Reid88]
and inseparability [Duan00] criteria, with observed results of and , respectively. Both results clearly beat the standard quantum
limit of unity. We experimentally analyze the effect of decoherence on each
criterion and demonstrate qualitative differences. We also characterize the
number of required and excess photons present in the entangled beams and
provide contour plots of the efficacy of quantum information protocols in terms
of these variables.Comment: 4 pages, 5 figure
Biased EPR entanglement and its application to teleportation
We consider pure continuous variable entanglement with non-equal correlations
between orthogonal quadratures. We introduce a simple protocol which equates
these correlations and in the process transforms the entanglement onto a state
with the minimum allowed number of photons. As an example we show that our
protocol transforms, through unitary local operations, a single squeezed beam
split on a beam splitter into the same entanglement that is produced when two
squeezed beams are mixed orthogonally. We demonstrate that this technique can
in principle facilitate perfect teleportation utilising only one squeezed beam.Comment: 8 pages, 5 figure
Increasing entanglement between Gaussian states by coherent photon subtraction
We experimentally demonstrate that the entanglement between Gaussian
entangled states can be increased by non-Gaussian operations. Coherent
subtraction of single photons from Gaussian quadrature-entangled light pulses,
created by a non-degenerate parametric amplifier, produces delocalized states
with negative Wigner functions and complex structures, more entangled than the
initial states in terms of negativity. The experimental results are in very
good agreement with the theoretical predictions
Time-resolved homodyne characterization of individual quadrature-entangled pulses
We describe a simple and efficient setup to generate and characterize
femtosecond quadrature-entangled pulses. Quantum correlations equivalent to
about 2.5 dB squeezing are efficiently and easily reached using the
non-degenerate parametric amplification of femtosecond pulses through a
single-pass in a thin (0.1 mm) potassium niobate crystal. The entangled pulses
are then individually sampled to characterize the non-separability and the
entropy of formation of the states. The complete experiment is analysed in the
time-domain, from the pulsed source of quadrature entanglement to the
time-resolved homodyne detection. This particularity allows for applications in
quantum communication protocols using continuous-variable entanglement.Comment: 7 pages, 5 figure
Demonstrating various quantum effects with two entangled laser beams
We report on the preparation of entangled two mode squeezed states of yet
unseen quality. Based on a measurement of the covariance matrix we found a
violation of the Reid and Drummond EPR-criterion at a value of only 0.36\pm0.03
compared to the threshold of 1. Furthermore, quantum state tomography was used
to extract a single photon Fock state solely based on homodyne detection,
demonstrating the strong quantum features of this pair of laser-beams. The
probability for a single photon in this ensemble measurement exceeded 2/3
Experimental characterization of continuous-variable entanglement
We present an experimental analysis of quadrature entanglement produced from a pair of amplitude squeezed beams. The correlation matrix of the state is characterized within a set of reasonable assumptions, and the strength of the entanglement is gauged using measures of the degree of inseparability and the degree of Einstein-Podolsky-Rosen (EPR) paradox. We introduce controlled decoherence in the form of optical loss to the entangled state, and demonstrate qualitative differences in the response of the degrees of inseparability and EPR paradox to this loss. The entanglement is represented on a photon number diagram that provides an intuitive and physically relevant description of the state. We calculate efficacy contours for several quantum information protocols on this diagram, and use them to predict the effectiveness of our entanglement in those protocols
Continuous variable entanglement distillation of Non-Gaussian Mixed States
Many different quantum information communication protocols such as
teleportation, dense coding and entanglement based quantum key distribution are
based on the faithful transmission of entanglement between distant location in
an optical network. The distribution of entanglement in such a network is
however hampered by loss and noise that is inherent in all practical quantum
channels. Thus, to enable faithful transmission one must resort to the protocol
of entanglement distillation. In this paper we present a detailed theoretical
analysis and an experimental realization of continuous variable entanglement
distillation in a channel that is inflicted by different kinds of non-Gaussian
noise. The continuous variable entangled states are generated by exploiting the
third order non-linearity in optical fibers, and the states are sent through a
free-space laboratory channel in which the losses are altered to simulate a
free-space atmospheric channel with varying losses. We use linear optical
components, homodyne measurements and classical communication to distill the
entanglement, and we find that by using this method the entanglement can be
probabilistically increased for some specific non-Gaussian noise channels
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