4,129 research outputs found
Experimental test of strongly non-classical character of a noisy squeezed single-photon state
We experimentally verify the quantum non-Gaussian character of a
conditionally generated noisy squeezed single-photon state with positive Wigner
function. Employing an optimized witness based on probabilities of squeezed
vacuum and squeezed single-photon states we prove that the state cannot be
expressed as a mixture of Gaussian states. In our experiment, the non-Gaussian
state is generated by conditional subtraction of a single photon from squeezed
vacuum state. The state is probed with a homodyne detector and the witness is
determined by averaging a suitable pattern function over the measured homodyne
data. Our experimental results are in good agreement with a theoretical fit
obtained from a simple yet realistic model of the experimental setup.Comment: 10 pages, 8 figures, REVTeX
Transformations of symmetric multipartite Gaussian states by Gaussian LOCC
Multipartite quantum correlations, in spite of years of intensive research,
still leave many questions unanswered. While bipartite entanglement is
relatively well understood for Gaussian states, the complexity of mere
qualitative characterization grows rapidly with increasing number of parties.
Here, we present two schemes for transformations of multipartite permutation
invariant Gaussian states by Gaussian local operations and classical
communication. To this end, we use a scheme for possible experimental
realization, making use of the fact, that in this picture, the whole N -
partite state can be described using two separable modes. Numerically, we study
entanglement transformations of tripartite states. Finally, we look at the
effect our protocols have on fidelity of assisted quantum teleportation and
find that while adding correlated noise does not affect the fidelity at all,
there is strong evidence that partial non-demolition measurement leads to a
drop in teleportation fidelity.Comment: 9 page
Homodyne estimation of Gaussian quantum discord
We address the experimental estimation of Gaussian quantum discord for
two-mode squeezed thermal state, and demonstrate a measurement scheme based on
a pair of homodyne detectors assisted by Bayesian analysis which provides
nearly optimal estimation for small value of discord. Besides, though homodyne
detection is not optimal for Gaussian discord, the noise ratio to the ultimate
quantum limit, as dictacted by the quantum Cramer-Rao bound, is limited to
about 10 dB.Comment: 5+3 pages, 3 figures, published versio
Cram\'er-Rao bound for time-continuous measurements in linear Gaussian quantum systems
We describe a compact and reliable method to calculate the Fisher information
for the estimation of a dynamical parameter in a continuously measured linear
Gaussian quantum system. Unlike previous methods in the literature, which
involve the numerical integration of a stochastic master equation for the
corresponding density operator in a Hilbert space of infinite dimension, the
formulas here derived depends only on the evolution of first and second moments
of the quantum states, and thus can be easily evaluated without the need of any
approximation. We also present some basic but physically meaningful examples
where this result is exploited, calculating analytical and numerical bounds on
the estimation of the squeezing parameter for a quantum parametric amplifier,
and of a constant force acting on a mechanical oscillator in a standard
optomechanical scenario.Comment: 9 pages, 2 figure
On the distillation and purification of phase-diffused squeezed states
Recently it was discovered that non-Gaussian decoherence processes, such as
phase-diffusion, can be counteracted by purification and distillation protocols
that are solely built on Gaussian operations. Here, we make use of this
experimentally highly accessible regime, and provide a detailed experimental
and theoretical analysis of several strategies for purification/distillation
protocols on phase-diffused squeezed states. Our results provide valuable
information for the optimization of such protocols with respect to the choice
of the trigger quadrature, the trigger threshold value and the probability of
generating a distilled state
Computation of generalized matrix functions
We develop numerical algorithms for the efficient evaluation of quantities
associated with generalized matrix functions [J. B. Hawkins and A. Ben-Israel,
Linear and Multilinear Algebra 1(2), 1973, pp. 163-171]. Our algorithms are
based on Gaussian quadrature and Golub--Kahan bidiagonalization. Block variants
are also investigated. Numerical experiments are performed to illustrate the
effectiveness and efficiency of our techniques in computing generalized matrix
functions arising in the analysis of networks.Comment: 25 paged, 2 figure
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