468 research outputs found
Gradient Scan Gibbs Sampler: an efficient algorithm for high-dimensional Gaussian distributions
This paper deals with Gibbs samplers that include high dimensional
conditional Gaussian distributions. It proposes an efficient algorithm that
avoids the high dimensional Gaussian sampling and relies on a random excursion
along a small set of directions. The algorithm is proved to converge, i.e. the
drawn samples are asymptotically distributed according to the target
distribution. Our main motivation is in inverse problems related to general
linear observation models and their solution in a hierarchical Bayesian
framework implemented through sampling algorithms. It finds direct applications
in semi-blind/unsupervised methods as well as in some non-Gaussian methods. The
paper provides an illustration focused on the unsupervised estimation for
super-resolution methods.Comment: 18 page
Estimating hyperparameters and instrument parameters in regularized inversion. Illustration for SPIRE/Herschel map making
We describe regularized methods for image reconstruction and focus on the
question of hyperparameter and instrument parameter estimation, i.e.
unsupervised and myopic problems. We developed a Bayesian framework that is
based on the \post density for all unknown quantities, given the observations.
This density is explored by a Markov Chain Monte-Carlo sampling technique based
on a Gibbs loop and including a Metropolis-Hastings step. The numerical
evaluation relies on the SPIRE instrument of the Herschel observatory. Using
simulated and real observations, we show that the hyperparameters and
instrument parameters are correctly estimated, which opens up many perspectives
for imaging in astrophysics
Tras las rejas: mamĂferos en los parques zoolĂłgicos y su bienestar
Treball presentat a l'assignatura de Deontologia i VeterinĂ ria Legal (21223
Experimental detection of steerability in Bell local states with two measurement settings
Steering, a quantum property stronger than entanglement but weaker than
non-locality in the quantum correlation hierarchy, is a key resource for
one-sided device-independent quantum key distribution applications, in which
only one of the communicating parties is trusted. A fine-grained steering
inequality was introduced in [PRA 90 050305(R) (2014)], enabling for the first
time the detection of steering in all steerable two-qubit Werner states using
only two measurement settings. Here we numerically and experimentally
investigate this inequality for generalized Werner states and successfully
detect steerability in a wide range of two-photon polarization-entangled Bell
local states generated by a parametric down-conversion source.Comment: 9 pages, 7 figures (including Appendix
Experimental Detection of Quantum Channels
We demonstrate experimentally the possibility of efficiently detecting
properties of quantum channels and quantum gates. The optimal detection scheme
is first achieved for non entanglement breaking channels of the depolarizing
form and is based on the generation and detection of polarized entangled
photons. We then demonstrate channel detection for non separable maps by
considering the CNOT gate and employing two-photon hyperentangled states.Comment: 8 pages, 9 figure
Experimental investigation of practical unforgeable quantum money
Wiesner's unforgeable quantum money scheme is widely celebrated as the first
quantum information application. Based on the no-cloning property of quantum
mechanics, this scheme allows for the creation of credit cards used in
authenticated transactions offering security guarantees impossible to achieve
by classical means. However, despite its central role in quantum cryptography,
its experimental implementation has remained elusive because of the lack of
quantum memories and of practical verification techniques. Here, we
experimentally implement a quantum money protocol relying on classical
verification that rigorously satisfies the security condition for
unforgeability. Our system exploits polarization encoding of weak coherent
states of light and operates under conditions that ensure compatibility with
state-of-the-art quantum memories. We derive working regimes for our system
using a security analysis taking into account all practical imperfections. Our
results constitute a major step towards a real-world realization of this
milestone protocol.Comment: 10 pages, 5 figure
Experimental generation of entanglement from classical correlations via non-unital local noise
We experimentally show how classical correlations can be turned into quantum
entanglement, via the presence of non-unital local noise and the action of a
CNOT gate. We first implement a simple two-qubit protocol in which entanglement
production is not possible in the absence of local non-unital noise, while
entanglement arises with the introduction of noise, and is proportional to the
degree of noisiness. We then perform a more elaborate four-qubit experiment, by
employing two hyperentangled photons initially carrying only classical
correlations. We demonstrate a scheme where the entanglement is generated via
local non-unital noise, with the advantage to be robust against local unitaries
performed by an adversary.Comment: 8 pages, 4 figure
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