3,298 research outputs found
Enhancing the Violation of the Einstein-Podolsky-Rosen Local Realism by Quantum Hyper-entanglement
Mermin's observation [Phys. Rev. Lett. {\bf 65}, 1838 (1990)] that the
magnitude of the violation of local realism, defined as the ratio between the
quantum prediction and the classical bound, can grow exponentially with the
size of the system is demonstrated using two-photon hyper-entangled states
entangled in polarization and path degrees of freedom, and local measurements
of polarization and path simultaneously.Comment: Minor errors corrected. To appear on Physical Review Letter
Proofs of nonlocality without inequalities revisited
We discuss critically the so-called nonlocality without inequalities proofs
for bipartite quantum states, we generalize them and we analyze their relation
with the Clauser-Horne inequality.Comment: 8 pages, RevTex; to be published on PL
Bell scenarios in which nonlocality and entanglement are inversely related
We show that for two-qubit chained Bell inequalities with an arbitrary number
of measurement settings, nonlocality and entanglement are not only different
properties but are inversely related. Specifically, we analytically prove that
in absence of noise, robustness of nonlocality, defined as the maximum fraction
of detection events that can be lost such that the remaining ones still do not
admit a local model, and concurrence are inversely related for any chained Bell
inequality with an arbitrary number of settings. The closer quantum states are
to product states, the harder it is to reproduce quantum correlations with
local models. We also show that, in presence of noise, nonlocality and
entanglement are simultaneously maximized only when the noise level is equal to
the maximum level tolerated by the inequality; in any other case, a more
nonlocal state is always obtained by reducing the entanglement. In addition, we
observed that robustness of nonlocality and concurrence are also inversely
related for the Bell scenarios defined by the tight two-qubit three-setting
inequality, and the tight two-qutrit inequality .Comment: 9 page
Unified view of correlations using the square norm distance
The distance between a quantum state and its closest state not having a
certain property has been used to quantify the amount of correlations
corresponding to that property. This approach allows a unified view of the
various kinds of correlations present in a quantum system. In particular, using
relative entropy as a distance measure, total correlations can be meaningfully
separated in a quantum and a classical part thanks to an additive relation
involving only distances between states. Here, we investigate a unified view of
correlations using as distance measure the square norm, already used to define
the so-called geometric quantum discord. We thus consider geometric quantifiers
also for total and classical correlations finding, for a quite general class of
bipartite states, their explicit expressions. We analyze the relationship among
geometric total, quantum and classical correlations and we find that they do
not satisfy anymore a closed additivity relation.Comment: 10 pages, 3 figures (to appear in Phys. Rev. A
Fully nonlocal quantum correlations
Quantum mechanics is a nonlocal theory, but not as nonlocal as the
no-signalling principle allows. However, there exist quantum correlations that
exhibit maximal nonlocality: they are as nonlocal as any non-signalling
correlations and thus have a local content, quantified by the fraction of
events admitting a local description, equal to zero. Exploiting the link
between the Kochen-Specker and Bell's theorems, we derive, from every
Kochen-Specker proof, Bell inequalities maximally violated by quantum
correlations. We then show that these Bell inequalities lead to experimental
bounds on the local content of quantum correlations which are significantly
better than those based on other constructions. We perform the experimental
demonstration of a Bell test originating from the Peres-Mermin Kochen-Specker
proof, providing an upper bound on the local content .Comment: 9 pages, 5 figures and three tables. To appear in PR
Experimental noise-resistant Bell-inequality violations for polarization-entangled photons
We experimentally demonstrate that violations of Bell's inequalities for
two-photon polarization-entangled states with colored noise are extremely
robust, whereas this is not the case for states with white noise. Controlling
the amount of noise by using the timing compensation scheme introduced by Kim
et al. [Phys. Rev. A 67, 010301(R) (2003)], we have observed violations even
for states with very high noise, in excellent agrement with the predictions of
Cabello et al. [Phys. Rev. A 72, 052112 (2005)].Comment: REVTeX4, 5 pages, 4 figure
Non-local quantum correlations and detection processes in QFT
Quantum detection processes in QFT must play a key role in the description of
quantum field correlations, such as the appearance of entanglement, and of
causal effects. We consider the detection in the case of a simple QFT model
with a suitable interaction to exact treatment, consisting of a quantum scalar
field coupled linearly to a classical scalar source. We then evaluate the
response function to the field quanta of two-level point-like quantum model
detectors, and analyze the effects of the approximation adopted in standard
detection theory. We show that the use of the RWA, that characterizes the
Glauber detection model, leads in the detector response to non-local terms
corresponding to an instantaneously spreading of source effects over the whole
space. Other detector models, obtained with non-standard or the no-application
of RWA, give instead local responses to field quanta, apart from source
independent vacuum contribution linked to preexisting correlations of
zero-point field.Comment: 23 page
Device-independent entanglement-based Bennett 1992 protocol
In this paper we set forth a novel connection between the Bennett 1992
protocol and a Bell inequality. This allows us to extend the usual
prepare-and-measure protocol to its entanglement-based formulation. We exploit
a recent result in the frame of device-independent quantum key distribution to
provide a simple, model-independent, security proof for the new protocol. The
minimum efficiency required for a practical implementation of the scheme is the
lowest reported to date.Comment: 7 pages, 3 figures. 3nd version: published versio
The Nature of Angular Momentum Transport in Radiative Self-Gravitating Protostellar Discs
Semi-analytic models of self-gravitating discs often approximate the angular
momentum transport generated by the gravitational instability using the
phenomenology of viscosity. This allows the employment of the standard viscous
evolution equations, and gives promising results. It is, however, still not
clear when such an approximation is appropriate. This paper tests this
approximation using high resolution 3D smoothed particle hydrodynamics (SPH)
simulations of self-gravitating protostellar discs with radiative transfer. The
nature of angular momentum transport associated with the gravitational
instability is characterised as a function of both the stellar mass and the
disc-to-star mass ratio. The effective viscosity is calculated from the
Reynolds and gravitational stresses in the disc. This is then compared to what
would be expected if the effective viscosity were determined by assuming local
thermodynamic equilibrium or, equivalently, that the local dissipation rate
matches the local cooling rate. In general, all the discs considered here
settle into a self-regulated state where the heating generated by the
gravitational instability is modulated by the local radiative cooling. It is
found that low-mass discs can indeed be represented by a local
"alpha-parametrisation", provided that the disc aspect ratio is small (H/R <
0.1) which is generally the case when the disc-to-star mass ratio q <0.5.
However, this result does not extend to discs with masses approaching that of
the central object. These are subject to transient burst events and global wave
transport, and the effective viscosity is not well modelled by assuming local
thermodynamic equilibrium. In spite of these effects, it is shown that massive
(compact) discs can remain stable and not fragment, evolving rapidly to reduce
their disc-to-star mass ratios through stellar accretion and radial spreading.Comment: 13 pages, 44 figures, accepted for publication in MNRA
Violation of Bell's Inequality with Photons from Independent Sources
We report a violation of Bell's inequality using one photon from a parametric
down-conversion source and a second photon from an attenuated laser beam. The
two photons were entangled at a beam splitter using the post-selection
technique of Shih and Alley [Phys. Rev. Lett. 61, 2921 (1988)]. A quantum
interference pattern with a visibility of 91% was obtained using the photons
from these independent sources, as compared with a visibility of 99.4% using
two photons from a central parametric down-conversion source.Comment: 4 pages, 5 figures; minor change
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