2,150 research outputs found
On the local and non-local content of bipartite qubit and qutrit correlations
The local and non-local contents of non-local probability distributions are
studied using the approach of Elitzur, Popescu and Rohrlich [Phys. Lett. A
\textbf{162}, 25 (1992)]. This work focuses on distributions that can be
obtained by single-copy von Neumann measurements on bipartite quantum systems.
For pure two-qubit states Psi(theta)=cos(theta)|00>+sin(theta)|11>, with
cos(theta)>=sin(theta), the local content of the corresponding probability
distribution is found to lie between 1-sin(2*theta) and cos(2*theta). For the
family Psi(gamma)= (|00>+|11>+gamma*|22>)/sqrt(2+gamma^2) of two-qutrit states,
non-zero local content is found for gamma>2.Comment: Accepted version. Previous title "The Elitzur-Popescu-Rohrlich
approach to quantum non-locality" changed for editorial reason
Multipartite fully-nonlocal quantum states
We present a general method to characterize the quantum correlations obtained
after local measurements on multipartite systems. Sufficient conditions for a
quantum system to be fully-nonlocal according to a given partition, as well as
being (genuinely) multipartite fully-nonlocal, are derived. These conditions
allow us to identify all completely-connected graph states as multipartite
fully-nonlocal quantum states. Moreover, we show that this feature can also be
observed in mixed states: the tensor product of five copies of the Smolin
state, a biseparable and bound entangled state, is multipartite fully-nonlocal.Comment: 5 pages, 1 figure. Version published in PRA. Note that it does not
contain all the results from the previous version; these will be included in
a later, more general, pape
Does entanglement depend on the timing of the impacts at the beam-splitters?
A new nonlocality experiment with moving beam-splitters is proposed. The
experiment is analysed according to conventional quantum mechanics, and to an
alternative nonlocal description in which superposition depends not only on
indistinguishability but also on the timing of the impacts at the
beam-splitters.Comment: 5 pages of Latex and 2 eps figures. Submitted to Phys. Lett.
The local content of bipartite qubit correlations
One of the last open problems concerning two qubits in a pure state is to
find the exact local content of their correlation, in the sense of Elitzur,
Popescu and Rohrlich (EPR2) [Phys. Lett. A 162, 25 (1992)]. We propose a new
EPR2 decomposition that allows us to prove, for a wide range of states
psi(theta)=cos(theta)|00>+sin(theta)|11>, that their local content is
pL(theta)=cos(2theta). We also share reflections on how to possibly extend our
result to all two-qubit pure states
Device-independent bounds for Hardy's experiment
In this Letter we compute an analogue of Tsirelson's bound for Hardy's test
of nonlocality, that is, the maximum violation of locality constraints allowed
by the quantum formalism, irrespective of the dimension of the system. The
value is found to be the same as the one achievable already with two-qubit
systems, and we show that only a very specific class of states can lead to such
maximal value, thus highlighting Hardy's test as a device-independent self-test
protocol for such states. By considering realistic constraints in Hardy's test,
we also compute device-independent upper bounds on this violation and show that
these bounds are saturated by two-qubit systems, thus showing that there is no
advantage in using higher-dimensional systems in experimental implementations
of such test.Comment: 4 pages, 2 figure
Experimental Falsification of Leggett's Non-Local Variable Model
Bell's theorem guarantees that no model based on local variables can
reproduce quantum correlations. Also some models based on non-local variables,
if subject to apparently "reasonable" constraints, may fail to reproduce
quantum physics. In this paper, we introduce a family of inequalities, which
allow testing Leggett's non-local model versus quantum physics, and which can
be tested in an experiment without additional assumptions. Our experimental
data falsify Leggett's model and are in agreement with quantum predictions.Comment: 5 pages, 3 figures, 1 tabl
Nonlocality tests enhanced by a third observer
We consider Bell tests involving bipartite states shared between three
parties. We show that the simple inclusion of a third part may greatly simplify
the measurement scenario (in terms of the number of measurement settings per
part) and allows the identification of previously unknown nonlocal resources.Comment: 4 pags, 4 figs. v2: fig 3 corrected. V3: refs corrected. Close to
published versio
Vulnerability of Temperate Mesophotic Ecosystems (TMEs) to environmental impacts: Rapid ecosystem changes at Lough Hyne Marine Nature Reserve, Ireland
Quantum Communication between N partners and Bell's inequalities
We consider a family of quantum communication protocols involving
partners. We demonstrate the existence of a link between the security of these
protocols against individual attacks by the eavesdropper, and the violation of
some Bell's inequalities, generalizing the link that was noticed some years ago
for two-partners quantum cryptography. The arguments are independent of the
local hidden variable debate.Comment: 4 pages, 2 figure
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