99 research outputs found
Can one see entanglement ?
The human eye can detect optical signals containing only a few photons. We
investigate the possibility to demonstrate entanglement with such biological
detectors. While one person could not detect entanglement by simply observing
photons, we discuss the possibility for several observers to demonstrate
entanglement in a Bell-type experiment, in which standard detectors are
replaced by human eyes. Using a toy model for biological detectors that
captures their main characteristic, namely a detection threshold, we show that
Bell inequalities can be violated, thus demonstrating entanglement. Remarkably,
when the response function of the detector is close to a step function, quantum
non-locality can be demonstrated without any further assumptions. For smoother
response functions, as for the human eye, post-selection is required.Comment: 5 pages, 5 figure
Multipartite quantum nonlocality under local decoherence
We study the nonlocal properties of two-qubit maximally-entangled and N-qubit
Greenberger-Horne-Zeilinger states under local decoherence. We show that the
(non)resilience of entanglement under local depolarization or dephasing is not
necessarily equivalent to the (non)resilience of Bell-inequality violations.
Apart from entanglement and Bell-inequality violations, we consider also
nonlocality as quantified by the nonlocal content of correlations, and provide
several examples of anomalous behaviors, both in the bipartite and multipartite
cases. In addition, we study the practical implications of these anomalies on
the usefulness of noisy Greenberger-Horne-Zeilinger states as resources for
nonlocality-based physical protocols given by communication complexity
problems. There, we provide examples of quantum gains improving with the number
of particles that coexist with exponentially-decaying entanglement and
non-local contents.Comment: 6 pages, 4 figure
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
Maximal violation of the I3322 inequality using infinite dimensional quantum systems
The I3322 inequality is the simplest bipartite two-outcome Bell inequality
beyond the Clauser-Horne-Shimony-Holt (CHSH) inequality, consisting of three
two-outcome measurements per party. In case of the CHSH inequality the maximal
quantum violation can already be attained with local two-dimensional quantum
systems, however, there is no such evidence for the I3322 inequality. In this
paper a family of measurement operators and states is given which enables us to
attain the largest possible quantum value in an infinite dimensional Hilbert
space. Further, it is conjectured that our construction is optimal in the sense
that measuring finite dimensional quantum systems is not enough to achieve the
true quantum maximum. We also describe an efficient iterative algorithm for
computing quantum maximum of an arbitrary two-outcome Bell inequality in any
given Hilbert space dimension. This algorithm played a key role to obtain our
results for the I3322 inequality, and we also applied it to improve on our
previous results concerning the maximum quantum violation of several bipartite
two-outcome Bell inequalities with up to five settings per party.Comment: 9 pages, 3 figures, 1 tabl
All Entangled Quantum States Are Nonlocal
Departing from the usual paradigm of local operations and classical
communication adopted in entanglement theory, here we study the interconversion
of quantum states by means of local operations and shared randomness. A set of
necessary and sufficient conditions for the existence of such a transformation
between two given quantum states is given in terms of the payoff they yield in
a suitable class of nonlocal games. It is shown that, as a consequence of our
result, such a class of nonlocal games is able to witness quantum entanglement,
however weak, and reveal nonlocality in any entangled quantum state. An example
illustrating this fact is provided.Comment: 4+2 pages. Final version published in PRL. The related APS Physics
Viewpoint can be found at http://dx.doi.org/10.1103/Physics.5.5
On local-hidden-variable no-go theorems
The strongest attack against quantum mechanics came in 1935 in the form of a
paper by Einstein, Podolsky and Rosen. It was argued that the theory of quantum
mechanics could not be called a complete theory of Nature, for every element of
reality is not represented in the formalism as such. The authors then put forth
a proposition: we must search for a theory where, upon knowing everything about
the system, including possible hidden variables, one could make precise
predictions concerning elements of reality. This project was ultimatly doomed
in 1964 with the work of Bell Bell, who showed that the most general local
hidden variable theory could not reproduce correlations that arise in quantum
mechanics. There exist mainly three forms of no-go theorems for local hidden
variable theories. Although almost every physicist knows the consequences of
these no-go theorems, not every physicist is aware of the distinctions between
the three or even their exact definitions. Thus we will discuss here the three
principal forms of no-go theorems for local hidden variable theories of Nature.
We will define Bell inequalities, Bell inequalities without inequalities and
pseudo-telepathy. A discussion of the similarities and differences will follow.Comment: 7 pages, no figure, replaced "Bell inequalities" with "Bell theorems"
and updated the reference
On the logical structure of Bell theorems without inequalities
Bell theorems show how to experimentally falsify local realism. Conclusive
falsification is highly desirable as it would provide support for the most
profoundly counterintuitive feature of quantum theory - nonlocality. Despite
the preponderance of evidence for quantum mechanics, practical limits on
detector efficiency and the difficulty of coordinating space-like separated
measurements have provided loopholes for a classical worldview; these loopholes
have never been simultaneously closed. A number of new experiments have
recently been proposed to close both loopholes at once. We show some of these
novel designs fail in the most basic way, by not ruling out local hidden
variable models, and we provide an explicit classical model to demonstrate
this. They share a common flaw, which reveals a basic misunderstanding of how
nonlocality proofs work. Given the time and resources now being devoted to such
experiments, theoretical clarity is essential. Our explanation is presented in
terms of simple logic and should serve to correct misconceptions and avoid
future mistakes. We also show a nonlocality proof involving four participants
which has interesting theoretical properties.Comment: 8 pages, text clarified, explicit LHV model provided for flawed
nonlocality tes
Quantifying the nonlocality of GHZ quantum correlations by a bounded communication simulation protocol
The simulation of quantum correlations with alternative nonlocal resources,
such as classical communication, gives a natural way to quantify their
nonlocality. While multipartite nonlocal correlations appear to be useful
resources, very little is known on how to simulate multipartite quantum
correlations. We present the first known protocol that reproduces 3-partite GHZ
correlations with bounded communication: 3 bits in total turn out to be
sufficient to simulate all equatorial Von Neumann measurements on the 3-partite
GHZ state.Comment: 7 pages, 1 figur
The local content of all pure two-qubit states
The (non-)local content in the sense of Elitzur, Popescu, and Rohrlich (EPR2)
[Phys. Lett. A 162, 25 (1992)] is a natural measure for the (non-)locality of
quantum states. Its computation is in general difficult, even in low
dimensions, and is one of the few open questions about pure two-qubit states.
We present a complete solution to this long-lasting problem.Comment: 9 pages, 3 figure
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