1,972 research outputs found
Ladder proof of nonlocality for two spin-half particles revisited
In this paper we extend the ladder proof of nonlocality without inequalities
for two spin-half particles given by Boschi et al [PRL 79, 2755 (1997)] to the
case in which the measurement settings of the apparatus measuring one of the
particles are different from the measurement settings of the apparatus
measuring the other particle. It is shown that, in any case, the proportion of
particle pairs for which the contradiction with local realism goes through is
maximized when the measurement settings are the same for each apparatus. Also
we write down a Bell inequality for the experiment in question which is
violated by quantum mechanics by an amount which is twice as much as the amount
by which quantum mechanics violates the Bell inequality considered in the above
paper by Boschi et al.Comment: LaTeX, 7 pages, 1 figure, journal versio
A feasible quantum optical experiment capable of refuting noncontextuality for single photons
Elaborating on a previous work by Simon et al. [PRL 85, 1783 (2000)] we
propose a realizable quantum optical single-photon experiment using standard
present day technology, capable of discriminating maximally between the
predictions of quantum mechanics (QM) and noncontextual hidden variable
theories (NCHV). Quantum mechanics predicts a gross violation (up to a factor
of 2) of the noncontextual Bell-like inequality associated with the proposed
experiment. An actual maximal violation of this inequality would demonstrate
(modulo fair sampling) an all-or-nothing type contradiction between QM and
NCHV.Comment: LaTeX file, 8 pages, 1 figur
Preparation of n-qubit Greenberger-Horne-Zeilinger entangled states in cavity QED: An approach with tolerance to nonidentical qubit-cavity coupling constants
We propose a way for generating -qubit Greenberger-Horne-Zeilinger (GHZ)
entangled states with a three-level qubit system and (n-1) four-level qubit
systems in a cavity. This proposal does not require identical qubit-cavity
coupling constants, and thus is tolerant to qubit-system parameter
nonuniformity and nonexact placement of qubits in a cavity. The proposal does
not require adjustment of the qubit-system level spacings during the entire
operation. Moreover, it is shown that entanglement can be deterministically
generated using this method and the operation time is independent of the number
of qubits. The present proposal is quite general, which can be applied to
physical systems such as various types of superconducting devices coupled to a
resonator or atoms trapped in a cavity.Comment: 3 figures, accepted by Phys. Rev.
Bell's theorem without inequalities and without alignments
A proof of Bell's theorem without inequalities is presented which exhibits
three remarkable properties: (a) reduced local states are immune to collective
decoherence; (b) distant local setups do not need to be aligned, since the
required perfect correlations are achieved for any local rotation of the local
setups; (c) local measurements require only individual measurements on the
qubits. Indeed, it is shown that this proof is essentially the only one which
fulfils (a), (b), and (c).Comment: REVTeX4, 4 page
Comment on "Bell's Theorem without Inequalities and without Probabilities for Two Observers"
In this Comment we show that Cabello's argument [Phys. Rev. Lett. 86, 1911
(2001)] which proves the nonlocal feature of any classical model of quantum
mechanics based on Einstein-Podolsky-Rosen (EPR) criterion of elements of
reality, must involve at least four distant observers rather than the two
employed by the author. Moreover we raise a remark on the necessity of
performing a real experiment confirming Cabello's argument.Comment: 1 page, REVTex4 fil
Conditions for the confirmation of three-particle non-locality
The notion of genuine three-particle non-locality introduced by Svetlichny
\cite{Svetlichny} is discussed. Svetlichny's inequality which can distinguish
between genuine three-particle non-locality and two-particle non-locality is
analyzed by reinterpreting it as a frustrated network of correlations. Its
quantum mechanical maximum violation is derived and a situation is presented
that produces the maximum violation. It is shown that the measurements
performed in recent experiments to demonstrate GHZ entanglement
\cite{Bouwmeester}, \cite{Pan} do not allow this inequality to be violated, and
hence can not be taken as confirmation of genuine three-particle non-locality.
Modifications to the experiments that would make such a confirmation possible
are discussed.Comment: minor revisions, references adde
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
Robust and Scalable Scheme to Generate Large-Scale Entanglement Webs
We propose a robust and scalable scheme to generate an -qubit state
among separated quantum nodes (cavity-QED systems) by using linear optics and
postselections. The present scheme inherits the robustness of the Barrett-Kok
scheme [Phys. Rev. A {\bf 71}, 060310(R) (2005)]. The scalability is also
ensured in the sense that an arbitrarily large -qubit state can be
generated with a quasi-polynomial overhead . The
process to breed the states, which we introduce to achieve the scalability,
is quite simple and efficient, and can be applied for other physical systems.Comment: 5 pages, 3 figure
Testing the Structure of Multipartite Entanglement with Bell Inequalities
We show that the rich structure of multipartite entanglement can be tested
following a device-independent approach. Specifically we present Bell
inequalities for distinguishing between different types of multipartite
entanglement, without placing any assumptions on the measurement devices used
in the protocol, in contrast with usual entanglement witnesses. We first
address the case of three qubits and present Bell inequalities that can be
violated by W states but not by GHZ states, and vice versa. Next, we devise
'sub-correlation Bell inequalities' for any number of parties, which can
provably not be violated by a broad class of multipartite entangled states
(generalizations of GHZ states), but for which violations can be obtained for W
states. Our results give insight into the nonlocality of W states. The
simplicity and robustness of our tests make them appealing for experiments.Comment: 7 page
Package of facts and theorems for efficiently generating entanglement criteria for many qubits
We present a package of mathematical theorems, which allow to construct
multipartite entanglement criteria. Importantly, establishing bounds for
certain classes of entanglement does not take an optimization over continuous
sets of states. These bonds are found from the properties of commutativity
graphs of operators used in the criterion. We present two examples of criteria
constructed according to our method. One of them detects genuine 5-qubit
entanglement without ever referring to correlations between all five qubits.Comment: 5 pages, 4 figure
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