1,698 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 $n$-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

### 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

### 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

### Useful entanglement can be extracted from all nonseparable states

We consider entanglement distillation from a single-copy of a multipartite
state, and instead of rates we analyze the "quality" of the distilled
entanglement. This "quality" is quantified by the fidelity with the GHZ-state.
We show that each not fully-separable state $\sigma$ can increase the "quality"
of the entanglement distilled from other states, no matter how weakly entangled
is $\sigma$. We also generalize this to the case where the goal is distilling
states different than the GHZ. These results provide new insights on the
geometry of the set of separable states and its dual (the set of entanglement
witnesses).Comment: 7 page

### Creating maximally entangled atomic states in a Bose-Einstein condensate

We propose a protocol to create maximally entangled pairs, triplets,
quartiles, and other clusters of Bose condensed atoms starting from a
condensate in the Mott insulator state. The essential element is to drive
single atom Raman transitions using laser pulses. Our scheme is simple,
efficient, and can be readily applied to the recent experimental system as
reported by Greiner {\it et al.} [ Nature {\bf 413}, 44 (2002)].Comment: 4 pages, 2 figures. revised version as to be publishe

### Teleportation with a Mixed State of Four Qubits and the Generalized Singlet Fraction

Recently, an explicit protocol ${\cal E}_0$ for faithfully teleporting
arbitrary two-qubit states using genuine four-qubit entangled states was
presented by us [Phys. Rev. Lett. {\bf 96}, 060502 (2006)]. Here, we show that
${\cal E}_0$ with an arbitrary four-qubit mixed state resource $\Xi$ is
equivalent to a generalized depolarizing bichannel with probabilities given by
the maximally entangled components of the resource. These are defined in terms
of our four-qubit entangled states. We define the generalized singlet fraction
${\cal G}[\Xi]$, and illustrate its physical significance with several
examples. We argue that in order to teleport arbitrary two-qubit states with
average fidelity better than is classically possible, we have to demand that
${\cal G}[\Xi] > 1/2$. In addition, we conjecture that when ${\cal G}[\Xi] <
1/4$ then no entanglement can be teleported. It is shown that to determine the
usefulness of $\Xi$ for ${\cal E}_0$, it is necessary to analyze ${\cal
G}[\Xi]$.Comment: 11 page

### Nested entangled states for distributed quantum channels

We find a coupling-strength configuration for a linear chain of N spins which
gives rise to simultaneous multiple Bell states. We suggest a way such an
interesting entanglement pattern can be used in order to distribute maximally
entangled channels to remote locations and generate multipartite entanglement
with a minimum-control approach. Our proposal thus provides a way to achieve
the core resources in distributed information processing. The schemes we
describe can be efficiently tested in chains of coupled cavities interacting
with three-level atoms.Comment: 4 pages, 2 figures, RevTeX

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