52,487 research outputs found
Constructing N-qubit entanglement monotones from anti-linear operators
We present a method to construct entanglement measures for pure states of
multipartite qubit systems. The key element of our approach is an antilinear
operator that we call {\em comb} in reference to the {\em hairy-ball theorem}.
For qubits (or spin 1/2) the combs are automatically invariant under
SL(2,\CC). This implies that the {\em filters} obtained from the combs are
entanglement monotones by construction. We give alternative formulae for the
concurrence and the 3-tangle as expectation values of certain antilinear
operators. As an application we discuss inequivalent types of genuine
four-qubit entanglement.Comment: 5 pages, revtex4; more detailed illustration of the metho
Faithful test of non-local realism with entangled coherent states
We investigate the violation of Leggett's inequality for non-local realism
using entangled coherent states and various types of local measurements. We
prove mathematically the relation between the violation of the
Clauser-Horne-Shimony-Holt form of Bell's inequality and Leggett's one when
tested by the same resources. For Leggett inequalities, we generalize the
non-local realistic bound to systems in Hilbert spaces larger than
bidimensional ones and introduce an optimization technique that allows to
achieve larger degrees of violation by adjusting the local measurement
settings. Our work describes the steps that should be performed to produce a
self-consistent generalization of Leggett's original arguments to
continuous-variable states.Comment: 8 pages, 6 figures, to be published in Phys. Rev.
Causal Quantum Theory and the Collapse Locality Loophole
Causal quantum theory is an umbrella term for ordinary quantum theory
modified by two hypotheses: state vector reduction is a well-defined process,
and strict local causality applies. The first of these holds in some versions
of Copenhagen quantum theory and need not necessarily imply practically
testable deviations from ordinary quantum theory. The second implies that
measurement events which are spacelike separated have no non-local
correlations. To test this prediction, which sharply differs from standard
quantum theory, requires a precise theory of state vector reduction.
Formally speaking, any precise version of causal quantum theory defines a
local hidden variable theory. However, causal quantum theory is most naturally
seen as a variant of standard quantum theory. For that reason it seems a more
serious rival to standard quantum theory than local hidden variable models
relying on the locality or detector efficiency loopholes.
Some plausible versions of causal quantum theory are not refuted by any Bell
experiments to date, nor is it obvious that they are inconsistent with other
experiments. They evade refutation via a neglected loophole in Bell experiments
-- the {\it collapse locality loophole} -- which exists because of the possible
time lag between a particle entering a measuring device and a collapse taking
place. Fairly definitive tests of causal versus standard quantum theory could
be made by observing entangled particles separated by light
seconds.Comment: Discussion expanded; typos corrected; references adde
Bell's Jump Process in Discrete Time
The jump process introduced by J. S. Bell in 1986, for defining a quantum
field theory without observers, presupposes that space is discrete whereas time
is continuous. In this letter, our interest is to find an analogous process in
discrete time. We argue that a genuine analog does not exist, but provide
examples of processes in discrete time that could be used as a replacement.Comment: 7 pages LaTeX, no figure
On Fast Linear Gravitational Dragging
A new formula is given for the fast linear gravitational dragging of the
inertial frame within a rapidly accelerated spherical shell of deep potential.
The shell is charged and is electrically accelerated by an electric field whose
sources are included in the solution.Comment: 4 pages, 1 figur
Super-activation of quantum non-locality
In this paper we show that quantum non-locality can be super-activated. That
is, one can obtain violations of Bell inequalities by tensorizing a local state
with itself. Moreover, previous results suggest that such Bell violations can
be very large.Comment: v2: Refs added. Same results, v3: Minor corrections. Close to the
published versio
Two-Photon Beatings Using Biphotons Generated from a Two-Level System
We propose a two-photon beating experiment based upon biphotons generated
from a resonant pumping two-level system operating in a backward geometry. On
the one hand, the linear optical-response leads biphotons produced from two
sidebands in the Mollow triplet to propagate with tunable refractive indices,
while the central-component propagates with unity refractive index. The
relative phase difference due to different refractive indices is analogous to
the pathway-length difference between long-long and short-short in the original
Franson interferometer. By subtracting the linear Rayleigh scattering of the
pump, the visibility in the center part of the two-photon beating interference
can be ideally manipulated among [0, 100%] by varying the pump power, the
material length, and the atomic density, which indicates a Bell-type inequality
violation. On the other hand, the proposed experiment may be an interesting way
of probing the quantum nature of the detection process. The interference will
disappear when the separation of the Mollow peaks approaches the fundamental
timescales for photon absorption in the detector.Comment: to appear in Phys. Rev. A (2008
No Signalling and Quantum Key Distribution
Standard quantum key distribution protocols are provably secure against
eavesdropping attacks, if quantum theory is correct. It is theoretically
interesting to know if we need to assume the validity of quantum theory to
prove the security of quantum key distribution, or whether its security can be
based on other physical principles. The question would also be of practical
interest if quantum mechanics were ever to fail in some regime, because a
scientifically and technologically advanced eavesdropper could perhaps use
post-quantum physics to extract information from quantum communications without
necessarily causing the quantum state disturbances on which existing security
proofs rely. Here we describe a key distribution scheme provably secure against
general attacks by a post-quantum eavesdropper who is limited only by the
impossibility of superluminal signalling. The security of the scheme stems from
violation of a Bell inequality.Comment: Clarifications and minor revisions in response to comments. Final
version; to appear in Phys. Rev. Let
Testing non-local realism with entangled coherent states
We investigate the violation of non-local realism using entangled coherent
states (ECS) under nonlinear operations and homodyne measurements. We address
recently proposed Leggett-type inequalities, including a class of optimized
incompatibility ones and thoroughly assess the effects of detection
inefficiency.Comment: 7 pages, 6 figures, RevTeX4, accepted for publication in Phys. Rev.
Do Rotations Beyond the Cosmological Horizon Affect the Local Inertial Frame?
If perturbations beyond the horizon have the velocities prescribed everywhere
then the dragging of inertial frames near the origin is suppressed by an
exponential factor. However if perturbations are prescribed in terms of their
angular momenta there is no such suppression.
We resolve this paradox and in doing so give new explicit results on the
dragging of inertial frames in closed, flat and open universe with and without
a cosmological constant.Comment: 12 page
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