54,041 research outputs found
Amplitude control of quantum interference
Usually, the oscillations of interference effects are controlled by relative
phases. We show that varying the amplitudes of quantum waves, for instance by
changing the reflectivity of beam splitters, can also lead to quantum
oscillations and even to Bell violations of local realism. We first study
theoretically a generalization of the Hong-Ou-Mandel experiment to arbitrary
source numbers and beam splitter transmittivity. We then consider a Bell type
experiment with two independent sources, and find strong violations of local
realism for arbitrarily large source number ; for small , one operator
measures essentially the relative phase of the sources and the other their
intensities. Since, experimentally, one can measure the parity of the number of
atoms in an optical lattice more easily than the number itself, we assume that
the detectors measure parity.Comment: 4 pages; 4 figure
Sudden Critical Current Drops Induced in S/F Structures
In the search for new physical properties of S/F structures, we have found
that the superconductor critical current can be controlled by the domain state
of the neighboring ferromagnet. The superconductor is a thin wire of thickness
d_{s} ~ 2 xi_{S}. Nb/Co and Nb/Py (Permalloy Ni_{80}Fe_{20}) bilayer structures
were grown with a significant magnetic anisotropy. Critical current
measurements of Nb/Co structures with ferromagnet thickness d_{F} > 30nm show
sudden drops in two very defined steps when the measurements are made along the
hard axes direction (i.e. current track parallel to hard anisotropy axes
direction). These drops disappear when they are made along the easy axis
direction or when the ferromagnet thickness is below 30nm. The drops are
accompanied by vortex flux flow. In addition magnetorestistance measurements
close to Tc show a sharp increase near saturation fields of the ferromagnet.
Similar results are reproduced in Nb/Py bilayer structure with the ferromagnet
thickness d_{F} ~ 50nm along the easy anisotropy axes. These results are
explained as being due to spontaneous vortex formation and flow induced by
Bloch domain walls of the ferromagnet underneath. We argue these Bloch domain
walls produce a 2D vortex-antivortex lattice structure.Comment: 6 pages, 6 figure
Class of bipartite quantum states satisfying the original Bell inequality
In a general setting, we introduce a new bipartite state property sufficient
for the validity of the perfect correlation form of the original Bell
inequality for any three bounded quantum observables. A bipartite quantum state
with this property does not necessarily exhibit perfect correlations. The class
of bipartite states specified by this property includes both separable and
nonseparable states. We prove analytically that, for any dimension d>2, every
Werner state, separable or nonseparable, belongs to this class.Comment: 6 pages, v.2: one reference added, the statement on Werner states
essentially extended; v.3: details of proofs inserte
Bell's theorem as a signature of nonlocality: a classical counterexample
For a system composed of two particles Bell's theorem asserts that averages
of physical quantities determined from local variables must conform to a family
of inequalities. In this work we show that a classical model containing a local
probabilistic interaction in the measurement process can lead to a violation of
the Bell inequalities. We first introduce two-particle phase-space
distributions in classical mechanics constructed to be the analogs of quantum
mechanical angular momentum eigenstates. These distributions are then employed
in four schemes characterized by different types of detectors measuring the
angular momenta. When the model includes an interaction between the detector
and the measured particle leading to ensemble dependencies, the relevant Bell
inequalities are violated if total angular momentum is required to be
conserved. The violation is explained by identifying assumptions made in the
derivation of Bell's theorem that are not fulfilled by the model. These
assumptions will be argued to be too restrictive to see in the violation of the
Bell inequalities a faithful signature of nonlocality.Comment: Extended manuscript. Significant change
Bounds for Non-Locality Distillation Protocols
Non-locality can be quantified by the violation of a Bell inequality. Since
this violation may be amplified by local operations an alternative measure has
been proposed - distillable non-locality. The alternative measure is difficult
to calculate exactly due to the double exponential growth of the parameter
space. In this article we give a way to bound the distillable non-locality of a
resource by the solutions to a related optimization problem. Our upper bounds
are exponentially easier to compute than the exact value and are shown to be
meaningful in general and tight in some cases.Comment: 8 pages, 3 figures; small changes in introduction and application
section due to the exact verification of distillation bounds using a symbolic
computation package (Maple 14); added journal re
The calculated effect of trailing-edge flaps on the take-off of flying boats
The results of take-off calculations are given for an application of simple trailing-edge flaps to two hypothetical flying boats, one having medium wing and power loading and consequently considerable excess of thrust over total resistance during the take-off run, the other having high wing and power loading and a very low excess thrust. For these seaplanes the effect of downward flap settings was: (1) to increase the total resistance below the stalling speed, (2) to decrease the get-away speed, (3) to improve the take-off performance of the seaplane having considerable excess thrust, and (4) to hinder the take-off of the seaplane having low excess thrust. It is indicated that flaps would allow a decrease in the high angles of wing setting necessary with most seaplanes, provided that the excess thrust is not too low
Threshold bounds for noisy bipartite states
For a nonseparable bipartite quantum state violating the
Clauser-Horne-Shimony-Holt (CHSH) inequality, we evaluate amounts of noise
breaking the quantum character of its statistical correlations under any
generalized quantum measurements of Alice and Bob. Expressed in terms of the
reduced states, these new threshold bounds can be easily calculated for any
concrete bipartite state. A noisy bipartite state, satisfying the extended CHSH
inequality and the perfect correlation form of the original Bell inequality for
any quantum observables, neither necessarily admits a local hidden variable
model nor exhibits the perfect correlation of outcomes whenever the same
quantum observable is measured on both "sides".Comment: 9 pages; v.2: minor editing corrections; to appear in J. Phys. A:
Math. Ge
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
Unified criteria for multipartite quantum nonlocality
Wiseman and co-workers (Phys. Rev. Lett. 98, 140402, 2007) proposed a
distinction between the nonlocality classes of Bell's nonlocality, steering and
entanglement based on whether or not an overseer trusts each party in a
bipartite scenario where they are asked to demonstrate entanglement. Here we
extend that concept to the multipartite case and derive inequalities that
progressively test for those classes of nonlocality, with different thresholds
for each level. This framework includes the three classes of nonlocality above
in special cases and introduces a family of others.Comment: V2: corrected image display; V3: substantial changes including new
proofs, arguments, and result
The bimodal spiral galaxy surface brightness distribution
We have assessed the significance of Tully and Verheijen's (1997) bimodal
Ursa Major Cluster spiral galaxy near-infrared surface brightness distribution,
focussing on whether this bimodality is simply an artifact of small number
statistics. A Kolmogorov-Smirnov style of significance test shows that the
total distribution is fairly represented by a single-peaked distribution, but
that their isolated galaxy subsample (with no significant neighbours within a
projected distance of around 80 kpc) is bimodal at the 96 per cent level. We
have also investigated the assumptions underlying the isolated galaxy surface
brightness distribution, finding that the (often large) inclination corrections
used in the construction of this distribution reduce the significance of the
bimodality. We conclude that the Ursa Major Cluster dataset is insufficient to
establish the presence of a bimodal near-infrared surface brightness
distribution: an independent sample of around 100 isolated, low inclination
galaxies is required to establish bimodality at the 99 per cent level.Comment: 5 pages LaTeX; 2 embedded figures; re-submitted to MNRAS after
replying to referee's comment
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