2,395 research outputs found
Lorentz invariant intrinsic decoherence
Quantum decoherence can arise due to classical fluctuations in the parameters
which define the dynamics of the system. In this case decoherence, and
complementary noise, is manifest when data from repeated measurement trials are
combined. Recently a number of authors have suggested that fluctuations in the
space-time metric arising from quantum gravity effects would correspond to a
source of intrinsic noise, which would necessarily be accompanied by intrinsic
decoherence. This work extends a previous heuristic modification of
Schr\"{o}dinger dynamics based on discrete time intervals with an intrinsic
uncertainty. The extension uses unital semigroup representations of space and
time translations rather than the more usual unitary representation, and does
the least violence to physically important invariance principles. Physical
consequences include a modification of the uncertainty principle and a
modification of field dispersion relations, in a way consistent with other
modifications suggested by quantum gravity and string theory .Comment: This paper generalises an earlier model published as Phys. Rev. A
vol44, 5401 (1991
The apparent roughness of a sand surface blown by wind from an analytical model of saltation
We present an analytical model of aeolian sand transport. The model
quantifies the momentum transfer from the wind to the transported sand by
providing expressions for the thickness of the saltation layer and the apparent
surface roughness. These expressions are derived from basic physical principles
and a small number of assumptions. The model further predicts the sand
transport rate (mass flux) and the impact threshold (the smallest value of the
wind shear velocity at which saltation can be sustained). We show that, in
contrast to previous studies, the present model's predictions are in very good
agreement with a range of experiments, as well as with numerical simulations of
aeolian saltation. Because of its physical basis, we anticipate that our model
will find application in studies of aeolian sand transport on both Earth and
Mars
Quantum repeaters and quantum key distribution: analysis of secret key rates
We analyze various prominent quantum repeater protocols in the context of
long-distance quantum key distribution. These protocols are the original
quantum repeater proposal by Briegel, D\"ur, Cirac and Zoller, the so-called
hybrid quantum repeater using optical coherent states dispersively interacting
with atomic spin qubits, and the Duan-Lukin-Cirac-Zoller-type repeater using
atomic ensembles together with linear optics and, in its most recent extension,
heralded qubit amplifiers. For our analysis, we investigate the most important
experimental parameters of every repeater component and find their minimally
required values for obtaining a nonzero secret key. Additionally, we examine in
detail the impact of device imperfections on the final secret key rate and on
the optimal number of rounds of distillation when the entangled states are
purified right after their initial distribution.Comment: Published versio
Reduced Deadtime and Higher Rate Photon-Counting Detection using a Multiplexed Detector Array
We present a scheme for a photon-counting detection system that can be
operated at incident photon rates higher than otherwise possible by suppressing
the effects of detector deadtime. The method uses an array of N detectors and a
1-by-N optical switch with a control circuit to direct input light to live
detectors. Our calculations and models highlight the advantages of the
technique. In particular, using this scheme, a group of N detectors provides an
improvement in operation rate that can exceed the improvement that would be
obtained by a single detector with deadtime reduced by 1/N, even if it were
feasible to produce a single detector with such a large improvement in
deadtime. We model the system for continuous and pulsed light sources, both of
which are important for quantum metrology and quantum key distribution
applications.Comment: 6 figure
Influence of the bias voltage on the structure and the tribological performance of nanoscale multilayer C/Cr PVD coatings
Nanoscale multilayer C/Cr coatings have been deposited by utilising the combined steered cathodic arc/unbalanced magnetron sputtering technique. The coating microstructure and tribological performance have been investigated as a function of the bias voltage, ranging from U-b=-65 to -350 V. The XRD results revealed that C/Cr coatings are amorphous at low U-b, but became more crystalline when the Ub increased to -350 V. High-resolution XTEM analysis indicated coating densification and smoothening as well as formation of novel amorphous nanostructure, in which carbon-rich clusters are surrounded by a Cr-rich matrix, leading to the formation of self-organised multilayer structure as the bias voltage was increased from -65 to -350 V. An increase of the bias voltage from -65 to -350 V resulted in an increase in the hardness from 8 to 25 CiPa and Young's modulus, E from 186 to 319 GPa. A pin-on-disc test showed that the friction coefficient was reduced from 0.22 to 0.16 when the bias voltage was increased from -65 to -95 V However, a further increase in the bias voltage to -350 V led to an increase in the friction coefficient to 0.31. The lowest wear coefficient K(c)similar to6.25 x 10(-17) m(3) N-1 m(-1) was achieved at U-b = - 120 V. Standard HSS drills, 8 mm in diameter, coated with C/Cr have been tested using solution annealed AISI 304 stainless steel as the work piece material. An improvement of the lifetime by a factor of similar to9 has been achieved as compared to the uncoated tools. In this test, the C/Cr coating outperformed a number of commercially available PVD coatings, such as TiCN, TiAlCrN and showed similar performance to TiAlCrYN. (C) 2004 Elsevier B.V. All rights reserved
Nano-displacement measurements using spatially multimode squeezed light
We demonstrate the possibility of surpassing the quantum noise limit for
simultaneous multi-axis spatial displacement measurements that have zero mean
values. The requisite resources for these measurements are squeezed light beams
with exotic transverse mode profiles. We show that, in principle, lossless
combination of these modes can be achieved using the non-degenerate Gouy phase
shift of optical resonators. When the combined squeezed beams are measured with
quadrant detectors, we experimentally demonstrate a simultaneous reduction in
the transverse x- and y- displacement fluctuations of 2.2 dB and 3.1 dB below
the quantum noise limit.Comment: 21 pages, 9 figures, submitted to "Special Issue on Fluctuations &
Noise in Photonics & Quantum Optics" of J. Opt.
There is no unmet requirement of optical coherence for continuous-variable quantum teleportation
It has been argued [T. Rudolph and B.C. Sanders, Phys. Rev. Lett. 87, 077903
(2001)] that continuous-variable quantum teleportation at optical frequencies
has not been achieved because the source used (a laser) was not `truly
coherent'. Here I show that `true coherence' is always illusory, as the concept
of absolute time on a scale beyond direct human experience is meaningless. A
laser is as good a clock as any other, even in principle, and this objection to
teleportation experiments is baseless.Comment: 6 pages, no figures, no equations, to be published in Journal of
Modern Optics. This is a long version of quant-ph/0104004. I have not
replaced that paper with this one because some authors have referenced that
one approvingly who may feel differently about doing so to this versio
Theory of monolayers with boundaries: Exact results and Perturbative analysis
Domains and bubbles in tilted phases of Langmuir monolayers contain a class
of textures knows as boojums. The boundaries of such domains and bubbles may
display either cusp-like features or indentations. We derive analytic
expressions for the textures within domains and surrounding bubbles, and for
the shapes of the boundaries of these regions. The derivation is perturbative
in the deviation of the bounding curve from a circle. This method is not
expected to be accurate when the boundary suffers large distortions, but it
does provide important clues with regard to the influence of various energetic
terms on the order-parameter texture and the shape of the domain or bubble
bounding curve. We also look into the effects of thermal fluctuations, which
include a sample-size-dependent effective line tension.Comment: replaced with published version, 21 pages, 16 figures include
Universal continuous-variable quantum computation: Requirement of optical nonlinearity for photon counting
Although universal continuous-variable quantum computation cannot be achieved
via linear optics (including squeezing), homodyne detection and feed-forward,
inclusion of ideal photon counting measurements overcomes this obstacle. These
measurements are sometimes described by arrays of beam splitters to distribute
the photons across several modes. We show that such a scheme cannot be used to
implement ideal photon counting and that such measurements necessarily involve
nonlinear evolution. However, this requirement of nonlinearity can be moved
"off-line," thereby permitting universal continuous-variable quantum
computation with linear optics.Comment: 6 pages, no figures, replaced with published versio
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