627 research outputs found
Low Timing Jitter Detector for Gigahertz Quantum Key Distribution
A superconducting single-photon detector based on a niobium nitride nanowire
is demonstrated in an optical-fibre-based quantum key distribution test bed
operating at a clock rate of 3.3 GHz and a transmission wavelength of 850 nm.
The low jitter of the detector leads to significant reduction in the estimated
quantum bit error rate and a resultant improvement in the secrecy efficiency
compared to previous estimates made by use of silicon single-photon avalanche
detectors.Comment: 11 pages, including 2 figure
Entanglement can increase asymptotic rates of zero-error classical communication over classical channels
It is known that the number of different classical messages which can be
communicated with a single use of a classical channel with zero probability of
decoding error can sometimes be increased by using entanglement shared between
sender and receiver. It has been an open question to determine whether
entanglement can ever increase the zero-error communication rates achievable in
the limit of many channel uses. In this paper we show, by explicit examples,
that entanglement can indeed increase asymptotic zero-error capacity, even to
the extent that it is equal to the normal capacity of the channel.
Interestingly, our examples are based on the exceptional simple root systems E7
and E8.Comment: 14 pages, 2 figur
Weak force detection with superposed coherent states
We investigate the utility of non classical states of simple harmonic
oscillators, particularly a superposition of coherent states, for sensitive
force detection. We find that like squeezed states a superposition of coherent
states allows displacement measurements at the Heisenberg limit. Entangling
many superpositions of coherent states offers a significant advantage over a
single mode superposition states with the same mean photon number.Comment: 6 pages, no figures: New section added on entangled resources.
Changes to discussions and conclusio
Evolution of cosmic string configurations
We extend and develop our previous work on the evolution of a network of
cosmic strings. The new treatment is based on an analysis of the probability
distribution of the end-to-end distance of a randomly chosen segment of
left-moving string of given length. The description involves three distinct
length scales: , related to the overall string density, , the
persistence length along the string, and , describing the small-scale
structure, which is an important feature of the numerical simulations that have
been done of this problem. An evolution equation is derived describing how the
distribution develops in time due to the combined effects of the universal
expansion, of intercommuting and loop formation, and of gravitational
radiation. With plausible assumptions about the unknown parameters in the
model, we confirm the conclusions of our previous study, that if gravitational
radiation and small-scale structure effects are neglected, the two dominant
length scales both scale in proportion to the horizon size. When the extra
effects are included, we find that while and grow,
initially does not. Eventually, however, it does appear to scale, at a much
lower level, due to the effects of gravitational back-reaction.Comment: 61 pages, requires RevTex v3.0, SUSSEX-TH-93/3-4,
IMPERIAL/TP/92-93/4
Unconventional Cosmology
I review two cosmological paradigms which are alternative to the current
inflationary scenario. The first alternative is the "matter bounce", a
non-singular bouncing cosmology with a matter-dominated phase of contraction.
The second is an "emergent" scenario, which can be implemented in the context
of "string gas cosmology". I will compare these scenarios with the inflationary
one and demonstrate that all three lead to an approximately scale-invariant
spectrum of cosmological perturbations.Comment: 45 pages, 10 figures; invited lectures at the 6th Aegean Summer
School "Quantum Gravity and Quantum Cosmology", Chora, Naxos, Greece, Sept.
12 - 17 2012, to be publ. in the proceedings; these lecture notes form an
updated version of arXiv:1003.1745 and arXiv:1103.227
Cosmic String Cusps with Small-Scale Structure: Their Forms and Gravitational Waveforms
We present a method for the introduction of small-scale structure into
strings constructed from products of rotation matrices. We use this method to
illustrate a range of possibilities for the shape of cusps that depends on the
properties of the small-scale structure. We further argue that the presence of
structure at cusps under most circumstances leads to the formation of loops at
the size of the smallest scales. On the other hand we show that the
gravitational waveform of a cusp remains generally unchanged; the primary
effect of small-scale structure is to smooth out the sharp waveform emitted in
the direction of cusp motion.Comment: RevTeX, 8 pages. Replaced with version accepted for publication by
PR
Looking Beyond Inflationary Cosmology
In spite of the phenomenological successes of the inflationary universe
scenario, the current realizations of inflation making use of scalar fields
lead to serious conceptual problems which are reviewed in this lecture. String
theory may provide an avenue towards addressing these problems. One particular
approach to combining string theory and cosmology is String Gas Cosmology. The
basic principles of this approach are summarized.Comment: invited talk at "Theory Canada 1" (Univ. of British Columbia,
Vancouver, Canada, June 2 - 4, 2005) (references updated
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