14,528 research outputs found
The Exact Ground State of the Frenkel-Kontorova Model with Repeated Parabolic Potential: II. Numerical Treatment
A procedure is described for efficiently finding the ground state energy and
configuration for a Frenkel-Kontorova model in a periodic potential, consisting
of N parabolic segments of identical curvature in each period, through a
numerical solution of the convex minimization problem described in the
preceding paper. The key elements are the use of subdifferentials to describe
the structure of the minimization problem; an intuitive picture of how to solve
it, based on motion of quasiparticles; and a fast linear optimization method
with a reduced memory requirement. The procedure has been tested for N up to
200.Comment: 9 RevTeX pages, using AMS-Fonts (amssym.tex,amssym.def), 3 Postscript
figures, accepted by Phys.Rev.B to be published together with
cond-mat/970722
Optimal Eavesdropping in Quantum Cryptography. II. Quantum Circuit
It is shown that the optimum strategy of the eavesdropper, as described in
the preceding paper, can be expressed in terms of a quantum circuit in a way
which makes it obvious why certain parameters take on particular values, and
why obtaining information in one basis gives rise to noise in the conjugate
basis.Comment: 7 pages, 1 figure, Latex, the second part of quant-ph/970103
Two qubit copying machine for economical quantum eavesdropping
We study the mapping which occurs when a single qubit in an arbitrary state
interacts with another qubit in a given, fixed state resulting in some unitary
transformation on the two qubit system which, in effect, makes two copies of
the first qubit. The general problem of the quality of the resulting copies is
discussed using a special representation, a generalization of the usual Schmidt
decomposition, of an arbitrary two-dimensional subspace of a tensor product of
two 2-dimensional Hilbert spaces. We exhibit quantum circuits which can
reproduce the results of any two qubit copying machine of this type. A simple
stochastic generalization (using a ``classical'' random signal) of the copying
machine is also considered. These copying machines provide simple embodiments
of previously proposed optimal eavesdropping schemes for the BB84 and B92
quantum cryptography protocols.Comment: Minor changes. 26 pages RevTex including 7 PS figure
Ultra-fine beryllium powder by amalgam process Progress report, period ending 31 Oct. 1966
Metallurgical evaluation of beryllium powdered metal, and electron microscope studies of agglomerate particle size
Constraints on the distance to SGR 1806-20 from HI absorption
The giant flare detected from the magnetar SGR 1806-20 on 2004 December 27
had a fluence more than 100 times higher than the only two other SGR flares
ever recorded. Whereas the fluence is independent of distance, an estimate for
the luminosity of the burst depends on the source's distance, which has
previously been argued to be ~15 kpc. The burst produced a bright radio
afterglow, against which Cameron et al. (2005) have measured an HI absorption
spectrum. This has been used to propose a revised distance to SGR 1806-20 of
between 6.4 and 9.8 kpc. Here we analyze this absorption spectrum, and compare
it both to HI emission data from the Southern Galactic Plane Survey and to
archival 12-CO survey data. We confirm ~6 kpc, as a likely lower limit on the
distance to SGR 1806-20, but argue that it is difficult to place an upper limit
on the distance to SGR 1806-20 from the HI data currently available. The
previous value of ~15 kpc thus remains the best estimate of the distance to the
source.Comment: 3 pages, 1 embedded EPS figure. Added sentences to end of Abstract
and Conclusion, clarifying that most likely distance is 15 kpc. ApJ Letters,
in pres
Trapped Ion Imaging with a High Numerical Aperture Spherical Mirror
Efficient collection and analysis of trapped ion qubit fluorescence is
essential for robust qubit state detection in trapped ion quantum computing
schemes. We discuss simple techniques of improving photon collection efficiency
using high numerical aperture (N.A.) reflective optics. To test these
techniques we placed a spherical mirror with an effective N.A. of about 0.9
inside a vacuum chamber in the vicinity of a linear Paul trap. We demonstrate
stable and reliable trapping of single barium ions, in excellent agreement with
our simulations of the electric field in this setup. While a large N.A.
spherical mirror introduces significant spherical aberration, the ion image
quality can be greatly improved by a specially designed aspheric corrector lens
located outside the vacuum system. Our simulations show that the spherical
mirror/corrector design is an easy and cost-effective way to achieve high
photon collection rates when compared to a more sophisticated parabolic mirror
setup.Comment: 5 figure
A physical distinction between a covariant and non covariant reduction process in relativistic quantum theories
Causality imposes strong restrictions on the type of operators that may be
observables in relativistic quantum theories. In fact, causal violations arise
when computing conditional probabilities for certain partial causally connected
measurements using the standard non covariant procedure. Here we introduce
another way of computing conditional probabilities, based on an intrinsic
covariant relational order of the events, which differs from the standard one
when these type of measurements are included. This alternative procedure is
compatible with a wider and very natural class of operators without breaking
causality. If some of these measurements could be implemented in practice as
predicted by our formalism, the non covariant, conventional approach should be
abandoned. Furthermore, the description we promote here would imply a new
physical effect where interference terms are suppressed as a consequence of the
covariant order in the measurement process.Comment: 7 pages, latex file, 1 ps figure. Major presentation changes. To
appear in New Journal of Physic
Classicality of quantum information processing
The ultimate goal of the classicality programme is to quantify the amount of
quantumness of certain processes. Here, classicality is studied for a
restricted type of process: quantum information processing (QIP). Under special
conditions, one can force some qubits of a quantum computer into a classical
state without affecting the outcome of the computation. The minimal set of
conditions is described and its structure is studied. Some implications of this
formalism are the increase of noise robustness, a proof of the quantumness of
mixed state quantum computing and a step forward in understanding the very
foundation of QIP.Comment: Minor changes, published in Phys. Rev. A 65, 42319 (2002
A new look at the Plebanski-Demianski family of solutions
The Plebanski-Demianski metric, and those that can be obtained from it by
taking coordinate transformations in certain limits, include the complete
family of space-times of type D with an aligned electromagnetic field and a
possibly non-zero cosmological constant. Starting with a new form of the line
element which is better suited both for physical interpretation and for
identifying different subfamilies, we review this entire family of solutions.
Our metric for the expanding case explicitly includes two parameters which
represent the acceleration of the sources and the twist of the repeated
principal null congruences, the twist being directly related to both the
angular velocity of the sources and their NUT-like properties. The
non-expanding type D solutions are also identified. All special cases are
derived in a simple and transparent way.Comment: 33 pages, 2 figures. To appear in Int. J. Mod. Phys.
Gambling in Great Britain:a response to Rogers
A recent issue of Practice: Social Work in Action featured a paper by Rogers that examined whether the issue of problem gambling was a suitable case for social work. Rogers’ overview was (in various places) out of date, highly selective, contradictory, presented unsupported claims and somewhat misleading. Rogers’ paper is to be commended for putting the issue of problem gambling on the social work agenda. However, social workers need up-to-date information and contextually situated information if they are to make informed decisions in helping problem gamblers
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