251 research outputs found
Saffman-Taylor fingers with kinetic undercooling
The mathematical model of a steadily propagating Saffman-Taylor finger in a
Hele-Shaw channel has applications to two-dimensional interacting streamer
discharges which are aligned in a periodic array. In the streamer context, the
relevant regularisation on the interface is not provided by surface tension,
but instead has been postulated to involve a mechanism equivalent to kinetic
undercooling, which acts to penalise high velocities and prevent blow-up of the
unregularised solution. Previous asymptotic results for the Hele-Shaw finger
problem with kinetic undercooling suggest that for a given value of the kinetic
undercooling parameter, there is a discrete set of possible finger shapes, each
analytic at the nose and occupying a different fraction of the channel width.
In the limit in which the kinetic undercooling parameter vanishes, the fraction
for each family approaches 1/2, suggesting that this 'selection' of 1/2 by
kinetic undercooling is qualitatively similar to the well-known analogue with
surface tension. We treat the numerical problem of computing these
Saffman-Taylor fingers with kinetic undercooling, which turns out to be more
subtle than the analogue with surface tension, since kinetic undercooling
permits finger shapes which are corner-free but not analytic. We provide
numerical evidence for the selection mechanism by setting up a problem with
both kinetic undercooling and surface tension, and numerically taking the limit
that the surface tension vanishes.Comment: 10 pages, 6 figures, accepted for publication by Physical Review
Quantum state transfer and entanglement distribution among distant nodes in a quantum network
We propose a scheme to utilize photons for ideal quantum transmission between
atoms located at spatially-separated nodes of a quantum network. The
transmission protocol employs special laser pulses which excite an atom inside
an optical cavity at the sending node so that its state is mapped into a
time-symmetric photon wavepacket that will enter a cavity at the receiving node
and be absorbed by an atom there with unit probability. Implementation of our
scheme would enable reliable transfer or sharing of entanglement among
spatially distant atoms.Comment: 4 pages, 3 postscript figure
Parametrization of projector-based witnesses for bipartite systems
Entanglement witnesses are nonpositive Hermitian operators which can detect
the presence of entanglement. In this paper, we provide a general
parametrization for orthonormal basis of and use it to
construct projector-based witness operators for entanglement detection in the
vicinity of pure bipartite states. Our method to parameterize entanglement
witnesses is operationally simple and could be used for doing symbolic and
numerical calculations. As an example we use the method for detecting
entanglement between an atom and the single mode of quantized field, described
by the Jaynes-Cummings model. We also compare the detection of witnesses with
the negativity of the state, and show that in the vicinity of pure stats such
constructed witnesses able to detect entanglement of the state.Comment: 12 pages, four figure
Quantum computing with neutral atoms
We develop a method to entangle neutral atoms using cold controlled
collisions. We analyze this method in two particular set-ups: optical lattices
and magnetic micro-traps. Both offer the possibility of performing certain
multi-particle operations in parallel. Using this fact, we show how to
implement efficient quantum error correction and schemes for fault-tolerant
computing.Comment: 21 pages, 19 figure
Quantum Communication and Decoherence
In this contribution we will give a brief overview on the methods used to
overcome decoherence in quantum communication protocols. We give an
introduction to quantum error correction, entanglement purification and quantum
cryptography. It is shown that entanglement purification can be used to create
``private entanglement'', which makes it a useful tool for cryptographic
protocols.Comment: 31 pages, 10 figures, LaTeX, book chapter to appear in ``Coherent
Evolution in Noisy Environments'', Lecture Notes in Physics, (Springer
Verlag, Berlin-Heidelberg-New York). Minor typos correcte
Inseparability criterion for continuous variable systems
An inseparability criterion based on the total variance of a pair of
Einstein-Podolsky-Rosen type operators is proposed for continuous variable
systems. The criterion provides a sufficient condition for entanglement of any
two-party continuous variable states. Furthermore, for all the Gaussian states,
this criterion turns out to be a necessary and sufficient condition for
inseparability.Comment: minor changes in the introduction and ref
Regional Hemodynamic Effects of Neutral Endopeptidase Inhibition and Angiotensin (AT 1 ) Receptor Antagonism Alone or in Combination in Conscious Spontaneously Hypertensive Rats
ABSTRACT We tested the hypothesis that angiotensin (AT 1 ) receptor antagonism (with losartan) would enhance the cardiovascular actions of neutral endopeptidase (NEP) inhibition [with candoxatrilat or (2S)-2-{[1-({[(1S)-1-carboxy-2-(5-phenyl-1,3-oxazol-2-yl)ethyl]amino}carbonyl)cyclopentyl]methyl}-4-methoxybutanoic acid (UK-489,329)] in conscious spontaneously hypertensive rats (SHR). Four-day continuous intravenous infusion of candoxatrilat (1.9 g kg Ϫ1 min Ϫ1 ) or UK-489,329 (0.15 g kg Ϫ1 min Ϫ1 ) had no significant cardiovascular effects, whereas candoxatrilat (6.4 g kg Ϫ1 min Ϫ1 ) had a modest antihypertensive effect (Ϫ10.9 mm Hg on day 4) but no significant sustained effects on regional hemodynamics. Losartan caused a fall in blood pressure (maximum Ϫ29.2 mm Hg on day 4) that was associated with renal, mesenteric, and, to a lesser extent, hindquarters vasodilatation. The combination of losartan with either dose of candoxatrilat had no greater antihypertensive or vasodilator effects than losartan alone, with the exception of the increase in renal vascular conductance, which was greater with the combination of the drugs than with either drug alone (significant only in the lower dose study). Losartan combined with UK-489,329 showed a greater antihypertensive effect than losartan alone (Ϫ14.6 mm Hg greater on day 4), although the effects of the combination were not significantly greater than the sum of the effects of both agents administered separately. However, losartan combined with UK-489,329 caused increases in renal and hindquarters vascular conductance that were significantly greater with the combination than with either agent given alone. Thus, in conscious SHR, the renin-angiotensin system may act to oppose a vasodilator action of NEP inhibition, particularly in the renal vascular bed. Neutral endopeptidase 24.11 (NEP) is a zinc metalloprotease responsible for the breakdown of a number of short linear or cyclic peptides, such as the natriuretic peptides, bradykinin, angiotensin II, and endothelin. Other members of the zinc metalloprotease family that may be involved in the metabolism of biologically active peptides include endothelin-converting enzyme and soluble secreted endopeptidase (SEP) The development of "vasopeptidase" inhibitors, which simultaneously inhibit the two zinc metallopeptidases angiotensin-converting enzyme (ACE) and NEP, was based on the This study was supported by Pfizer Ltd. Article, publication date, and citation information can be found a
Entanglement of atoms via cold controlled collisions
We show that by using cold controlled collisions between two atoms one can
achieve conditional dynamics in moving trap potentials. We discuss implementing
two qubit quantum--gates and efficient creation of highly entangled states of
many atoms in optical lattices.Comment: 4 pages 3 figure
Effects of noise on quantum error correction algorithms
It has recently been shown that there are efficient algorithms for quantum
computers to solve certain problems, such as prime factorization, which are
intractable to date on classical computers. The chances for practical
implementation, however, are limited by decoherence, in which the effect of an
external environment causes random errors in the quantum calculation. To combat
this problem, quantum error correction schemes have been proposed, in which a
single quantum bit (qubit) is ``encoded'' as a state of some larger number of
qubits, chosen to resist particular types of errors. Most such schemes are
vulnerable, however, to errors in the encoding and decoding itself. We examine
two such schemes, in which a single qubit is encoded in a state of qubits
while subject to dephasing or to arbitrary isotropic noise. Using both
analytical and numerical calculations, we argue that error correction remains
beneficial in the presence of weak noise, and that there is an optimal time
between error correction steps, determined by the strength of the interaction
with the environment and the parameters set by the encoding.Comment: 26 pages, LaTeX, 4 PS figures embedded. Reprints available from the
authors or http://eve.physics.ox.ac.uk/QChome.htm
A position-momentum EPR state of distantly-separated trapped atoms
We propose a scheme for preparing an EPR state in position and momentum of a
pair of distantly-separated trapped atoms. The scheme utilizes the entangled
light fields output from a nondegenerate optical parametric amplifier. Quantum
state exchange between these fields and the motional states of the trapped
atoms is accomplished via interactions in cavity QED.Comment: 5 pages, 2 figures, submitted to Phys. Rev.
- …