7,486 research outputs found
Close Approach during Hard Binary--Binary Scattering
We report on an extensive series of numerical experiments of binary--binary
scattering, analysing the cross--section for close approach during interactions
for a range of hard binary parameters of interest in globular cluster cores. We
consider the implied rate for tidal interactions for different globular
clusters and compare our results with previous, complementary estimates of
stellar collision rates in globular clusters. We find that the collision rate
for binary--binary encounters dominates in low density clusters if the binary
fraction in the cluster is larger than for wide main--sequence binaries.
In dense clusters binary--single interactions dominate the collision rate and
the core binary fraction must be \ltorder 0.1 per decade in semi--major axis
or too many collisions take place compared to observations. The rates are
consistent if binaries with semi--major axes are overabundant in
low density clusters or if breakup and ejection substantially lowers the binary
fraction in denser clusters. Given reasonable assumptions about fractions of
binaries in the cores of low density clusters such as NGC~5053, we cannot
account for all the observed blue stragglers by stellar collisions during
binary encounters, suggesting a substantial fraction may be due to coalescence
of tight primordial binaries.Comment: 13 pages including 13 ps figures. MNRAS in pres
The Optimal Single Copy Measurement for the Hidden Subgroup Problem
The optimization of measurements for the state distinction problem has
recently been applied to the theory of quantum algorithms with considerable
successes, including efficient new quantum algorithms for the non-abelian
hidden subgroup problem. Previous work has identified the optimal single copy
measurement for the hidden subgroup problem over abelian groups as well as for
the non-abelian problem in the setting where the subgroups are restricted to be
all conjugate to each other. Here we describe the optimal single copy
measurement for the hidden subgroup problem when all of the subgroups of the
group are given with equal a priori probability. The optimal measurement is
seen to be a hybrid of the two previously discovered single copy optimal
measurements for the hidden subgroup problem.Comment: 8 pages. Error in main proof fixe
Coherence-Preserving Quantum Bits
Real quantum systems couple to their environment and lose their intrinsic
quantum nature through the process known as decoherence. Here we present a
method for minimizing decoherence by making it energetically unfavorable. We
present a Hamiltonian made up solely of two-body interactions between four
two-level systems (qubits) which has a two-fold degenerate ground state. This
degenerate ground state has the property that any decoherence process acting on
an individual physical qubit must supply energy from the bath to the system.
Quantum information can be encoded into the degeneracy of the ground state and
such coherence-preserving qubits will then be robust to local decoherence at
low bath temperatures. We show how this quantum information can be universally
manipulated and indicate how this approach may be applied to a quantum dot
quantum computer.Comment: 5 pages, 1 figur
Weak Gravitational Flexion
Flexion is the significant third-order weak gravitational lensing effect
responsible for the weakly skewed and arc-like appearance of lensed galaxies.
Here we demonstrate how flexion measurements can be used to measure galaxy halo
density profiles and large-scale structure on non-linear scales, via
galaxy-galaxy lensing, dark matter mapping and cosmic flexion correlation
functions. We describe the origin of gravitational flexion, and discuss its
four components, two of which are first described here. We also introduce an
efficient complex formalism for all orders of lensing distortion. We proceed to
examine the flexion predictions for galaxy-galaxy lensing, examining isothermal
sphere and Navarro, Frenk & White (NFW) profiles and both circularly symmetric
and elliptical cases. We show that in combination with shear we can precisely
measure galaxy masses and NFW halo concentrations. We also show how flexion
measurements can be used to reconstruct mass maps in 2-D projection on the sky,
and in 3-D in combination with redshift data. Finally, we examine the
predictions for cosmic flexion, including convergence-flexion
cross-correlations, and find that the signal is an effective probe of structure
on non-linear scales.Comment: 17 pages, including 12 figures, submitted to MNRA
The reduced cost of providing a nationally recognised service for familial hypercholesterolaemia
OBJECTIVE: Familial hypercholesterolaemia (FH) affects 1 in 500 people in the UK population and is associated with premature morbidity and mortality from coronary heart disease. In 2008, National Institute for Health and Care Excellence (NICE) recommended genetic testing of potential FH index cases and cascade testing of their relatives. Commissioners have been slow to respond although there is strong evidence of cost and clinical effectiveness. Our study quantifies the recent reduced cost of providing a FH service using generic atorvastatin and compares NICE costing estimates with three suggested alternative models of care (a specialist-led service, a dual model service where general practitioners (GPs) can access specialist advice, and a GP-led service).METHODS: Revision of existing 3?year costing template provided by NICE for FH services, and prediction of costs for running a programme over 10?years. Costs were modelled for the first population-based FH service in England which covers Southampton, Hampshire, Isle of Wight and Portsmouth (SHIP). Population 1.95 million.RESULTS: With expiry of the Lipitor (Pfizer atorvastatin) patent the cost of providing a 10-year FH service in SHIP reduces by 42.5% (£4.88 million on patent vs £2.80 million off patent). Further cost reductions are possible as a result of the reduced cost of DNA testing, more management in general practice, and lower referral rates to specialists. For instance a dual-care model with GP management of patients supported by specialist advice when required, costs £1.89 million.CONCLUSIONS: The three alternative models of care are now <50% of the cost of the original estimates undertaken by NICE
Finite automata for caching in matrix product algorithms
A diagram is introduced for visualizing matrix product states which makes
transparent a connection between matrix product factorizations of states and
operators, and complex weighted finite state automata. It is then shown how one
can proceed in the opposite direction: writing an automaton that ``generates''
an operator gives one an immediate matrix product factorization of it. Matrix
product factorizations have the advantage of reducing the cost of computing
expectation values by facilitating caching of intermediate calculations. Thus
our connection to complex weighted finite state automata yields insight into
what allows for efficient caching in matrix product algorithms. Finally, these
techniques are generalized to the case of multiple dimensions.Comment: 18 pages, 19 figures, LaTeX; numerous improvements have been made to
the manuscript in response to referee feedbac
Few-body spin couplings and their implications for universal quantum computation
Electron spins in semiconductor quantum dots are promising candidates for the
experimental realization of solid-state qubits. We analyze the dynamics of a
system of three qubits arranged in a linear geometry and a system of four
qubits arranged in a square geometry. Calculations are performed for several
quantum dot confining potentials. In the three-qubit case, three-body effects
are identified that have an important quantitative influence upon quantum
computation. In the four-qubit case, the full Hamiltonian is found to include
both three-body and four-body interactions that significantly influence the
dynamics in physically relevant parameter regimes. We consider the implications
of these results for the encoded universality paradigm applied to the
four-electron qubit code; in particular, we consider what is required to
circumvent the four-body effects in an encoded system (four spins per encoded
qubit) by the appropriate tuning of experimental parameters.Comment: 1st version: 33 pages, 25 figures. Described at APS March Meeting in
2004 (P36.010) and 2005 (B17.00009). Most figures made uglier here to reduce
file size. 2nd version: 19 pages, 9 figures. Much mathematical detail chopped
away after hearing from journal referee; a few typos correcte
A double-helix neutron detector using micron-size B-10 powder
A double-helix electrode configuration is combined with a B powder
coating technique to build large-area (9 in 36 in) neutron detectors.
The neutron detection efficiency for each of the four prototypes is comparable
to a single 2-bar He drift tube of the same length (36 in). One unit has
been operational continuously for 18 months and the change of efficiency is
less than 1%. An analytic model for pulse heigh spectra is described and the
predicted mean film thickness agrees with the experiment to within 30%. Further
detector optimization is possible through film texture, power size, moderator
box and gas. The estimated production cost per unit is less than 3k US\$ and
the technology is thus suitable for deployment in large numbers
Probing the presence of a single or binary black hole in the globular cluster NGC 6752 with pulsar dynamics
The five millisecond pulsars that inhabit NGC 6752 display locations or
accelerations that are quite unusual compared to all other pulsars known in
globular clusters. In particular PSR-A, a binary pulsar, lives in the cluster
halo, while PSR-B and PSR-E, located in the core, show remarkably high negative
spin derivatives. This is suggestive that some uncommon dynamical process is at
play in the cluster core that we attribute to the presence of a massive
perturber. We here investigate whether a single intermediate-mass black hole,
lying on the extrapolation of the Mass versus Sigma relation observed in galaxy
spheroids, or a less massive binary consisting of two black holes could play
the requested role. To this purpose we simulated binary-binary encounters
involving PSR-A, its companion star, and the black hole(s). Various scenarios
are discussed in detail. In our close 4-body encounters, a black hole-black
hole binary may attract on a long-term stable orbit a millisecond pulsar.
Timing measurements on the captured satellite-pulsar, member of a hierarchical
triplet, could unambiguously unveil the presence of a black hole(s) in the core
of a globular cluster.Comment: 13 pages, 8 figures, Accepted for publication in The Astrophysical
Journa
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