59,760 research outputs found
A quantum-inspired algorithm for estimating the permanent of positive semidefinite matrices
We construct a quantum-inspired classical algorithm for computing the
permanent of Hermitian positive semidefinite matrices, by exploiting a
connection between these mathematical structures and the boson sampling model.
Specifically, the permanent of a Hermitian positive semidefinite matrix can be
expressed in terms of the expected value of a random variable, which stands for
a specific photon-counting probability when measuring a linear-optically
evolved random multimode coherent state. Our algorithm then approximates the
matrix permanent from the corresponding sample mean and is shown to run in
polynomial time for various sets of Hermitian positive semidefinite matrices,
achieving a precision that improves over known techniques. This work
illustrates how quantum optics may benefit algorithms development.Comment: 9 pages, 1 figure. Updated version for publicatio
Kalikow-type decomposition for multicolor infinite range particle systems
We consider a particle system on with real state space and
interactions of infinite range. Assuming that the rate of change is continuous
we obtain a Kalikow-type decomposition of the infinite range change rates as a
mixture of finite range change rates. Furthermore, if a high noise condition
holds, as an application of this decomposition, we design a feasible perfect
simulation algorithm to sample from the stationary process. Finally, the
perfect simulation scheme allows us to forge an algorithm to obtain an explicit
construction of a coupling attaining Ornstein's -distance for two
ordered Ising probability measures.Comment: Published in at http://dx.doi.org/10.1214/12-AAP882 the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Quantum spin Hall phase in multilayer graphene
The so called quantum spin Hall phase is a topologically non trivial
insulating phase that is predicted to appear in graphene and graphene-like
systems. In this work we address the question of whether this topological
property persists in multilayered systems. We consider two situations: purely
multilayer graphene and heterostructures where graphene is encapsulated by
trivial insulators with a strong spin-orbit coupling. We use a four orbital
tight-binding model that includes the full atomic spin-orbit coupling and we
calculate the topological invariant of the bulk states as well as the
edge states of semi-infinite crystals with armchair termination. For
homogeneous multilayers we find that even when the spin-orbit interaction opens
a gap for all the possible stackings, only those with odd number of layers host
gapless edge states while those with even number of layers are trivial
insulators. For the heterostructures where graphene is encapsulated by trivial
insulators, it turns out that the interlayer coupling is able to induce a
topological gap whose size is controlled by the spin-orbit coupling of the
encapsulating materials, indicating that the quantum spin Hall phase can be
induced by proximity to trivial insulators.Comment: 7 pages, 6 figure
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Dust Around R Coronae Borealis Stars. I. Spitzer/Infrared Spectrograph Observations
Spitzer/infrared spectrograph (IRS) spectra from 5 to 37 mu m for a complete sample of 31 R Coronae Borealis stars (RCBs) are presented. These spectra are combined with optical and near-infrared photometry of each RCB at maximum light to compile a spectral energy distribution (SED). The SEDs are fitted with blackbody flux distributions and estimates are made of the ratio of the infrared flux from circumstellar dust to the flux emitted by the star. Comparisons for 29 of the 31 stars are made with the Infrared Astronomical Satellite (IRAS) fluxes from three decades earlier: Spitzer and IRAS fluxes at 12 mu m and 25 mu m are essentially equal for all but a minority of the sample. For this minority, the IRAS to Spitzer flux ratio exceeds a factor of three. The outliers are suggested to be stars where formation of a dust cloud or dust puff is a rare event. A single puff ejected prior to the IRAS observations may have been reobserved by Spitzer as a cooler puff at a greater distance from the RCB. RCBs which experience more frequent optical declines have, in general, a circumstellar environment containing puffs subtending a larger solid angle at the star and a quasi-constant infrared flux. Yet, the estimated subtended solid angles and the blackbody temperatures of the dust show a systematic evolution to lower solid angles and cooler temperatures in the interval between IRAS and Spitzer. Dust emission by these RCBs and those in the LMC is similar in terms of total 24 mu m luminosity and [8.0]-[24.0] color index.Spanish Ministry of Science and Innovation (MICINN) AYA-2007-64748NASA GO 50212, 1407Robert A. Welch Foundation of Houston, Texas F-634McDonald Observator
High-resolution optical spectroscopy of DY Cen: diffuse interstellar bands in a proto-fullerene circumstellar environment?
We search high-resolution and high-quality VLT/UVES optical spectra of the
hot R Coronae Borealis (RCB) star DY Cen for electronic transitions of the C60
molecule and diffuse interstellar bands (DIBs). We report the non-detection of
the strongest C60 electronic transitions (e.g., those at ~3760, 3980, and 4024
A). Absence of C60 absorption bands may support recent laboratory results,
which show that the ~7.0, 8.5, 17.4, and 18.8 um emission features seen in DY
Cen - and other similar objects with PAH-like dominated IR spectra - are
attributable to proto-fullerenes or fullerene precursors rather than to C60.
DIBs towards DY Cen are normal for its reddening; the only exception is the DIB
at 6284 A (possibly also the 7223A DIB) that is found to be unusually strong.
We also report the detection of a new broad (FWHM~2 A) and unidentified feature
centered at ~4000 A. We suggest that this new band may be related to the
circumstellar proto-fullerenes seen at infrared wavelengths.Comment: Accepted for publication in The Astrophysical Journal Letters (15
pages, 3 figures, and 2 tables
Superburst oscillations: ocean and crustal modes excited by Carbon-triggered Type I X-ray bursts
Accreting neutron stars (NS) can exhibit high frequency modulations in their
lightcurves during thermonuclear X-ray bursts, known as burst oscillations. The
frequencies can be offset from the spin frequency of the NS by several Hz, and
can drift by 1-3 Hz. One possible explanation is a mode in the bursting ocean,
the frequency of which would decrease (in the rotating frame) as the burst
cools, hence explaining the drifts. Most burst oscillations have been observed
during H/He triggered bursts, however there has been one observation of
oscillations during a superburst; hours' long Type I X-ray bursts caused by
unstable carbon burning deeper in the ocean. This paper calculates the
frequency evolution of an oceanic r-mode during a superburst. The rotating
frame frequency varies during the burst from 4-14 Hz, and is sensitive to the
background parameters, in particular the temperature of the ocean and ignition
depth. This calculation is compared to the superburst oscillations observed on
4U-1636-536. The predicted mode frequencies ( 10 Hz) would require a spin
frequency of 592 Hz to match observations; 6 Hz higher than the spin
inferred from an oceanic r-mode model for the H/He triggered burst
oscillations. This model also over-predicts the frequency drift during the
superburst by 90 %.Comment: Accepted for publication in MNRA
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