577 research outputs found
First-order transitions and the performance of quantum algorithms in random optimization problems
We present a study of the phase diagram of a random optimization problem in
presence of quantum fluctuations. Our main result is the characterization of
the nature of the phase transition, which we find to be a first-order quantum
phase transition. We provide evidence that the gap vanishes exponentially with
the system size at the transition. This indicates that the Quantum Adiabatic
Algorithm requires a time growing exponentially with system size to find the
ground state of this problem.Comment: 4 pages, 4 figures; final version accepted on Phys.Rev.Let
Sparse sampling, galaxy bias, and voids
To study the impact of sparsity and galaxy bias on void statistics, we use a
single large-volume, high-resolution N-body simulation to compare voids in
multiple levels of subsampled dark matter, halo populations, and mock galaxies
from a Halo Occupation Distribution model tuned to different galaxy survey
densities. We focus our comparison on three key observational statistics:
number functions, ellipticity distributions, and radial density profiles. We
use the hierarchical tree structure of voids to interpret the impacts of
sampling density and galaxy bias, and theoretical and empirical functions to
describe the statistics in all our sample populations. We are able to make
simple adjustments to theoretical expectations to offer prescriptions for
translating from analytics to the void properties measured in realistic
observations. We find that sampling density has a much larger effect on void
sizes than galaxy bias. At lower tracer density, small voids disappear and the
remaining voids are larger, more spherical, and have slightly steeper profiles.
When a proper lower mass threshold is chosen, voids in halo distributions
largely mimic those found in galaxy populations, except for ellipticities,
where galaxy bias leads to higher values. We use the void density profile of
Hamaus et al. (2014) to show that voids follow a self-similar and universal
trend, allowing simple translations between voids studied in dark matter and
voids identified in galaxy surveys. We have added the mock void catalogs used
in this work to the Public Cosmic Void Catalog at http://www.cosmicvoids.net.Comment: 11 pages, 7 figures, MNRAS accepted. Minor changes from previous
version. Public catalog available at http://www.cosmicvoids.ne
Tailoring Chirp in Spin-Lasers
The usefulness of semiconductor lasers is often limited by the undesired
frequency modulation, or chirp, a direct consequence of the intensity
modulation and carrier dependence of the refractive index in the gain medium.
In spin-lasers, realized by injecting, optically or electrically,
spin-polarized carriers, we elucidate paths to tailoring chirp. We provide a
generalized expression for chirp in spin-lasers and introduce modulation
schemes that could simultaneously eliminate chirp and enhance the bandwidth, as
compared to the conventional (spin-unpolarized) lasers.Comment: 4 pages, 3 figure
Low temperature tunneling current enhancement in silicide/Si Schottky contacts with nanoscale barrier width
The low temperature electrical behavior of adjacent silicide/Si Schottky
contacts with or without dopant segregation is investigated. The electrical
characteristics are very well modeled by thermionic-field emission for
non-segregated contacts separated by micrometer-sized gaps. Still, an excess of
current occurs at low temperature for short contact separations or
dopant-segregated contacts when the voltage applied to the device is
sufficiently high. From two-dimensional self-consistent non-equilibrium Green's
function simulations, the dependence of the Schottky barrier profile on the
applied voltage, unaccounted for in usual thermionic-field emission models, is
found to be the source of this deviation
Exact solution of the Bose-Hubbard model on the Bethe lattice
The exact solution of a quantum Bethe lattice model in the thermodynamic
limit amounts to solve a functional self-consistent equation. In this paper we
obtain this equation for the Bose-Hubbard model on the Bethe lattice, under two
equivalent forms. The first one, based on a coherent state path integral, leads
in the large connectivity limit to the mean field treatment of Fisher et al.
[Phys. Rev. B {\bf 40}, 546 (1989)] at the leading order, and to the bosonic
Dynamical Mean Field Theory as a first correction, as recently derived by
Byczuk and Vollhardt [Phys. Rev. B {\bf 77}, 235106 (2008)]. We obtain an
alternative form of the equation using the occupation number representation,
which can be easily solved with an arbitrary numerical precision, for any
finite connectivity. We thus compute the transition line between the superfluid
and Mott insulator phases of the model, along with thermodynamic observables
and the space and imaginary time dependence of correlation functions. The
finite connectivity of the Bethe lattice induces a richer physical content with
respect to its infinitely connected counterpart: a notion of distance between
sites of the lattice is preserved, and the bosons are still weakly mobile in
the Mott insulator phase. The Bethe lattice construction can be viewed as an
approximation to the finite dimensional version of the model. We show indeed a
quantitatively reasonable agreement between our predictions and the results of
Quantum Monte Carlo simulations in two and three dimensions.Comment: 27 pages, 16 figures, minor correction
Moment tensors for rapid characterization of megathrust earthquakes: the example of the 2011 M9 Tohoku-oki, Japan earthquake
The rapid detection and characterization of megathrust earthquakes is a difficult task given their large rupture zone and duration. These events produce very strong ground vibrations in the near field that can cause weak motion instruments to clip, and they are also capable of generating large-scale tsunamis. The 2011 M9 Tohoku-oki earthquake that occurred offshore Japan is one member of a series of great earthquakes for which extended geophysical observations are available. Here, we test an automated scanning algorithm for great earthquakes using continuous very long-period (100-200 s) seismic records from K-NET strong-motion seismograms of the earthquake. By continuously performing the cross-correlation of data and Green's functions (GFs) in a moment tensor analysis, we show that the algorithm automatically detects, locates and determines source parameters including the moment magnitude and mechanism of the great Tohoku-oki earthquake within 8 min of its origin time. The method does not saturate. We also show that quasi-finite-source GFs, which take into account the effects of a finite-source, in a single-point source moment tensor algorithm better fit the data, especially in the near-field. We show that this technique allows the correct characterization of the earthquake using a limited number of stations. This can yield information usable for tsunami early warnin
Cavity-based single atom preparation and high-fidelity hyperfine state readout
We prepare and detect the hyperfine state of a single 87Rb atom coupled to a
fiber-based high finesse cavity on an atom chip. The atom is extracted from a
Bose-Einstein condensate and trapped at the maximum of the cavity field,
resulting in a reproducibly strong atom-cavity coupling. We use the cavity
reflection and transmission signal to infer the atomic hyperfine state with a
fidelity exceeding 99.92% in a read-out time of 100 microseconds. The atom is
still trapped after detection.Comment: 5 pages, 4 figure
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