21,741 research outputs found
Photon-pair generation by non-instantaneous spontaneous four-wave mixing
We present a general model, based on a Hamiltonian approach, for the joint
quantum state of photon pairs generated through pulsed spontaneous four-wave
mixing, including nonlinear phase-modulation and a finite material response
time. For the case of a silica fiber, it is found that the pair-production rate
depends weakly on the waveguide temperature, due to higher-order Raman
scattering events, and more strongly on pump-pair frequency detuning. From the
analytical model, a numerical scheme is derived, based on the well-known
split-step method. This scheme allows computation of joint states where
nontrivial effects are included, such as group-velocity dispersion and Raman
scattering. In this work, the numerical model is used to study the impact of
the non-instantaneous response on the pre-filtering purity of heralded single
photons. We find that for pump pulses shorter than 1 ps, a significant
detuning-dependent change in quantum-mechanical purity may be observed in
silica
High temperature thermoelectric efficiency in Ba8Ga16Ge30
The high thermoelectric figure of merit (zT) of Ba8Ga16Ge30 makes it one of the best n-type materials for thermoelectric power generation. Here, we describe the synthesis and characterization of a Czochralski pulled single crystal of Ba8Ga16Ge30 and polycrystalline disks. Measurements of the electrical conductivity, Hall effect, specific heat, coefficient of thermal expansion, thermal conductivity, and Seebeck coefficient were performed up to 1173 K and compared with literature results. Dilatometry measurements give a coefficient of thermal expansion of 16×10^−6 K^−1 up to 1175 K. The trend in electronic properties with composition is typical of a heavily doped semiconductor. The maximum in the thermoelectric figure of merit is found at 1050 K with a value of 0.8. The correction of zT due to thermal expansion is not significant compared to the measurement uncertainties involved. Comparing the thermoelectric efficiency of segmented materials, the effect of compatibility makes Ba8Ga16Ge30 more efficient than the higher zT n-type materials SiGe or skutterudite CoSb3
Engineering spectrally unentangled photon pairs from nonlinear microring resonators through pump manipulation
The future of integrated quantum photonics relies heavily on the ability to
engineer refined methods for preparing the quantum states needed to implement
various quantum protocols. An important example of such states are
quantum-correlated photon pairs, which can be efficiently generated using
spontaneous nonlinear processes in integrated microring-resonator structures.
In this work, we propose a method for generating spectrally unentangled photon
pairs from a standard microring resonator. The method utilizes interference
between a primary and a delayed secondary pump pulse to effectively increase
the pump spectral width inside the cavity. This enables on-chip generation of
heralded single photons with state purities in excess of 99 % without spectral
filtering.Comment: 5 pages, 5 figure
Spectrally pure heralded single photons by spontaneous four-wave mixing in a fiber: reducing impact of dispersion fluctuations
We model the spectral quantum-mechanical purity of heralded single photons
from a photon-pair source based on nondegenerate spontaneous four-wave mixing
taking the impact of distributed dispersion fluctuations into account. The
considered photon-pair-generation scheme utilizes pump-pulse walk-off to
produce pure heralded photons and phase matching is achieved through the
dispersion properties of distinct spatial modes in a few-mode silica step-index
fiber. We show that fiber-core-radius fluctuations in general severely impact
the single-photon purity. Furthermore, by optimizing the fiber design we show
that generation of single photons with very high spectral purity is feasible
even in the presence of large core-radius fluctuations. At the same time,
contamination from spontaneous Raman scattering is greatly mitigated by
separating the single-photon frequency by more than 32 THz from the pump
frequency
The Temperature Evolution of the Out-of-Plane Correlation Lengths of Charge-Stripe Ordered La(1.725)Sr(0.275)NiO(4)
The temperature dependence of the magnetic order of stripe-ordered
La(1.725)Sr(0.275)NiO(4) is investigated by neutron diffraction. Upon cooling,
the widths if the magnetic Bragg peaks are observed to broaden. The degree of
broadening is found to be very different for l = odd-integer and l =
even-integer magnetic peaks. We argue that the observed behaviour is a result
of competition between magnetic and charge order.Comment: 3 figure
Formation of antiwaves in gap-junction-coupled chains of neurons
Using network models consisting of gap junction coupled Wang-Buszaki neurons,
we demonstrate that it is possible to obtain not only synchronous activity
between neurons but also a variety of constant phase shifts between 0 and \pi.
We call these phase shifts intermediate stable phaselocked states. These phase
shifts can produce a large variety of wave-like activity patterns in
one-dimensional chains and two-dimensional arrays of neurons, which can be
studied by reducing the system of equations to a phase model. The 2\pi periodic
coupling functions of these models are characterized by prominent higher order
terms in their Fourier expansion, which can be varied by changing model
parameters. We study how the relative contribution of the odd and even terms
affect what solutions are possible, the basin of attraction of those solutions
and their stability. These models may be applicable to the spinal central
pattern generators of the dogfish and also to the developing neocortex of the
neonatal rat
Casimir energy density in closed hyperbolic universes
The original Casimir effect results from the difference in the vacuum
energies of the electromagnetic field, between that in a region of space with
boundary conditions and that in the same region without boundary conditions. In
this paper we develop the theory of a similar situation, involving a scalar
field in spacetimes with compact spatial sections of negative spatial
curvature.Comment: 10 pages. Contribution to the "Fifth Alexander Friedmann
International Seminar on Gravitation and Cosmology," Joao Pessoa, Brazil,
2002. Revised version, with altered Abstract and one new referenc
Shape-preserving and unidirectional frequency conversion using four-wave mixing Bragg scattering
In this work, we investigate the properties of four-wave mixing Bragg
scattering in a configuration that employs orthogonally polarized pumps in a
birefringent waveguide. This configuration enables a large signal conversion
bandwidth, and allows strongly unidirectional frequency conversion as undesired
Bragg-scattering processes are suppressed by waveguide birefringence. Moreover,
we show that this form of four-wave mixing Bragg scattering preserves the
(arbitrary) signal pulse shape, even when driven by pulsed pumps.Comment: 11 pages + refs, 5 figure
Genome-wide linkage analysis for aggressive prostate cancer in Utah high risk pedigrees
posterResearch has consistently shown that genetics plays a critical role in prostate cancer (CaP) development, but the identification of CaP genes has proven to be very difficult. Hereditary prostate cancer is a complex disease involving numerous genes and variable phenotypic expression. This heterogeneity has led researchers to pursue genes associated with alternative phenotypes for CaP, such as tumor aggressiveness. Several recent linkage studies have used clinical and pathological data to define CaP aggressiveness as a qualitative trait. The International Consortium for Prostate Cancer Genetics (ICPCG) recently completed such an analysis using pooled data from 11 member institutions. This analysis required all families be small to moderate in size in order to facilitate standard linkage analysis software. Hence, although the ICPCG analysis included data from the Utah prostate cancer pedigree resource, the Utah pedigrees were not analyzed in their complete form. Specifically, pedigrees were divided and trimmed before analysis, which reduced the power of the analysis to detect predisposition loci. Here we present the results of a genome-wide scan for aggressive prostate cancer predisposition loci utilizing the full Utah pedigrees
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