539 research outputs found
Optimal model averaging estimation for partially linear models
This article studies optimal model averaging for partially linear models with heteroscedasticity. A Mallows-type criterion is proposed to choose the weight. The resulting model averaging estimator is proved to be asymptotically optimal under some regularity conditions. Simulation experiments suggest that the proposed model averaging method is superior to other commonly used model selection and averaging methods. The proposed procedure is further applied to study Japan’s sovereign credit default swap spreads
Observation of a red-blue detuning asymmetry in matter-wave superradiance
We report the first experimental observations of strong suppression of
matter-wave superradiance using blue-detuned pump light and demonstrate a
pump-laser detuning asymmetry in the collective atomic recoil motion. In
contrast to all previous theoretical frameworks, which predict that the process
should be symmetric with respect to the sign of the pump-laser detuning, we
find that for condensates the symmetry is broken. With high condensate
densities and red-detuned light, the familiar distinctive multi-order,
matter-wave scattering pattern is clearly visible, whereas with blue-detuned
light superradiance is strongly suppressed. In the limit of a dilute atomic
gas, however, symmetry is restored.Comment: Accepted by Phys. Rev. Let
Non-Markovian Fermionic Stochastic Schr\"{o}dinger Equation for Open System Dynamics
In this paper we present an exact Grassmann stochastic Schr\"{o}dinger
equation for the dynamics of an open fermionic quantum system coupled to a
reservoir consisting of a finite or infinite number of fermions. We use this
stochastic approach to derive the exact master equation for a fermionic system
strongly coupled to electronic reservoirs. The generality and applicability of
this Grassmann stochastic approach is justified and exemplified by several
quantum open system problems concerning quantum decoherence and quantum
transport for both vacuum and finite-temperature fermionic reservoirs. We show
that the quantum coherence property of the quantum dot system can be profoundly
modified by the environment memory.Comment: 10.5 pages, 3 figure
Non-Markovian Quantum Trajectories of Many-Body Quantum Open Systems
A long-standing open problem in non-Markovian quantum state diffusion (QSD)
approach to open quantum systems is to establish the non-Markovian QSD
equations for multiple qubit systems. In this paper, we settle this important
question by explicitly constructing a set of exact time-local QSD equations for
-qubit systems. Our exact time-local (convolutionless) QSD equations have
paved the way towards simulating quantum dynamics of many-body open systems
interacting with a common bosonic environment. The applicability of this
multiple-qubit stochastic equation is exemplified by numerically solving
several quantum open many-body systems concerning quantum coherence dynamics
and dynamical control.Comment: 8 pages, 2 figures. manuscript revised and reference update
NuSTAR Hard X-ray Monitoring of Gravitationally Lensed Quasar RX J1131-1231
The X-ray emission from active galactic nuclei (AGN) is believed to come from
a combination of inverse Compton scattering of photons from the accretion disk
and reprocessing of the direct X-ray emission by reflection. We present hard
(10-80 keV) and soft (0.5-8 keV) X-ray monitoring of a gravitationally lensed
quasar RX J1131-1231 with NuSTAR, Swift, and XMM-Newton between 10 June 2016
and 30 November 2020. Comparing the amplitude of quasar microlensing
variability at the hard and soft bands allows a size comparison, where larger
sources lead to smaller microlensing variability. During the period between 6
June 2018 and 30 November 2020, where both the hard and soft light curves are
available, the hard and soft bands varied by factors of 3.7 and 5.5,
respectively, with rms variability of and . Both the
variability amplitude and rms are moderately smaller for the hard X-ray
emission, indicating that the hard X-ray emission is moderately larger than the
soft X-ray emission region. We found the reflection fraction from seven joint
hard and soft X-ray monitoring epochs is effectively consistent with a constant
with low significance variability. After decomposing the total X-ray flux into
direct and reprocessed components, we find a smaller variability amplitude for
the reprocessed flux compared to the direct emission. The power-law cutoff
energy is constrained at 96 keV, which position the system in the
allowable parameter space due to the pair production limit.Comment: 12 pages, 6 figures, 3 tables, accepted for publication by the
Astrophysical Journa
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