3,236 research outputs found
Finite-temperature Bell test for quasiparticle entanglement in the Fermi sea
We demonstrate that the Bell test cannot be realized at finite temperatures
in the vast majority of electronic setups proposed previously for quantum
entanglement generation. This fundamental difficulty is shown to originate in a
finite probability of quasiparticle emission from Fermi-sea detectors. In order
to overcome the feedback problem, we suggest a detection strategy, which takes
advantage of a resonant coupling to the quasiparticle drains. Unlike other
proposals, the designed Bell test provides a possibility to determine the
critical temperature for entanglement production in the solid state.Comment: 6 pages, 3 figures, essentially revised and extended versio
Paleoproductivity at the Antarctic continental margin: opal and barium records for the last 400 ka
Ground states in the Many Interacting Worlds approach
Recently the Many-Interacting-Worlds (MIW) approach to a quantum theory
without wave functions was proposed. This approach leads quite naturally to
numerical integrators of the Schr\"odinger equation. It has been suggested that
such integrators may feature advantages over fixed-grid methods for higher
numbers of degrees of freedom. However, as yet, little is known about concrete
MIW models for more than one spatial dimension and/or more than one particle.
In this work we develop the MIW approach further to treat arbitrary degrees of
freedom, and provide a systematic study of a corresponding numerical
implementation for computing one-particle ground and excited states in one
dimension, and ground states in two spatial dimensions. With this step towards
the treatment of higher degrees of freedom we hope to stimulate their further
study.Comment: 16 pages, 8 figure
Control and coherence of the optical transition of single defect centers in diamond
We demonstrate coherent control of the optical transition of single
Nitrogen-Vacancy defect centers in diamond. On applying short resonant laser
pulses, we observe optical Rabi oscillations with a half-period as short as 1
nanosecond, an order of magnitude shorter than the spontaneous emission time.
By studying the decay of Rabi oscillations, we find that the decoherence is
dominated by laser-induced spectral jumps. By using a low-power probe pulse as
a detuning sensor and applying post-selection, we demonstrate that spectral
diffusion can be overcome in this system to generate coherent photons.Comment: 5 pages,4 figure
Large Scale Variational Inference and Experimental Design for Sparse Generalized Linear Models
Many problems of low-level computer vision and image processing, such as
denoising, deconvolution, tomographic reconstruction or super-resolution, can
be addressed by maximizing the posterior distribution of a sparse linear model
(SLM). We show how higher-order Bayesian decision-making problems, such as
optimizing image acquisition in magnetic resonance scanners, can be addressed
by querying the SLM posterior covariance, unrelated to the density's mode. We
propose a scalable algorithmic framework, with which SLM posteriors over full,
high-resolution images can be approximated for the first time, solving a
variational optimization problem which is convex iff posterior mode finding is
convex. These methods successfully drive the optimization of sampling
trajectories for real-world magnetic resonance imaging through Bayesian
experimental design, which has not been attempted before. Our methodology
provides new insight into similarities and differences between sparse
reconstruction and approximate Bayesian inference, and has important
implications for compressive sensing of real-world images.Comment: 34 pages, 6 figures, technical report (submitted
Higher-order contributions and non-perturbative effects in the non-degenerate nonlinear optical absorption of direct-gap semiconductors
The semiconductor Bloch equations for a two-band model including inter- and
intraband excitation are used to study the nonlinear absorption of single and
multiple light pulses by direct-gap semiconductors. For a consistent analysis
the contributions to the absorption originating from both the interband
polarization and the intraband current need to be included. In the Bloch
equation approach theses contributions as well as different excitation pathways
in terms of sequences of inter- and intraband excitations can be evaluated
separately which allows for a transparent analysis, the identification of the
dominant terms, and analyzing their dependence on the excitation conditions. In
the perturbative regime, we obtain analytical expressions for the multi-photon
absorption coefficients for continuous-wave excitation. These results are shown
to agree well with numerical results for short pulses and/or finite dephasing
and relaxation times and we confirm the previously predicted strong enhancement
of two-photon absorption for non-degenerate conditions for pulsed excitation.
We discuss the dependencies on the light frequencies, initial band populations,
and the time delay between the pulses. The frequency dependence of the
two-photon absorption coefficient for non-degenerate excitation is evaluated
perturbatively in third-order. The higher-order contributions to the optical
absorption include three- and four-photon absorption and show a rich frequency
dependence including negative regions and dispersive lineshapes.
Non-perturbative solutions of the Bloch equations demonstrate a strongly
non-monotonous behavior of the intensity-dependent optical absorption for a
single incident pulse and in a pump-probe set-up
Fast Convergent Algorithms for Expectation Propagation Approximate Bayesian Inference
We propose a novel algorithm to solve the expectation propagation relaxation
of Bayesian inference for continuous-variable graphical models. In contrast to
most previous algorithms, our method is provably convergent. By marrying
convergent EP ideas from (Opper&Winther 05) with covariance decoupling
techniques (Wipf&Nagarajan 08, Nickisch&Seeger 09), it runs at least an order
of magnitude faster than the most commonly used EP solver.Comment: 16 pages, 3 figures, submitted for conference publicatio
Researching employee experiences and behavior in times of crisis: Theoretical and methodological considerations and implications for human resource management
Over the past 2 years, numerous empirical studies in the fields of human resource management, organizational behavior, and industrial, work, and organizational psychology have investigated employee experiences and behavior during the COVID-19 pandemic. The goal of this paper is to take a step back and to outline several theoretical and methodological considerations when researching employee experiences and behavior in times of crisis more generally. These insights may be useful when developing conceptual models, designing empirical studies, and managing people in the context of future crises. We first review theoretical approaches that could be applied to explain changes in employee experiences and behavior in times of crisis, including stress theories, theories of adjustment to work-related changes, career construction theory, event system theory, transition-adaptation theories, the crisis management and resilience framework, and the social identity model of identity change. Second, we outline methodological considerations and best practices regarding the research design of quantitative empirical studies, sampling, measurement, and analytic strategies. Throughout, we highlight empirical studies on employee experiences and behavior during the COVID-19 pandemic that have adopted these theoretical approaches and methodological best practices. We conclude with several suggestions for future theory development and empirical studies on employee experiences and behavior as well as human resource management in times of crisis
Dielectric screening in extended systems using the self-consistent Sternheimer equation and localized basis sets
We develop a first-principles computational method for investigating the
dielectric screening in extended systems using the self-consistent Sternheimer
equation and localized non-orthogonal basis sets. Our approach does not require
the explicit calculation of unoccupied electronic states, only uses two-center
integrals, and has a theoretical scaling of order O(N^3). We demonstrate this
method by comparing our calculations for silicon, germanium, diamond, and LiCl
with reference planewaves calculations. We show that accuracy comparable to
planewaves calculations can be achieved via a systematic optimization of the
basis set.Comment: 6 pages, 3 figure
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