2,232 research outputs found
Ground State Energy Fluctuations of a System Coupled to a Bath
It is often argued that a small non-degenerate quantum system coupled to a
bath has a fixed energy in its ground state since a fluctuation in energy would
require an energy supply from the bath. We consider a simple model of a
harmonic oscillator (the system) coupled to a linear string and determine the
mean squared energy fluctuations. We also analyze the two time correlator of
the energy and discuss its behavior for a finite string.Comment: 5 pages, 2 eps figures, minor change
Many-body dephasing in a trapped-ion quantum simulator
How a closed interacting quantum many-body system relaxes and dephases as a function of time is a fundamental question in thermodynamic and statistical physics. In this Letter, we analyze and observe the persistent temporal fluctuations after a quantum quench of a tunable long-range interacting transverse-field Ising Hamiltonian realized with a trapped-ion quantum simulator. We measure the temporal fluctuations in the average magnetization of a finite-size system of spin-1/2 particles. We experiment in a regime where the properties of the system are closely related to the integrable Hamiltonian with global spin-spin coupling, which enables analytical predictions for the long-time nonintegrable dynamics. The analytical expression for the temporal fluctuations predicts the exponential suppression of temporal fluctuations with increasing system size. Our measurement data is consistent with our theory predicting the regime of many-body dephasing
Algorithms and literate programs for weighted low-rank approximation with missing data
Linear models identification from data with missing values is posed as a weighted low-rank approximation problem with weights related to the missing values equal to zero. Alternating projections and variable projections methods for solving the resulting problem are outlined and implemented in a literate programming style, using Matlab/Octave's scripting language. The methods are evaluated on synthetic data and real data from the MovieLens data sets
Stub model for dephasing in a quantum dot
As an alternative to Buttiker's dephasing lead model, we examine a dephasing
stub. Both models are phenomenological ways to introduce decoherence in chaotic
scattering by a quantum dot. The difference is that the dephasing lead opens up
the quantum dot by connecting it to an electron reservoir, while the dephasing
stub is closed at one end. Voltage fluctuations in the stub take over the
dephasing role from the reservoir. Because the quantum dot with dephasing lead
is an open system, only expectation values of the current can be forced to
vanish at low frequencies, while the outcome of an individual measurement is
not so constrained. The quantum dot with dephasing stub, in contrast, remains a
closed system with a vanishing low-frequency current at each and every
measurement. This difference is a crucial one in the context of quantum
algorithms, which are based on the outcome of individual measurements rather
than on expectation values. We demonstrate that the dephasing stub model has a
parameter range in which the voltage fluctuations are sufficiently strong to
suppress quantum interference effects, while still being sufficiently weak that
classical current fluctuations can be neglected relative to the nonequilibrium
shot noise.Comment: 8 pages with 1 figure; contribution for the special issue of J.Phys.A
on "Trends in Quantum Chaotic Scattering
A LEED structural analysis of the Co(100) surface
The structure of the clean Co(1010) surface has been analysed by LEED. Application of a recently developed computational scheme reveals the prevalence of the termination A in which the two topmost layers exhibit a narrow spacing of 0.62 Ă
, corresponding to a 12.8(Âą0.5)% contraction with respect to the bulk value, while the spacing between the second and third layer is slightly expanded by 0.8(Âą0.2)%
Continuous mode cooling and phonon routers for phononic quantum networks
We study the implementation of quantum state transfer protocols in phonon
networks, where in analogy to optical networks, quantum information is
transmitted through propagating phonons in extended mechanical resonator arrays
or phonon waveguides. We describe how the problem of a non-vanishing thermal
occupation of the phononic quantum channel can be overcome by implementing
optomechanical multi- and continuous mode cooling schemes to create a 'cold'
frequency window for transmitting quantum states. In addition, we discuss the
implementation of phonon circulators and switchable phonon routers, which rely
on strong coherent optomechanical interactions only, and do not require strong
magnetic fields or specific materials. Both techniques can be applied and
adapted to various physical implementations, where phonons coupled to spin or
charge based qubits are used for on-chip networking applications.Comment: 33 pages, 8 figures. Final version, a few minor changes and updated
reference
Decoherence of Einstein-Podolsky-Rosen pairs in a noisy Andreev entangler
We investigate quantum noise effect on the transportation of nonlocal Cooper
pairs accross the realistic Andreev entangler which consists of an s-wave
superconductor coupled to two small quantum dots at resonance which themselves
are coupled to normal leads. The noise emerges due to voltage fluctuations felt
by the electrons residing on the two dots as a result of the finite resistances
in the gate leads or of any resistive lead capacitively coupled to the dots. In
the ideal noiseless case, the setup provides a trustable source of mobile and
nonlocal spin-entangled electrons and the transport is dominated by a
two-particle Breit-Wigner resonance that allows the injection of two
spin-entangled electrons into different leads at the same energy [P. Recher, E.
V. Sukhorukov, and D. Loss, Phys. Rev. B 63, 165314 (2001)]. We seek to revisit
the transport of those nonlocal Cooper pairs as well as the efficiency of such
an Andreev entangler when including the quantum noise (decoherence).Comment: 15 pages and 6 figures; final version to appear in Physical Review
The statistical mechanics of complex signaling networks : nerve growth factor signaling
It is becoming increasingly appreciated that the signal transduction systems
used by eukaryotic cells to achieve a variety of essential responses represent
highly complex networks rather than simple linear pathways. While significant
effort is being made to experimentally measure the rate constants for
individual steps in these signaling networks, many of the parameters required
to describe the behavior of these systems remain unknown, or at best,
estimates. With these goals and caveats in mind, we use methods of statistical
mechanics to extract useful predictions for complex cellular signaling
networks. To establish the usefulness of our approach, we have applied our
methods towards modeling the nerve growth factor (NGF)-induced differentiation
of neuronal cells. Using our approach, we are able to extract predictions that
are highly specific and accurate, thereby enabling us to predict the influence
of specific signaling modules in determining the integrated cellular response
to the two growth factors. We show that extracting biologically relevant
predictions from complex signaling models appears to be possible even in the
absence of measurements of all the individual rate constants. Our methods also
raise some interesting insights into the design and possible evolution of
cellular systems, highlighting an inherent property of these systems wherein
particular ''soft'' combinations of parameters can be varied over wide ranges
without impacting the final output and demonstrating that a few ''stiff''
parameter combinations center around the paramount regulatory steps of the
network. We refer to this property -- which is distinct from robustness -- as
''sloppiness.''Comment: 24 pages, 10 EPS figures, 1 GIF (makes 5 multi-panel figs + caption
for GIF), IOP style; supp. info/figs. included as brown_supp.pd
Picosecond photofragment spectroscopy. I. Microcanonical stateâtoâstate rates of the reaction NCNOâCN+NO
Neural Network-Based Equations for Predicting PGA and PGV in Texas, Oklahoma, and Kansas
Parts of Texas, Oklahoma, and Kansas have experienced increased rates of
seismicity in recent years, providing new datasets of earthquake recordings to
develop ground motion prediction models for this particular region of the
Central and Eastern North America (CENA). This paper outlines a framework for
using Artificial Neural Networks (ANNs) to develop attenuation models from the
ground motion recordings in this region. While attenuation models exist for the
CENA, concerns over the increased rate of seismicity in this region necessitate
investigation of ground motions prediction models particular to these states.
To do so, an ANN-based framework is proposed to predict peak ground
acceleration (PGA) and peak ground velocity (PGV) given magnitude, earthquake
source-to-site distance, and shear wave velocity. In this framework,
approximately 4,500 ground motions with magnitude greater than 3.0 recorded in
these three states (Texas, Oklahoma, and Kansas) since 2005 are considered.
Results from this study suggest that existing ground motion prediction models
developed for CENA do not accurately predict the ground motion intensity
measures for earthquakes in this region, especially for those with low
source-to-site distances or on very soft soil conditions. The proposed ANN
models provide much more accurate prediction of the ground motion intensity
measures at all distances and magnitudes. The proposed ANN models are also
converted to relatively simple mathematical equations so that engineers can
easily use them to predict the ground motion intensity measures for future
events. Finally, through a sensitivity analysis, the contributions of the
predictive parameters to the prediction of the considered intensity measures
are investigated.Comment: 5th Geotechnical Earthquake Engineering and Soil Dynamics Conference,
Austin, TX, USA, June 10-13. (2018
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