1,067 research outputs found
Entanglement vs. the quantum-to-classical transition
We analyze the quantum-to-classical transition (QCT) for coupled bipartite
quantum systems for which the position of one of the two subsystems is
continuously monitored. We obtain the surprising result that the QCT can emerge
concomitantly with the presence of highly entangled states in the bipartite
system. Furthermore the changing degree of entanglement is associated with the
back-action of the measurement on the system and is itself an indicator of the
QCT. Our analysis elucidates the role of entanglement in von Neumann's paradigm
of quantum measurements comprised of a system and a monitored measurement
apparatus
Recovering Classical Dynamics from Coupled Quantum Systems Through Continuous Measurement
We study the role of continuous measurement in the quantum to classical transition for a system with coupled internal (spin) and external (motional) degrees of freedom. Even when the measured motional degree of freedom can be treated classically, entanglement between spin and motion causes strong measurement back action on the quantum spin subsystem so that classical trajectories are not recovered in this mixed quantum classical regime. The measurement can extract localized quantum trajectories that behave classically only when the internal action also becomes large relative to ħ
The Landauer Resistance and Band Spectra for the Counting Quantum Turing Machine
The generalized counting quantum Turing machine (GCQTM) is a machine which,
for any N, enumerates the first integers in succession as binary
strings. The generalization consists of associating a potential with read-1
steps only. The Landauer Resistance (LR) and band spectra were determined for
the tight binding Hamiltonians associated with the GCQTM for energies both
above and below the potential height. For parameters and potentials in the
electron region, the LR fluctuates rapidly between very high and very low
values as a function of momentum. The rapidity and extent of the fluctuations
increases rapidly with increasing N. For N=18, the largest value considered,
the LR shows good transmission probability as a function of momentum with
numerous holes of very high LR values present. This is true for energies above
and below the potential height. It is suggested that the main features of the
LR can be explained by coherent superposition of the component waves reflected
from or transmitted through the potentials in the distribution. If
this explanation is correct, it provides a dramatic illustration of the effects
of quantum nonlocality.Comment: 19 pages Latex, elsart.sty file included, 12 postscript figures,
Submitted to PhysComp96 for publication in Physica-
Accurate and efficient linear scaling DFT calculations with universal applicability
Density Functional Theory calculations traditionally suffer from an inherent
cubic scaling with respect to the size of the system, making big calculations
extremely expensive. This cubic scaling can be avoided by the use of so-called
linear scaling algorithms, which have been developed during the last few
decades. In this way it becomes possible to perform ab-initio calculations for
several tens of thousands of atoms or even more within a reasonable time frame.
However, even though the use of linear scaling algorithms is physically well
justified, their implementation often introduces some small errors.
Consequently most implementations offering such a linear complexity either
yield only a limited accuracy or, if one wants to go beyond this restriction,
require a tedious fine tuning of many parameters. In our linear scaling
approach within the BigDFT package, we were able to overcome this restriction.
Using an ansatz based on localized support functions expressed in an underlying
Daubechies wavelet basis -- which offers ideal properties for accurate linear
scaling calculations -- we obtain an amazingly high accuracy and a universal
applicability while still keeping the possibility of simulating large systems
with only a moderate demand of computing resources
Atomic Motion in Magneto-Optical Double-Well Potentials: A Testing Ground for Quantum Chaos
We have identified ultracold atoms in magneto-optical double-well potentials as a very clean setting in which to study the quantum and classical dynamics of a nonlinear system with multiple degrees of freedom. In this system, entanglement at the quantum level and chaos at the classical level arise from nonseparable couplings between the atomic spin and its center of mass motion. The main features of the chaotic dynamics are analyzed using action-angle variables and Poincaré surfaces of section. We show that for the initial state prepared in current experiments [D. J. Haycock et al., Phys. Rev. Lett. 85, 3365 (2000)], classical and quantum expectation values diverge after a finite time, and the observed experimental dynamics is consistent with quantum-mechanical predictions. Furthermore, the motion corresponds to tunneling through a dynamical potential barrier. The coupling between the spin and the motional subsystems, which are very different in nature from one another, leads to interesting questions regarding the transition from regular quantum dynamics to chaotic classical motion
Controlling nuclear spin exchange via optical Feshbach resonances in Yb
Nuclear spin exchange occurs in ultracold collisions of fermionic
alkaline-earth-like atoms due to a difference between s- and p-wave phase
shifts. We study the use of an optical Feshbach resonance, excited on the
intercombination line of Yb, to affect a large
modification of the s-wave scattering phase shift, and thereby optically
mediate nuclear exchange forces. We perform a full multichannel calculation of
the photoassociation resonances and wave functions and from these calculate the
real and imaginary parts of the scattering length. As a figure of merit of this
interaction, we estimate the fidelity to implement a entangling
quantum logic gate for two atoms trapped in the same well of an optical
lattice. For moderate parameters one can achieve a gate fidelity of
in a time of s.Comment: 5 pages, 1 figur
The wandering mind and performance routines in golf
The past decade of research has brought about new understandings in the study of pre-shot routines, with multiple researchers advancing the field of knowledge surrounding the usage of pre-shot routines as a performance enhancement mechanism. Across golfers of novice to expert skill-levels, the results of peer-reviewed studies have clearly presented the potential benefits of incorporating pre-shot routines for all golfers in improving their play. However, with the current state of research serving as an indicator as to how far we have come in our learning of pre-shot routines in golf, researchers and practitioners in the field understand that there is still a long way to go in expanding our knowledge base on pre-shot routines and their role in the golf performance spectrum. The paper reviews the concept of the wandering mind, attentional control theory, performance routines in general, and more specifically, pre-shot routines in golf
Daubechies Wavelets for Linear Scaling Density Functional Theory
We demonstrate that Daubechies wavelets can be used to construct a minimal
set of optimized localized contracted basis functions in which the Kohn-Sham
orbitals can be represented with an arbitrarily high, controllable precision.
Ground state energies and the forces acting on the ions can be calculated in
this basis with the same accuracy as if they were calculated directly in a
Daubechies wavelets basis, provided that the amplitude of these contracted
basis functions is sufficiently small on the surface of the localization
region, which is guaranteed by the optimization procedure described in this
work. This approach reduces the computational costs of DFT calculations, and
can be combined with sparse matrix algebra to obtain linear scaling with
respect to the number of electrons in the system. Calculations on systems of
10,000 atoms or more thus become feasible in a systematic basis set with
moderate computational resources. Further computational savings can be achieved
by exploiting the similarity of the contracted basis functions for closely
related environments, e.g. in geometry optimizations or combined calculations
of neutral and charged systems
Interfaces in partly compatible polymer mixtures: A Monte Carlo simulation approach
The structure of polymer coils near interfaces between coexisting phases of
symmetrical polymer mixtures (AB) is discussed, as well as the structure of
symmetric diblock copolymers of the same chain length N adsorbed at the
interface. The problem is studied by Monte Carlo simulations of the bond
fluctuation model on the simple cubic lattice, using massively parallel
computers (CRAY T3D). While homopolymer coils in the strong segregation limit
are oriented parallel to the interface, the diblocks form ``dumbbells''
oriented perpendicular to the interface. However, in the dilute case
(``mushroom regime'' rather than ``brush regime''), the diblocks are only
weakly stretched. Distribution functions for monomers at the chain ends and in
the center of the polymer are obtained, and a comparison to the self consistent
field theory is made.Comment: to appear in Physica
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