6,226 research outputs found
Quantumness beyond quantum mechanics
Bohmian mechanics allows us to understand quantum systems in the light of
other quantum traits than the well-known ones (coherence, diffraction,
interference, tunneling, discreteness, entanglement, etc.). Here the discussion
focusses precisely on two of these interesting aspects, which arise when
quantum mechanics is though within this theoretical framework: the non-crossing
property, which allows for distinguishability without erasing interference
patterns, and the possibility to define quantum probability tubes, along which
the probability remains constant all the way. Furthermore, taking into account
this hydrodynamic-like description as a link, it is also shown how this
knowledge (concepts and ideas) can be straightforwardly transferred to other
fields of physics (for example, the transmission of light along waveguides).Comment: 11 pages, 4 figures; based on a talk at the Conference "Emergent
Quantum Mechanics" / 5th Heinz von Foerster Congress (Vienna, Nov 11-13,
2011
Quantum Zeno-based control mechanism for molecular fragmentation
A quantum control mechanism is proposed for molecular fragmentation processes
within a scenario grounded on the quantum Zeno effect. In particular, we focus
on the van der Waals Ne-Br complex, which displays two competing
dissociation channels via vibrational and electronic predissociation.
Accordingly, realistic three dimensional wave packet simulations are carried
out by using ab initio interaction potentials recently obtained to reproduce
available experimental data. Two numerical models to simulate the repeated
measurements are reported and analyzed. It is found that the otherwise fast
vibrational predissociation is slowed down in favor of the slow electronic
(double fragmentation) predissociation, which is enhanced by several orders of
magnitude. Based on these theoretical predictions, some hints to
experimentalists to confirm their validity are also proposed.Comment: 4 pages, 3 figure
Importance Sampling: Intrinsic Dimension and Computational Cost
The basic idea of importance sampling is to use independent samples from a
proposal measure in order to approximate expectations with respect to a target
measure. It is key to understand how many samples are required in order to
guarantee accurate approximations. Intuitively, some notion of distance between
the target and the proposal should determine the computational cost of the
method. A major challenge is to quantify this distance in terms of parameters
or statistics that are pertinent for the practitioner. The subject has
attracted substantial interest from within a variety of communities. The
objective of this paper is to overview and unify the resulting literature by
creating an overarching framework. A general theory is presented, with a focus
on the use of importance sampling in Bayesian inverse problems and filtering.Comment: Statistical Scienc
Vibrational effects in the quantum dynamics of the H + D_2^+ charge transfer reaction
The H + D_2^+(v=0,1 and 2) charge transfer reaction is studied using an
accurate wave packet method, using recently proposed coupled diabatic potential
energy surfaces. The state-to-state cross section is obtained for three
different channels: non-reactive charge transfer, reactive charge transfer, and
exchange reaction. The three processes proceed via the electronic transition
from the first excited to the ground electronic state. The cross section for
the three processes increases with the initial vibrational excitation. The
non-reactive charge transfer process is the dominant channel, whose branching
ratio increases with collision energy, and it compares well with experimental
measurements at collision energies around 0.5 eV. For lower energies the
experimental cross section is considerably higher, suggesting that it
corresponds to higher vibrational excitation of D_2^+(v) reactants. Further
experimental studies of this reaction and isotopic variants are needed, where
conditions are controlled to obtain a better analysis of the vibrational
effects of the D_2^+ reagents.Comment: 15 pages, 7 figure
Quantum Zeno effect: Quantum shuffling and Markovianity
The behavior displayed by a quantum system when it is perturbed by a series
of von Neumann measurements along time is analyzed. Because of the similarity
between this general process with giving a deck of playing cards a shuffle,
here it is referred to as quantum shuffling, showing that the quantum Zeno and
anti-Zeno effects emerge naturally as two time limits. Within this framework, a
connection between the gradual transition from anti-Zeno to Zeno behavior and
the appearance of an underlying Markovian dynamics is found. Accordingly,
although a priori it might result counterintuitive, the quantum Zeno effect
corresponds to a dynamical regime where any trace of knowledge on how the
unperturbed system should evolve initially is wiped out (very rapid shuffling).
This would explain why the system apparently does not evolve or decay for a
relatively long time, although it eventually undergoes an exponential decay. By
means of a simple working model, conditions characterizing the shuffling
dynamics have been determined, which can be of help to understand and to devise
quantum control mechanisms in a number of processes from the atomic, molecular
and optical physics.Comment: 12 pages, 2 figure
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Nanoimaging of Organic Charge Retention Effects: Implications for Nonvolatile Memory, Neuromorphic Computing, and High Dielectric Breakdown Devices
While a large variety of organic and molecular materials have been found to exhibit charge memory effects, the underlying mechanism is not well-understood, which hinders rational device design. Here, we study the charge retention mechanism of a nanoscale memory system, an organic monolayer on a silicon substrate, with Au nanoparticles on top serving as the electrical contact. Combining scanning probe imaging/manipulation and density functional simulations, we observe stable charge retention effects in the system and attributed it to polaron effects at the amine functional groups. Our findings can pave the way for applications in nonvolatile memory, neuromorphic computing, and high dielectric breakdown devices
Matrix Product States: Symmetries and Two-Body Hamiltonians
We characterize the conditions under which a translationally invariant matrix
product state (MPS) is invariant under local transformations. This allows us to
relate the symmetry group of a given state to the symmetry group of a simple
tensor. We exploit this result in order to prove and extend a version of the
Lieb-Schultz-Mattis theorem, one of the basic results in many-body physics, in
the context of MPS. We illustrate the results with an exhaustive search of
SU(2)--invariant two-body Hamiltonians which have such MPS as exact ground
states or excitations.Comment: PDFLatex, 12 pages and 6 figure
Non-interlaced solutions of 2-dimensional systems of linear ordinary differential equations
We consider a 2-dimensional system of linear ordinary differential
equations whose coefficients are definable in an o-minimal structure R. We
prove that either every pair of solutions at 0 of the system is interlaced or the
expansion of R by all solutions at 0 of the system is o-minimal. We also show
that if the coefficients of the system have a Taylor development of sufficiently
large finite order, then the question of which of the two cases holds can be
effectively determined in terms of the coefficients of this Taylor development.Second author was partially supported by Ministerio de Ciencia e Innovacióna, Spain, process MTM2010-1547
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