122 research outputs found
"Superluminal paradox" in wavepacket propagation and its quantum mechanical resolution
We analyse in detail the reshaping mechanism leading to apparently
"superluminal" advancement of a wave packet traversing a classically forbidden
region. In the coordinate representation, a barrier is shown to act as an
effective beamsplitter, recombining envelopes of the freely propagating pulse
with various spacial shifts. Causality ensures that none of the constituent
envelopes are advanced with respect to free propagation, yet the resulting
pulse is advanced due to a peculiar interference effect, similar to the one
responsible for "anomalous" values which occur in Aharonov's "weak
measurements". In the momentum space, the effect is understood as a bandwidth
phenomenon, where the incident pulse probes local, rather than global,
analytical properties of the transmission amplitude T (p). The advancement is
achieved when T (p) mimics locally an exponential behaviour, similar to the one
occurring in Berry's "superoscillations". Seen in a broader quantum mechanical
context, the "paradox" is but a consequence of an attempt to obtain "which
way?" information without destroying the interference between the pathways of
interest. This explains, to a large extent, the failure to adequately describe
tunnelling in terms of a single "tunnelling time"
No Time at the End of the Tunnel
Modern atto-second experiments seek to provide an insight into a long standing question: “how much time does a tunnelling particle spend in the barrier?” Traditionally, quantum theory relates this duration to the delay with which the particle emerges from the barrier. The link between these two times is self-evident in classical mechanics, but may or may not exist in the quantum case. Here we show that it does not, and give a detailed explanation why. The tunnelling process does not lend itself to classical analogies, and its duration cannot, in general, be guessed by observing the behaviour of the transmitted particle
Interference mechanism of seemingly superluminal tunnelling
Apparently 'superluminal' transmission, e.g., in quantum tunnelling and its
variants, occurs via a subtle interference mechanism which allows
reconstruction of the entire spacial shape of a wave packet from its front
tail. It is unlikely that the effect could be described adequately in simpler
terms
Dynamic modeling of the morphology of latex particles with in situ formation of graft copolymer
Modification of the polymer-polymer interfacial tension is a way to tailor-make particle morphology of waterborne polymer-polymer hybrids. This allows achieving a broader spectrum of application properties and maximizing the synergy of the positive properties of both polymers, avoiding their drawbacks. In situ formation of graft copolymer during polymerization is an efficient way to modify the polymer-polymer interfacial tension. Currently, no dynamic model is available for polymer-polymer hybrids in which a graft copolymer is generated during polymerization. In this article, a novel model based on stochastic dynamics is developed for predicting the dynamics of the development of particle morphology for composite waterborne systems in which a graft copolymer is produced in situ during the process
Adaptive multi-stage integrators for optimal energy conservation in molecular simulations
We introduce a new Adaptive Integration Approach (AIA) to be used in a wide
range of molecular simulations. Given a simulation problem and a step size, the
method automatically chooses the optimal scheme out of an available family of
numerical integrators. Although we focus on two-stage splitting integrators,
the idea may be used with more general families. In each instance, the
system-specific integrating scheme identified by our approach is optimal in the
sense that it provides the best conservation of energy for harmonic forces. The
AIA method has been implemented in the BCAM-modified GROMACS software package.
Numerical tests in molecular dynamics and hybrid Monte Carlo simulations of
constrained and unconstrained physical systems show that the method
successfully realises the fail-safe strategy. In all experiments, and for each
of the criteria employed, the AIA is at least as good as, and often
significantly outperforms the standard Verlet scheme, as well as fixed
parameter, optimized two-stage integrators. In particular, the sampling
efficiency found in simulations using the AIA is up to 5 times better than the
one achieved with other tested schemes
An even simpler understanding of quantum weak values
We explain the properties and clarify the meaning of quantum weak values using only the basic notions of elementary quantum mechanics.MTM2013-46553-C3-1-
Meso-GSHMC: A stochastic algorithm for meso-scale constant temperature simulations
We consider the problem of time-stepping/sampling for molecular and meso-scale particle dynamics. The aim of the work is to derive numerical time-stepping methods that generate samples exactly from the desired target temperature distribution. The numerical methods proposed in this paper rely on the well-known splitting of stochastic thermostat equations into conservative and fluctuation-dissipation parts. We propose a methodology to derive numerical approximation to the fluctuation-dissipation part that exactly samples from the underlying Boltzmann distribution. Our methodology applies to Langevin dynamics as well as Dissipative Particle Dynamics and, more generally, to arbitrary position dependent fluctuation-dissipation terms. A Metropolis criterion is introduced to correct for numerical inconsistency in the conservative dynamics part of the model. Shadow energies are used to increase the acceptance rate under the Metropolis criterion. We call the newly proposed method meso-GSHMC
The Hartman effect and weak measurements "which are not really weak"
We show that in wavepacket tunnelling localisation of the transmitted
particle amounts to a quantum measurement of the delay it experiences in the
barrier. With no external degree of freedom involved, the envelope of the
wavepacket plays the role of the initial pointer state. Under tunnelling
conditions such 'self measurement' is necessarily weak, and the Hartman effect
just reflects the general tendency of weak values to diverge, as post-selection
in the final state becomes improbable. We also demonstrate that it is a good
precision, or 'not really weak' quantum measurement: no matter how wide the
barrier d, it is possible to transmit a wavepacket with a width {\sigma} small
compared to the observed advancement. As is the case with all weak
measurements, the probability of transmission rapidly decreases with the ratio
{\sigma}/d.Comment: 6 pages, 1 figur
Wigner's friends, tunnelling times and Feynman's "only mystery of quantum mechanics"
Recent developments in elementary quantum mechanics have seen a number of extraordinary claims regarding quantum behaviour, and even questioning internal consistency of the theory. These are, we argue, different disguises of what Feynman described as quantum theory's "only mystery".ELKARTEK
KK-2021/00064;
KK-2021/00022;
KK-2020/0000
Dynamic modeling of the morphology of multiphase waterborne polymer particles
Multiphase waterborne polymer particles provide advantages in more demanding applications, and their performance depends on particle morphology. Currently, no dynamic model for the prediction of the development of the morphology of multiphase latex particles is available. In this work, a model was developed for the prediction of the dynamic development of the morphology of multiphase waterborne systems, such as polymer-polymer and polymer-polymer- inorganic hybrids
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