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Physics World

Quantum reflection shields ultracold atoms

Harald Friedrich

Published Aug 1, 2004• 2 citations
Publisher

Abstract

When a ball rolls down a hill, the last thing you expect is for it to suddenly stop and then roll straight back up the hill. At the atomic scale, however, the wave nature of particles becomes important and can lead to a counterintuitive phenomenon called quantum reflection. This is a purely quantum-mechanical process in which a particle moving in a given direction can reverse its direction of motion without reaching a classical turning point. It is thus the counterpart of quantum tunnelling, in which a particle can penetrate a potential barrier in a way that is forbidden by classical mechanics.

DOI
10.1088/2058-7058/17/8/30

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When a ball rolls down a hill, the last thing you expect is for it to suddenly stop and then roll straight back up the hill. At the atomic scale, however, the wave nature of particles becomes important and can lead to a counterintuitive phenomenon called quantum reflection. This is a purely quantum-mechanical process in which a particle moving in a given direction can reverse its direction of motion without reaching a classical turning point. It is thus the counterpart of quantum tunnelling, in which a particle can penetrate a potential barrier in a way that is forbidden by classical mechanics.
Everyday macroscopic objects follow the laws of classical mechanics. For microscopic objects, such as atoms or nuclei, the wave character of their dynamics has to be taken into account within the framework of quantum mechanics. While wave-like behaviour can show up in various ways, one of the most striking is the tunnelling effect: some events that are classically forbidden for energetic reasons happen because the quantum particles can tunnel under a potential barrier. Similarly, events that are prohibited due to some other dynamical constraints can occur via a process known as dynamical tunnelling. Indeed, tunnelling plays a major role in a variety of physical phenomena, ranging from α-particle radioactivity to the current–voltage characteristics of transistors.
Quantum reflection refers to the reflection of particles by attractive long-range potentials in nonclassical regions of coordinate space without reaching a classical turning point. Recent advances in nanotechnology and in cooling processes of atoms and molecules give rise to the creation of hybrid systems involving ultracold atoms and nanostructures, where quantum reflection might play an important role. In this work we review some fundamental aspects of quantum reflection. These include the description of atom-surface interactions -which are originated from the vacuum fluctuations of the electromagnetic field-, the near-threshold characterization of quantum reflection, possible applications for trapping and guiding ultracold atoms, and the description of scattering processes in a basic hybrid system: ultracold atoms interacting with nanotubes.
Parallel analyses of matter penetrating a barrier are analyzed, one using electrodynamics, the other using quantum mechanics. Mathematical operations are performed to show the identical nature of the phenomena of reflection above the barrier and of penetration of the barrier in quantum mechanics and in classical electrodynamics. It is shown that, in reality, the tunneling effect is not a “purely quantum phenomenon”. Проведено параллельное рассмотрение прохождения материи через барьер методами электродинамики и квантовой механики. Математически строго получена абсолютная идентичность надбарьерного отражения и подбарьерного прохождения, как в квантовой механике, так и в классической электродинамике. Показано, что определение туннельного эффекта как «чисто квантового явления» – не соответствует действительности. Проведено паралельне дослідження проходження матерії через бар’єр методами електродинаміки та квантової механіки. Математично виключно отримана абсолютна ідентичність надбар’єрного відбиття та підбар’єрного проходження як у квантовій механіці так і в класичній електродинаміці. Показано, що визначення тунельного ефекту як «чисто квантового явища» – не відповідає дійсності.
We are in the process of building an experiment to study the tunneling of laser-cooled Rubidium atoms through an optical barrier. A particularly thorny set of questions arises when one considers the possibility of observing a tunneling particle while it is in the ``forbidden'' region. In earlier work, we have discussed how one might probe a tunneling atom ``weakly,'' so as to prevent collapse. Here we make some observations about the implications of a more traditional quantum measurement. Considerations of energy conservation suggest that attempts to observe tunneling atoms will enhance inelastic scattering, but not in a way which can be directly observed. It is possible that attempts to make such measurements may lead to experimentally realizable ``observationally assisted barrier penetration.''
‘Tunnelling’ is the metaphorical name given to the process, possible in quantum mechanics, but not in classical mechanics, whereby a particle can disappear from one side of a potential-energy barrier and appear on the other side without having enough kinetic energy to mount the barrier. One can think of this as a manifestation of the wave-nature of particles. The wavelength is larger if a particle is lighter. In particular electrons, being very light compared to atoms, have wavelengths as large or larger than atoms at energies found in the valence shells of molecules. Thus, they easily ooze through and around atoms and molecules. We are also concerned with the tunnelling of heavy particles: nuclei, atoms, molecules.