Quantum reflection
A classically behaving neutron or molecule will strike the same surface much like a thrown ball, hitting only at one atomic-scale location where it is either absorbed or scattered.
Observation of quantum reflection has become possible thanks to recent advances in trapping and cooling atoms.
This repulsion is responsible for the classical scattering one would expect for particles incident on a surface.
To reduce this part of the physical process, a grazing angle of incidence is used; this enhances the quantum reflection.
This requirement of small incident velocities for the particles means that a non-relativistic approximation to quantum mechanics is appropriate.
So far, one usually considers the single-dimensional case of this phenomenon, that is when the potential has translational symmetry in two directions (
In this case one can examine the specular reflection of a slow neutral atom from a solid state surface .
[6][7] Where one has an atom in a region of free space close to a material capable of being polarized, a combination of the pure van der Waals interaction, and the related Casimir-Polder interaction attracts the atom to the surface of the material.
The intermediate region is controversial as it is dependent upon the specific nature and quantum state of the incident atom.
In accordance with this approximation the wavelength of the gross motion of the atom system toward the surface as a quantity local to every region along the
is the potential it experiences, then it is clear that we cannot give meaning to this quantity where, That is, in regions of space where the variation of the atomic wavelength is significant over its own length (i.e. the gradient of
Such a reflection may occur for slow atoms experiencing the comparatively rapid variation of the Van der Waals potential near the material surface.
Irrespective of the sign of the difference in index, there will be a reflected component of the light from the interface.
Practically, in many experiments with quantum reflection from Si, the grazing incidence angle is used (figure A).
The excitation of atoms is not essential for the quantum reflection but it allows the efficient trapping and cooling using optical frequencies.
In addition, the excitation of atoms allows the registration at the micro-channel plate (MCP) detector (bottom of the figure).
At the MCP, there was observed relatively intensive strip of atoms which come straightly (without reflection) from the MOT, by-passing the sample, strong shadow of the sample (the thickness of this shadow could be used for rough control of the grazing angle), and the relatively weak strip produced by the reflected atoms.
Practically, the mounting and maintaining of this facility (not shown in the figure) is the heaviest job in the experiments with quantum reflection of cold atoms.
The possibility of an experiment with the quantum reflection with just a pinhole instead of MOT are discussed in the literature.
As was briefly mentioned above, the potential in the intermediate region between the regions dominated by the Casimir-Polder and Van der Waals interactions requires an explicit Quantum Electrodynamical calculation for the particular state and type of atom incident on the surface.
Thus the reflection could simply be explained by a repulsive force, which would make the phenomenon not quite so surprising.
Furthermore, a similar dependence for reflectivity on the incident velocity is observed in the case of the absorption of particles in vicinity of a surface.
Until 2006, the published papers interpreted the reflection in terms of a Hermitian potential;[11] this assumption allows to build a quantitative theory.
[12] A qualitative estimate for the efficiency of quantum reflection can be made using dimensional analysis.
In other words, the wavelength is small compared to the distance at which the atom may become reflected from the surface.
If this condition holds, the aforementioned effect of the discrete character of the surface may be neglected.
, which shows good agreement with experimental data for excited neon and helium atoms, reflected from a flat silicon surface (fig.1), see [10] and references therein.
Such a fit is also in good agreement with a single-dimensional analysis of the scattering of atoms from an attractive potential,.
[14] If one produces a surface consisting of a set of narrow ridges then the resulting non-uniformity of the material allows the reduction of the effective van der Waals constant; this extends the working ranges of the grazing angle.
Similar enhancement of quantum reflection takes place where one has particles incident on an array of pillars .
