Quantum reflection

Quantum reflection is a phenomenon in quantum physics in which a matter wave is reflected by an attractive potential .

Such a phenomenon is not possible in classical physics . For example, if two magnets were attracted, one would not expect one magnet to be suddenly repelled (e.g. shortly before they touch) in the opposite direction.

definition

Quantum reflection became an important field in 21st century physics. In a workshop on quantum reflection, the following definition was proposed:

“Quantum reflection is a classically counterintuitive phenomenon whereby the motion of particles is reverted“ against the force ”acting on them. This effect manifests the wave nature of particles and influences collisions of ultracold atoms and interaction of atoms with solid surfaces. "

“Quantum reflection is a phenomenon that contradicts classical intuition, in which the movement of a particle is reversed“ opposite to the attacking force ”. This phenomenon confirms the wave character of particles and influences collisions between ultracold atoms and the interaction of atoms with solid surfaces. "

The observation of quantum reflections has been made possible by advances in holding and cooling atoms. The application of this effect has only just begun and has promising applications.

One-dimensional approximation

Usually one first considers the one-dimensional case of this phenomenon. The potential thus has translation symmetry in two directions (e.g. and ), so that only a single coordinate ( ) is important. In this case we can look at the reflection of a slow neutral atom by a solid surface. ${\displaystyle y}$ ${\displaystyle z}$ ${\displaystyle x}$

The atom is in free space close to the surface and polarizable . A combination of the pure van der Waals interaction with the associated Casimir interaction causes the atom to be attracted to the material surface. The Casimir force dominates at greater distances from the surface, while the interaction is determined by the Van der Waals force at a short distance. The transition area is not clearly defined as it depends on the specific nature and quantum state of the approaching atom.

A quantum reflection of the atom can now take place in the areas of space in which the WKB approximation of its wave functions collapses. This approximates the local wavelength of the movement towards the surface as:

${\displaystyle \lambda \left(x\right)={\frac {h}{\sqrt {2m\left(E-V\left(x\right)\right)}}}}$

in which

• ${\displaystyle h}$the Planck constant
• ${\displaystyle m}$ the mass of the atom
• ${\displaystyle E}$ his energy
• ${\displaystyle V(x)}$ the potential is.

The approximation makes no sense if the variation of the atomic wavelength is significantly higher than its own length (e.g. if the gradient is very steep, regardless of the sign of the potential): ${\displaystyle V(x)}$

${\displaystyle \left|{\frac {d\lambda (x)}{dx}}\right|\sim 1}$

Parts of the atomic wave functions can be reflected in these spatial areas. For slow atoms, such a reflection is possible in the region of the comparatively fast variation of the Van der Waals potential near the material surface.

The phenomenon is comparable to the situation when light passes from one material in a small area of ​​space into another material with a significantly different refractive index : regardless of the sign of the difference in the refractive indices, there is a reflection component of the light at the transition point.

The quantum reflection of the surface of a solid wafer enables a quantum optical analogy to a mirror - the atomic mirror - with high precision.

Individual evidence

1. ↑ Quantum Reflection, workshop; October 22-24, 2007, Cambridge, Massachusetts, USA
2. ^ F. Shimizu: Specular Reflection of Very Slow Metastable Neon Atoms from a Solid Surface . In: Physical Review Letters . 86, 2001, pp. 987-990. doi : 10.1103 / PhysRevLett.86.987 .
3. ^ H. Oberst, Y. Tashiro, K. Shimizu, F. Shimizu: Quantum reflection of He * on silicon . In: Physical Review A . 71, 2005, p. 052901. doi : 10.1103 / PhysRevA.71.052901 .