Tunnelling of the 3rd kind (Vol. 44 No. 4)
The incoming atom a emits a virtual photon c and enters an excited state b. After traversing the barrier they recombine and return to the original state. The mirrors on either side represent an optical cavity used to enhance the coupling of photons to the atom. The black walls denote potential barriers blocking both a and b, confining the atom and preventing contact with the mirrors.
It is a fundamental property of quantum field theory that averaging over quantum fluctuations results in an effective theory that is non-local. Furthermore, it is possible that this effect can be demonstrated using a cavity quantum electrodynamics setup. In the proposed configuration (sketched in figure) the effective non-locality becomes evident in that an atom has a finite probability to traverse an arbitrarily high potential barrier. The atom, a, can "split" into a virtual excited state, b, and a virtual photon, c, which do not interact with the barrier, and can consequently cross it and recombine into the original atomic internal state. This so-called "tunnelling of the third kind" is distinct from regular quantum tunnelling (the "first kind") in that it relies upon the many-body interactions inherent to quantum field theory but absent from non-relativistic quantum mechanics; it is a purely quantum field theoretic effect.
Aside from its novelty as a gedankenexperiment, the process has the potential to stimulate some very interesting experiments in quantum optics and cold atom physics. Moreover, it may serve as a demonstration for "light-shining-through-walls" experiments that use comparable effects (real particle conversion - "tunnelling of the second kind") to search for particles beyond the Standard Model.
S. A. Gardiner, H. Gies, J. Jaeckel and C. J. Wallace, ‘Tunnelling of the 3rd kind: A test of the effective non-locality of quantum field theory', EPL, 101, 61001 (2013)
[Abstract]
