Quantum Sonoluminescence

A reddit comment I made a year ago:

Cavitation produces sonoluminescence, which is a quantum phenomenon. Some people will tell you that it’s actually just plasma or something “like totally classical.” Those people are stuck in the 90s. The spectrum produced by cavitation is discrete and changes based on the pressure, temperature, solutes, and the overall geometry of the multibubble system. It probably has something to do with the surface at the last stages of collapse resembling distinct quantum states that only emit specific photon energies due to symmetry conditions or somesuch.
A paper submitted to arxiv Jul 30, 2012:

Environment-induced heating in sonoluminescence experiments

In the next section, we consider composite quantum systems with repeated energy-absorbing measurements on one systems component. A simple example of such a system is a well localised atom (or ion) in free space which interacts with the surrounding free radiation field. Another example is a trapped atom (or ion) with quantised motional states in a free radiation field. In these two systems, repeated energy-absorbing measurements on one system component – the free radiation field – occur naturally in the presence of a photon-absorbing environment, like the walls of the laboratory. When the atom (or ion) is driven by external laser fields, these measurements manifest themselves in the spontaneous emission of photons. Quantum optical models based on the assumption of environment induced photon-absorbing measurements are in general in very good agreement with experiments.

In the third section of this paper, we emphasise common features between ion trap and sonoluminescence experiments. More concretely, we assume that the particles inside the cavitating bubble are so strongly confined that their motion becomes quantised. In addition, we assume that a weak but highly inhomogenous electric field establishes a relatively strong coupling between the electronic states and the motional states of the trapped particles. Such a field occurs naturally during rapid bubble deformations.

[…]

What supports our thesis of the presence of a quantum optical heating mechanism in sonoluminescence experiments is the importance of the presence of atomic species inside the cavitating bubble. High temperatures in sonoluminescence experiments are achieved, for example, when there is a high concentration of nobel gas atoms. Compared to normal atoms and ions, nobel gas atoms are unlikely to be involved in chemical reactions. However, the electronic transitions of the valence electrons of nobel gas atoms lie in the ultraviolet regime [20]. This means, their w0 is significantly larger than the typical transition frequencies w0 of other particle species which lie in in the optical regime. For example, typical transition frequencies of Argon atoms are above 13eV, while transitions frequencies in the optical regime are below 1eV. As a result, particles with transitions in the optical regime can result in higher heating rates l (cf. Eq. (20)) then the typical l of a nobel gas atom. Recently, it has been found that sonoluminescence experiments in ionic liquids can exhibit much higher photon emission rates than sonoluminescence experiments with nobel gas atoms.
:P