Polarization measurements and their perspectives at the low energy frontier.

Polarization measurements at low energy (1-2 eV) will soon probably be able to directly detect QED processes, such as photon-photon scattering. In principle, these techniques could also be used to probe particle production (Axion-Like Particles, Mini-Charged Par- ticles, Chameleons, to mention a few) at the Low Energy Frontier. Reaching an interesting unexplored zone in the ALP parameter space is, however, currently beyond their power, even in very optimistic scenarios. Photon regeneration experiments, on the other hand, have the potential to extend the reach of laser experiments beyond what is possible with polarization detection schemes. Using the resonant regeneration idea one can exploit the coherence properties of the ALP and photon fields to enhance the ALP-photon conversion probability by a factor which can be as large as 1010, or more, by using two frequency- locked Fabry-Perot optical resonators. The PVLAS-Phase II group in Trieste is attempting to build a table-top resonant regeneration pilot apparatus. At the moment, resonant re- generation appears as the sole purely laboratory-based method capable of investigating a region of the ALP parameter space now accessible, in part, only to astrophysical obser- vations, such as those from the CAST magnetic helioscope for ALPs. In the optimistic, though not a priori excluded, case of a positive signal one would obtain a discovery of great scientific value.