Data acquisition and signal processing in the PVLAS experiment

Photonic crystals are commonly used in quantum electronics to generate harmonic or subharmonic beams from primary laser. One surprising result of quantum field theory is that vacuum behaves like a photonic crystal with a very weak nonlinearity. The virtual particle pairs associated to quantum fluctuations of charged fields, can be polarized by an external field and vacuum can thus become birefringent: the PVLAS experiment was originally meant to explore this strange quantum regime with optical methods. Since its inception PVLAS has found a new, additional goal: in fact vacuum can become a dichroic medium if we assume that it is filled with light neutral particles that couple to two photons, and thus PVLAS can search for exotic particles as well. PVLAS implements a complex signal processing scheme with some unusual features, the most outstanding being the uneven time spacing of the samples. Here we describe the double data acquisition chain and the data analysis methods used to process the experimental data.