Nonlinear interactions of light with light are well known in quantum electronics, and it is quite common to generate harmonic or subharmonic beams from a primary laser with photonic crystals. One suprising result of quantum field theory is that because of the quantum fluctuations of charged fields, the same can happen in vacuum. The virtual charged particle pairs 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.
