PVLAS
PVLAS aims to carry out a test of quantum electrodynamics and possibly detect dark matter at the Department of Physics and National Institute of Nuclear Physics in Ferrara, Italy. It searches for vacuum polarization causing nonlinear optical behavior in magnetic fields. Experiments began in 2001 at the INFN Laboratory in Legnaro and continue today with new equipment.
Background
Nonlinear electrodynamic effects in vacuum have been predicted since the earliest days of quantum electrodynamics, a few years after the discovery of positrons. One such effect is vacuum magnetic birefringence, closely connected to elastic light-by-light interaction. The effect is extremely small and has never yet been observed directly.Although today QED is a very well-tested theory, the importance of detecting light-by-light interaction remains. First, QED has always been tested in the presence of charged particles either in the initial state or the final state. No tests exist in systems with only photons. More generally, no interaction has ever been observed directly with only gauge bosons present in the initial and final states. Second, to date, the evidence for zero-point quantum fluctuations relies entirely on the observation of the Casimir effect, which applies to photons only. PVLAS deals with the fluctuations of virtual charged particle-antiparticle pairs and therefore the structure of fermionic quantum vacuum: to leading order, it would be a direct detection of loop diagrams. Finally, the observation of light-by-light interaction would be an evidence of the breakdown of the superposition principle and of Maxwell's equations. One important consequence of a nonlinearity is that the velocity of light would depend on the presence or not of other electromagnetic fields. PVLAS carries out its search by looking at changes in the polarisation state of a linearly polarised laser beam after it passes through a vacuum with an intense magnetic field. The birefringence of the vacuum in quantum electrodynamics by an external field is generally credited to Stephen L. Adler, who presented the first general derivation in in 1971. Experimental investigation of the photon splitting in atomic field was carried out at the facility at the Budker institute in 1993-96.