Detection of elastic photon-photon scattering through four-wave coupling
  According to the theory of quantum electrodynamics, photon-photon scattering can take place via exchange of virtual electron-positron pairs. Effectively, the interaction can be formulated in terms of non-linear corrections to Maxwell's equations, and hence may be treated by classical non-linear electrodynamics. Due to the strong electromagnetic fields needed to reach any noticeable effect, photon-photon scattering has not yet been observed experimentally, but recent improvements in laser technology have increased the possibility of direct detection. A verification of the phenomena would be of great scientific value as a confirmation of quantum electrodynamics. In this thesis the possibility of direct detection of elastic photon-photon scattering through four-wave coupling is investigated, both for current and future systems. It is shown how three colliding laser pulses satisfying certain matching conditions, can generate scattered radiation in a fourth resonant direction. The interaction is modeled, and the number of scattered photons is estimated for optimized configurations of incoming pulses, both for the Vulcan laser at the Rutherford Appleton Laboratory as well as for the planned XFEL at DESY. An experiment using the Vulcan laser is predicted to produce 22 scattered photons per shot, an amount which is definitely detectable. The usefulness of the XFEL highly depends on future developments in laser technology, but a realistic estimation gives a result of 30000 scattered photons per second. 
E. Lundstrom