Signatures of quantum vacuum nonlinearity in two-beam laser collisions

One of the many astonishing predictions of quantum field theory is the nonlinear nature of the quantum vacuum. In the context of quantum electrodynamics this enables classically forbidden, nonlinear interactions between electromagnetic fields in the vacuum. One much pursued idea to test this prediction is to excite the quantum vacuum with high-intensity laser pulses to emit signal photons that can be distinguished from the driving laser photons, for example, by their polarization or photon energy. A major experimental challenge is the small number of signal photons compared to the background photons. With the aim of identifying a potential discovery experiment of quantum vacuum nonlinearity, we provide theoretically firm predictions for two different quantum vacuum signatures with an emphasize on experimentally feasible setups. This covers on the one side the much studied effect of vacuum birefringence, for which we analyze an innovative setup using a single X-ray free-electron laser. Special attention is paid to the influence of the optical components in the setup on the laser pulse properties. On the other side, we perform an in-depth study of a signature of quantum vacuum nonlinearity that has received less attention so far, namely laser photon merging. The signal photons here have the outstanding property that they differ in photon energy from the background photons and can thus be isolated efficiently. We show that, using state-of-the-art technology, the merging signal can compete with the vacuum birefringence signal in terms of its suitability for a potential discovery experiment of quantum vacuum nonlinearity.