In quantum description, vacuum is the state of lowest-possible energy density. In contrast to classical physics, that is at finite, non-zero energy density – due to quantum nature. Secondly, Heisenberg’s uncertainty principle allows for energy fluctuations on very short time scales, and observations are the outcome of many possibilities. In essence, vacuum contains fleeting fields and particles, know as virtual particles or vacuum fluctuations.
A classic thought experiment considering those pairs is the “Schwinger effect”: Imagine an external electric field strong enough to pull a transient virtual particle-antiparticle pair apart in such way that the work done is equivalent to the pair’s rest mass. Due to mass-energy equivalence, the pair becomes real and matter is effectively created out of nothing (well, converted from the field’s energy) – as inversion of particle-antiparticle annihilation.
Vacuum Birefringence — a Helmholtz lighthouse experiment
Before reaching field strengths for such direct observation of virtual pairs they also have indirect and rather weak effects. In particular, the virtual pairs can be partially aligned, rendering vacuum as optical medium with a refractive index different from unity. This is in general coined light-by-light scattering, in contrast to classic electrodynamics with its superposition principle. Light-by-light scattering contributes to many quantum effects as minor contribution. For isolated detection, many pathways are conceivable like frequency conversion (change of color), refraction (change of direction) or birefringence (change of polarization), upon which we focus together with HI Jena, DESY and XFEL.
This experiment relies substantially on precision X-ray polarimetry, being the expertise of the X-ray optics group at the Helmholtz-Institute Jena. With a very small probability, a polarization flip may occur due to the ultrastrong electric field in the optical laser focus. Hence an analyser, set after the interaction, should suppress a large amount of the unflipped photons. Also the initial polarization should have an extreme purity that no photons of the later flipped polarization are already in. The HI Jena group has gained the understanding of providing suppression ratios and purities of better than 1:1010 (status 2020), strongly depending on source and beam parameters.
A second challenge is the preservation of a high purity when using refractive X-ray lenses (CRLs). At such high level of polarization purity, the impact of material properties was so far not of interest and are currently being studied.
On the side of the optical laser, driving the effect, we have to put efforts into reaching the outmost intensity in the focus. Laser plasma particle acceleration experiments show that especially the temporal overlap in the focus is non-trivial. Furthermore an intensity benchmark would be highly recommended.
The last move in completing the puzzle is to precisely overlap the pulses in space and time and to preserve the overlap for the duration of the experiment.