Quantum ElectroDynamics experiments
Quantum electrodynamics (QED) is a physical theory that unifies quantum mechanics to electromagnetism. This theory enabled to explain very subtle phenomena in matter but its most astonishing predictions occur when physical conditions are extreme and the QED processes become nonlinear. In this regime, QED predicts (1) that the vacuum itself is a nonlinear medium which can modify the propagation of a light beam and (2) that it is possible to create matter from vacuum by concentrating enough energy. Such phenomena have never been observed directly, but their experimental study is crucial to verify the predictions of QED and better understand the matter.
One of the UPX group projects is to reach this extreme regime using the APOLLON laser, built on the Saclay plateau. To this end, two experimental approaches are considered.
- A) The “indirect” approach where the vacuum is not probed directly. The collision between a relativistic electron beam and an ultra-intense laser beam enables to drastically increase the laser field in the electrons’ frame. Despite their extreme energy, their behavior becomes quantic: they are discontinuously deflected by the laser field producing very high energy photons in the extreme ultraviolet (XUV) by Compton QED effect. These XUV photons will then collide with the high energy electrons for the creation of electron-positron pairs.
- B) The “direct” approach where the vacuum is directly probed. It requires concentrating enough light in a vacuum to reach extreme light fields. With the most powerful lasers in the world, more than 5 orders of magnitude are lacking to reach such intensities. Another strategy is to convert the laser energy to XUV wavelengths that can be focused more tightly. At such intensities, the most efficient wavelength “converters” are plasma mirrors. Plasma mirrors have a highly nonlinear response and reflect the laser field with (1) compression it in time, (2) conversion of a significant fraction of the energy towards the XUV, and (3) with plasma denting that produces a parabola that “over-focuses” the reflected beam. These combined effects enable to compress energy both temporally and spatially to increase the maximum intensity by several orders of magnitude.
This strategy will allow to probe the nonlinearity of the vacuum and to create electron-positron pairs in vacuum.
These two complementary strategies (A and B) will enable to probe nonlinear QED in different regimes.