Quantum electrodynamics (QED) predicts that the vacuum must be a non-linear optical medium: the speed of light in 
the vacuum should decrease when the vacuum is subjected to intense electromagnetic fields. This amazing property of
vacuum has never been observed before. The DeLLight (Deflection of Light by Light) experiment seeks to measure this
effect using ultra-intense femtosecond laser pulses produced by the LASERIX platform (E =2.5J, 30fs, 10Hz) at the
IJCLab (Université Paris-Saclay). The innovative method of DeLLIght is to measure by interferometry the refraction of a
laser pulse (probe) of low energy, induced by the vacuum index gradient produced by an external pulse (pump) of high
intensity. The refraction of the probe pulse is detected using a Sagnac interferometer.
In this colloquium, I will start by explaining the scientific motivations of the study of the optical vacuum index, and the
few rare existing experimental tests. I will then describe the experimental method and the technical challenges of the
DeLLight project. Finally I will present the recent results obtained with the DeLLight pilot experiment, with low energy
pump laser pulses in air, validating the experimental method.