Reaching the quantum noise limit for interferometric measurement of optical nonlinearity in vacuum

Quantum Electrodynamics predicts that the vacuum must behave as a nonlinear optical medium:the vacuum optical index should increase when vacuum is stressed by intense electromagnetic fields.The DeLLight (Deflection of Light by Light) project aims to measure it by using intense and ultra-short laser pulses delivered by the LASERIX facility at IJCLab (Paris-Saclay University). Theprinciple is to measure by interferometry the deflection of a low-intensity probe pulse when crossingthe vacuum optical index gradient produced by an external high-intensity pump pulse. The detectionof the expected signal requires measuring the position of the interference intensity profile with a highspatial resolution, limited by the ultimate quantum noise. However, the spatial resolution is highlydegraded by the phase noise induced by the mechanical vibrations of the interferometer. In order tosuppress this interferometric phase noise, we have developed a new method, named High-FrequencyPhase Noise Suppression (HFPNS) method, based on the use of a delayed reference signal to correctany noise-related signal appearing in the probe beam. In this work, we present the experimentalvalidation of this novel method. The results demonstrate a robust path toward picometer-scalesensitivity and provide a key step toward the observation of QED-induced vacuum refraction.