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

TL;DR

This work advances the experimental detection of the QED-predicted optical nonlinearity of vacuum by combining a Sagnac interferometer with a High-Frequency Phase Noise Suppression strategy. By splitting the probe into a prompt and a delayed pulse, the method exploits strong prompt–delay correlations to cancel interferometric phase noise from mechanical vibrations, achieving a near shot-noise spatial resolution of about $\sigma_y \approx 46$ nm and a measured displacement consistent with zero in the absence of pump. The study identifies residual noise sources, demonstrates partial suppression via back-reflections, and validates a notch-filter approach, collectively pushing toward quantum-limited sensitivity and a path to observing vacuum refraction under QED. These results establish a robust framework for high-precision, interferometric measurements of ultra-small vacuum nonlinearities with potential applications to fundamental QED tests.

Abstract

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.

Figures (14)

• Figure 1: Sagnac interferometer layout with pump injection.
• Figure 2: A simplified scheme of the optical design of DeLLight experiment with HFPNS method setup.
• Figure 3: A CCD image of the intensity profiles of the prompt (top) and delay (bottom) probe pulses, recorded in the dark output of the interferometer. The interference signals is located in the central part. The two opposite lateral spots correspond to the back refections on the rear side of the beamsplitter. The white dotted square areas correspond to the Region of Interest (RoI) used to calculate the barycenters.
• Figure 4: The measured barycenters of the interference intensity profiles within a square analysis window of size $w_{\mathrm{RoI}} = w_{\mathrm{FWHM}}$ for the prompt (upper plots) and delayed (bottom plots) beams, and for the OFF events (left plots) and the ON events (right plots). In all cases, a slow correlated temporal drift is clearly observed across all the measurement sequence.
• Figure 5: Distribution of the standard ON-OFF signal $\Delta y_{\mathrm{Standard}}$, obtained with direct subtraction between successive ON and OFF barycenter measurements of the interference intensity profile, applied on a sample dataset with 3500 successive laser shots at 10 Hz, corresponding to 1750 ON-OFF measurements. The right panel shows the fitted Gaussian distribution on the obtained data.