Quantum electrodynamics predicts that the vacuum must behave as a nonlinear optical medium: the speed of light should be modified when the vacuum is stressed by intense electromagnetic fields. This optical phenomenon has not yet been observed. The DeLLight (deflection of light by light) experiment aims to observe the optically induced index change of vacuum, a nonlinear effect which has never been explored. The experiment is installed in the LASERIX facility at IJCLab, which delivers ultrashort intense laser pulses (2.5 J per pulse, each of 30 fs duration, with a 10 Hz repetition rate). The proposal is to measure the refraction of a probe laser pulse when crossing a transverse vacuum index gradient, produced by a very intense pump pulse. The refraction induces a transverse shift in the intensity profile of the probe, whose signal is amplified by a Sagnac interferometer. In this article we describe the experimental method and setup, and present the complete theoretical calculations for the expected signal. With a minimum waist at focus of $5\mu \mathrm{m}$ (corresponding to a maximum intensity of $\sim 3\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$), and with the nonlinear vacuum index derived from QED, the expected refraction angle is 0.13 prad. First results of the interferometer prototype are presented. It is shown that an extinction factor $\mathcal{F}=0.4\times {10}^{-5}$ (corresponding to a signal amplification factor of 250) and a spatial resolution ${\sigma }_y=10\mathrm{nm}$ are achievable. The expected signal is then about 15 pm, and could be observed at a 5-sigma confidence level with about one month of collected data.

中文翻译：

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观察光学诱导的真空指数变化的实验

量子电动力学预测真空必须表现为一种非线性光学介质：当真空受到强电磁场的作用时，光速应该改变。尚未观察到这种光学现象。DeLLight（光偏转）实验旨在观察光学诱导的真空指数变化，这是一种从未探索过的非线性效应。实验安装在IJCLab的LASERIX设施中，该设施提供超短的强激光脉冲（每个脉冲2.5 J，持续时间为30 fs，重复频率为10 Hz）。该提议是在跨越由非常强的泵浦脉冲产生的横向真空指数梯度时测量探针激光脉冲的折射。折射会引起探针强度分布的横向偏移，Sagnac干涉仪会放大其信号。在本文中，我们描述了实验方法和设置，并给出了预期信号的完整理论计算。最小腰围为$5\mu \mathrm{米}$ （对应于最大强度为 $〜3\times {10}^{20}\mathrm{w\; ^}/{\mathrm{厘米}}^2$），并根据QED得出非线性真空指数，则预期的折射角为0.13 prad。介绍了干涉仪原型的初步结果。表明消光因子$\mathcal{F}=0.4\times {10}^{-5}$ （对应于250的信号放大系数）和空间分辨率 ${\sigma }_{ÿ}=10\mathrm{纳米}$是可以实现的。然后，预期信号约为15 pm，并且可以在5σ置信度下观察到大约一个月的数据。