Search for non linearity of the electromagnetic vacuum in intense field – DeLLight

The quantum electrodynamics theory (QED) predicts that the vacuum optical refractive index is modified when the vacuum is stressed by intense electromagnetic fields. The vacuum is expected to be a nonlinear optical medium, much like other standard optical media. This effect has never been observed experimentally. Today, the most sensitive experimental test is the search for vacuum birefringence with a sensitivity an order of magnitude above the QED prediction. The goal of the DeLLight project (Deflection of Light by Light) is to observe not birefringence but a global change of the vacuum refractive index by using ultra-short and ultra-intense laser pulses, delivered by the LASERIX facility in the Laboratoire de l’Accélérateur Linéaire (LAL). The novel idea is to measure the refraction (a coherent optical effect at macroscopic scale, enhancing the signal) of a probe laser pulse crossing a transverse vacuum index gradient. The latter is produced by the interaction of two counter-propagating intense pulses, used as pump pulses. The refraction of the probe pulse is detected with a Sagnac interferometer by measuring with a CCD camera a transversal shift of the intensity profile in the dark output of the interferometer.

In order to reach a high experimental sensitivity, one needs a strong focusing of the pump beams (a waist at focus as small as possible), a strong intensity extinction in the dark output of the interferometer, and a high spatial resolution for the position measurement of the intensity profile on the CCD. With challenging but realistic specifications of the DeLLight experiment, i.e. an extinction factor F=10^(-5) (value measured with the current prototype), a spatial resolution of 10nm (30 nm obtained today with the prototype), and a waist at focus w0=5micron, numerical simulations show that the predicted nonlinear QED signal can be observed at 3 sigma statistical confidence level with about 6 days of collected data. Conversely, if one assumes specifications already reached with our first prototype and a very conservative value of the waist at focus (w0=20micron), the expected sensitivity is identical to the best sensitivity obtained by the birefringence experiment. We emphasize that the search for a global change in the vacuum index has been performed only once by R.V. Jones in 1960, with a sensitivity 10 orders of magnitude lower than the theoretical prediction. The DeLLight project therefore offers a substantial improvement on sensitivity with a real potential for discovery, with results published in a high impact journal.

We propose a 3-year program. The first stage of the program is to complete the DeLLight prototype (currently under development) with the requested means for optical elements. The goal of this prototype (demonstrator) is to validate the DeLLight experimental method and its sensitivity by measuring the index gradient produced in a gas at low pressure (the so-called nonlinear Kerr effect) with femtosecond mJ pulses below the plasma threshold. Automatic control of the optical alignment will be developed and validated with this prototype. The second stage is to focus two counter-propagating intense pulses (1.25 J per pulse, 30 fs) with a stable overlap and a beam waist at focus as small as possible, and to install the final Sagnac interferometer (of larger dimensions) in the interaction area. These two stages completed, the DeLLight measurement in vacuum will start. Two phases of running are planned with different specifications for the focus of the two pump pulses and for the level of vacuum. Two thirds of the requested means are for equipment, and one third for the funding of a 3-years PhD.