  The aim of the DeLLight (Deflection of Light by Light) experiment is to
observe for the first time the optical nonlinearity in vacuum, as predicted by
Quantum Electrodynamics, by measuring the refraction of a low-intensity focused
laser pulse (probe) when crossing the effective vacuum index gradient induced
by a high-intensity focused laser pulse (pump). The deflection signal is
amplified by using a Sagnac interferometer. Here, we report the first
measurement performed with the DeLLight pilot interferometer, of the deflection
of light by light in air, with a low-intensity pump. We show that the
deflection signal measured by the interferometer is amplified, and is in
agreement with the expected signal induced by the optical Kerr effect in air.
Moreover, we verify that the signal varies as expected as a function of the
pump intensity, the temporal delay between the pump and the probe, and their
relative polarisation. These results represent a proof of concept of the
DeLLight experimental method based on interferometric amplification.
