Introduction

Since its prediction in 1936 by Euler, Heisenberg and Weisskopf in the earlier development of the Quantum Electrodynamic (QED) theory, the Vacuum Magnetic Birefringence (VMB) is still a challenge for optical metrology techniques. According to QED, the vacuum behaves as an optically active medium in the presence of an external magnetic field. It can be experimentally probed with a linearly polarized laser beam. After propagating through the vacuum submitted to a transverse magnetic field, the polarization of the laser beam will change to elliptical and the parameters of the polarization are directly related to fundamental constants such as the fine structure constant and the electron Compton wavelength. Contributions to the VMB could also arise from the existence of light scalar or pseudo-scalar particles like axions that couple to two photons and this would manifest itself as a sizeable deviation from the initial QED prediction. On one side, the interest in axion search, providing an answer to the strong-CP problem lies beyond particle physics since such hypothetical neutral light spin-zero particle is considered as one of the good dark matter candidates, and the only non-supersymmetric one. The cosmological problems concerning dark matter and dark energy could then profit from results obtained from the purely laboratory experiment, like OSQAR. On the other side, the domain of physics that will be investigated with OSQAR is guaranteed by the QED vacuum polarization. The test of QED by measuring a predicted ellipticity of the order of 2x10-11 rad for a light beam propagating over 25 km in a 9.5 T field constitutes the best test of a theory never achieved so far i.e. at the level of 10-22 that corresponds to the absolute relative change of the vacuum refractive index. To measure both magnetic birefringence and linear dichroism of the vacuum, the experimental approach must focus on two main requirements, a strong transverse magnetic field and an efficient tool for the optical metrology. This project focuses on the development of the required optical measurement technique for which a breakthrough is expected to be achieved with respect to the present state of the art. Today, optical techniques enable to reach a sensitivity for ellipticity measurement of the order of 10-8 rad/(Hz)12. With this project a limit value below 10-10 rad/(Hz)12 is aimed to be achieved. One of the key idea of the proposed detection scheme is based on a high frequency modulation of the polarization of the laser beam that will probe the vacuum under high transverse magnetic field, leading to the creation of two sidebands far enough from the optical carrier. Then, with a hyper selective optical filter centred on one of the two sidebands, the optical carrier can be strongly rejected and consequently the modulation depth of the signal to measure will be increased by the same ratio. The development of this rejection filter which does not affect the sideband containing the relevant information will be one of the innovative achievements of this project. Concerning the strong transverse magnetic field required to obtain measurable effects from the polarized vacuum, one of the ideal implementations for the experiment is within long superconducting accelerator dipolar magnets such as the ones developed and manufactured for the Large Hadron Collider (LHC) under construction at CERN. This possibility was already addressed in the feasibility study published in two parts and a Letter of Intent was submitted to the CERN-SPSC committee to propose a re-use of long LHC prototype magnets providing a magnetic field up to 9.5 T over 14.3 meters long as well as existing CERN infrastructure. The feasibility study and the technical proposal was also presented during five international conferences or workshops - the later one being hosted at the Institute for Advance Study at Princeton - and received each time a positive feedback from the scientific community. In addition to fundamental scientific interests, laser measurement techniques developed for this project will impact on optical metrology techniques and a patent is aimed to be registered. Various scientific domains could profit from this development such as the precise characterisation of field strength, field angle and transfer function of magnets in general, and in particular those dedicated to future accelerator projects. The characterisation of plasmas that will be produced by ITER or the W7-X stellator could also profit from the novel optical metrology techniques developed for this project. Let us also notice that with this technique, coupled to pigtailed electro-optic or magneto-optic probes, a wide field of applications can be considered.