Optical polarimetry for fundamental Physics

Abstract:

The research project aims at developing an experimental method for measuring minute birefringences based on a polarimeter with heterodyne detection. The originality of the proposed instrument resides in a novel way of modulating the effect: two half-wave plates WP's co-rotating at a frequency  are placed inside a Fabry-Perot cavity (mirrors M's) enclosing the birefringent region, making the polarisation rotate at a frequency  = 4 while keeping the polarisation fixed on the mirrors. In this way the intrinsic birefringence of the reflective mirror's surface does not affect the measurement. As the measurement noise decreases as 1/ν, high modulation frequencies are pursued.

The project is motivated by the intent do design a polarimeter capable of performing the first measurement of the vacuum magnetic birefringence (VMB) [MBV], a minute quantum effect never observed directly. The method will allow also the determination of the birefringence of unaccessible optical elements like the large cryogenic mirrors of the next generation gravitational waves interferometers (Einstein Telescope).

VMB is related to the existence of the vacuum fluctuations. It stems from the 1935-36 Euler-Kockel-Heisenberg-Weisskopf Lagrangian density of the electromagnetic field [ED] corresponding to a non-linear set of Maxwell equations in vacuum. This Lagrangian, predicting photon-photon interaction, has been later confirmed in the framework of the Quantum Electrodynamics [QED]. With reference to the polarisation directions parallel and perpendicular to the magnetic field, VMB is given by


Magnetic dichroism and birefringence of vacuum may also derive from the existence of a neutral scalar light particle coupling to two photons. Such a particle has been postulated in Quantum Chromodynamics with the name of "axion" to solve the so called strong CP problem, namely the absence of a violation of the CP symmetry in the QCD sector in spite of the theory not requiring it [PQ]. Today axion-like particles are candidate constituents of dark matter. Another source of birefringence and dichroism might be the existence of light particles with fractional charge [MCP], both Dirac fermions or scalar bosons.