GINGER: an array of ring lasers for testing fundamental physics

Ring laser (RL) gyroscopes are, at present, the most precise sensors of absolute angular velocity. In the near future, their application is foreseen to provide ground based tests of General Relativity. We have recently proposed a tri-axial array of RLs that can reach the sensitivity, accuracy, and long term stability required to measure the inertial frame dragging induced by the rotating Earth, as predicted by General Relativity. The effect, also known Lense-Thirring effect, amounts for the Earth to 1 part in 109 of its rotation rate, thus requiring an unprecedented sensitivity and accuracy of experimental apparatus. An array of at least 3 RLs would allow us to measure not only the rotation rate, i.e. the angular velocity modulus, but also the angular velocity vector. In this way, having at disposal the time series of the daily estimate of Earth rotation vector from the International Earth Rotation and Reference System Service, it would be possible to isolate the Geodetic and Lense-Thirring contributions. Our proposal GINGER (Gyroscopes IN GEneral Relativity) is intended to push the present knowledge of RL physics and technology to achieve an accuracy in the estimation of the Earth rotation rate of 1 part in 109. In the experimental apparatus we have to account for systematic errors resulting from non linear dynamics of the active laser medium, and changes of the optical cavity geometry. The redundancy of the array, e.g. the addition of a ring almost parallel to the Earth rotation axis, should allow for the reduction of such errors at the level of the geometry control. In this contribution we describe the intermediate prototypes GP2 and GEMS (GINGER External Metrology System) devoted to control the geometrical fluctuations of a RL cavity and the 3D geometry of the RL array (dihedral angles among RLs), respectively.