  We have built a microwave Fabry-Perot resonator made of diamond-machined
copper mirrors coated with superconducting niobium. Its damping time (Tc = 130
ms at 51 GHz and 0.8 K) corresponds to a &#64257;nesse of 4.6 x 109, the
highest ever reached for a Fabry-Perot in any frequency range. This result
opens novel perspectives for quantum information, decoherence and non-locality
studies.
