Ultrahigh finesse Fabry-Pérot superconducting resonator

Abstract

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 finesse of 4.6 × 10 9 , the highest ever reached for a Fabry-Perot in any frequency range. This result opens many perspectives for quantum information processing, decoherence and non-locality studies.PACS numbers: 42.50. Pq, Since Bohr-Einstein's photon box thought experiment, storing a photon for a long time has been a dream of physicists. Cavity quantum electrodynamics (CQED) in the microwave domain comes closest to this goal. Photons are trapped in a superconducting cavity and probed by atoms crossing the field one at a time. Experiments with circular Rydberg atoms and Fabry-Perot resonators have led to fundamental tests of quantum theory and various demonstrations of quantum information procedures [1]. The open geometry of the cavity is essential to allow a perturbation-free propagation of long-lived atomic coherences through the mode. With this cavity structure, however, the field energy damping time T c is very sensitive to geometrical mirror defects, limiting T c to ≃ 1 ms in previous experiments. We report here the realization of a Fabry-Perot resonator at ω/2π = 51 GHz, with T c = 130 ms. The cavity quality factor Q is 4.2 × 10 10 and its finesse 4.6 × 10 9 , the highest ever achieved in any frequency domain for this geometry. This important step opens the way to many CQED experiments. Quantum non-demolition detection of a single photon [2] and generation of mesoscopic non-local quantum superpositions [3] are now accessible. Long term storage of single photon fields opens bright perspectives for quantum information processing. These high-Q cavities are also promising for the stabilization of microwave oscillators or for the search of exotic particles [4].A picture of the cavity C with the top mirror removed is shown in Fig. 1. The mirrors have a diameter D 0 = 50 mm. The distance between their apexes is L = 27.57 mm. Their surface is toroidal (radii of curvature 39.4 and 40.6 mm in two orthogonal planes). The two TEM 900 modes near 51.099 GHz with orthogonal linear polarizations are separated by 1.2 MHz. This large frequency splitting is essential to ensure that atoms are efficiently coupled to a single mode only. The mirrors are electrically insulated. A static electric field parallel to the cavity axis is applied between them to preserve the circular states and to tune the atomic transition via the Stark effect [1]. The 1 cm spacing between mirror edges is partly closed by two guard rings improving the static field homogeneity in C. The atoms of a thermal beam enter and exit the cavity through two large ports (1 cm × 2 cm) so that they never come close to metallic surfaces, preserving them from patch effect stray fields. This ensures a good transmission of atomic coherences through the cavity [2]. Four piezoelectric actuators are employed to translate one of the mirrors and to tune the cavity (within ±5 MHz) wit...

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