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Cavité
Cavités Fabry-Perot supraconductrices
We use high quality Fabry Perot Niobium superconducting cavity. We have devoted considerable efforts for the improvement of these resonators in the past years. They reach now extremely long photon storage times, in the second range (0.13 s). We recall here the important steps in this development.
Very long damping times can be obtained straightforwardly with closed superconducting cavities. $Q$ values up to $10^12$ have been reported in the GHz frequency range, and cavity field lifetimes (...)
Miroirs supraconducteurs et assemblage
The very long cavity lifetimes reached recently rely on a new technology for the cavity mirrors, based on Niobium sputtering on a high quality copper surface.
Obtaining a high-quality niobium mirror is not an easy task. Niobium polishing is extremely difficult, since it has at the same time a large Young modulus and a very low plastic deformation limit. Our early mirrors made up of massive niobium always exhibit large-scale surface defects, wich severly limited the field lifetime. The (...)
Mesure de la durée de vie
We measure directly the lifetime of a classical coherent field stored in the cavity. We use atomic probes to detect the field amplitude decay .
In order to minimize surface defects, the superconducting mirrors do not have coupling irises at their apex. It is still possible to couple microwave into the cavity mode trough the residual diffraction loss channels. A fraction of the stored photons finally escape through the gap between the mirrors, diffused by surface defects. In the same way, a (...)
Accord et stabilité de fréquence
A very high frequency stability is required to preserve the phase coherence of the stored field over the cavity lifteime. At the same time, the cavity mode frequency must be adjusted easily.
The precise shape and thickness of the cavity mirrors is inferred from measurements at room temperature. The finite precision of the cavity assembly and of the thermal contraction measurements results in an uncertainty of a few MHz of the final cavity frequency.
We perform the cavity frequency tuning (...)
