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Superconducting microwave cavities have been proposed for the detection of gravitational waves in the high frequency range of the spectrum. The interaction of the gravitational wave with the cavity walls, and the resulting motion, induces the transition of some electromagnetic energy from an initially excited cavity mode to an empty one. The energy transfer is maximum when the frequency of the wave is equal to the frequency difference of the two cavity modes. In this paper, some ideas for the development of a tunable detector of gravitational waves around 10 kHz are discussed; the outline of a possible detector design and its expected sensitivity are also shown.

The design and test of a detector of small harmonic displacements is presented. The detector is based on the principle of the parametric conversion of power between the resonant modes of two superconducting coupled microwave cavities. The work is based on the original ideas of Bernard, Pegoraro, Picasso and Radicati, who, in 1978, suggested that superconducting coupled cavities could be used as sensitive detectors of gravitational waves, and on the work of Reece, Reiner and Melissinos, who, in 1984, built a detector of this kind. They showed that an harmonic modulation of the cavity length ℓ produced an energy transfer between two modes of the cavity, provided that the frequency of the modulation was equal to the frequency difference of the two modes. They achieved a sensitivity to fractional deformations of δℓ/ℓ ≈ 10 −17 Hz −1/2 . We repeated the Reece, Reiner and Melissinos experiment, and with an improved experimental configuration and better cavity quality, increased the sensitivity to δℓ/ℓ ≈ 10 −20 Hz −1/2 . In this paper the basic principles of the device are discussed and the experimental technique is explained in detail. Possible future developments, aiming at gravitational waves detection, are also outlined.

The electromagnetic properties of a prototype gravitational wave detector, based on two coupled superconducting microwave cavities, were tested. The radio-frequency (rf) detection system was carefully analysed. With the use of piezoelectric crystals small harmonic displacements of the cavity walls were induced and the parametric conversion of the electromagnetic field inside the cavities explored. Experimental results of bandwidth and sensitivity of the parametric converter versus stored energy and voltage applied to the piezoelectric crystal are reported. A rf control loop, developed to stabilize phase changes on signal paths, gave a 125 dBc rejection of the drive mode on a time scale of 1 h.

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