“Multi-frequency Superconducting Cavity Stabilized Oscillators (SCSO) for Quantum-Gas Measurements and Gravitational Physics”, T. A. Corcovilos., D. M. Strayer, N. Asplund. and N.-C. Yeh. Journal of Low Temperature Physics134, 431 (2004).
http://link.springer.com/article/10.1023/B%3AJOLT.0000012591.58110.9c