Thermal noise in a high Q cryogenic resonator

In order to evaluate the feasibility of a mixed mechanical and electrical multimode matching network for a resonant gravitational wave detector, current noise measurements were performed on a high quality factor inductance-capacitance resonator, based on a superconducting coil, by using a dc superconducting quantum interference device weakly coupled to the coil. We present a method to identify different noise sources in the system by their dependence on the temperature or on the resonator quality factor. Measurements performed at quality factors up to 10(6) in the temperature range 1.2-4.2 K, show that the total measured noise is only contributed by the resonator thermal noise. (C) 1999 American Institute of Physics. [S0034-6748(99)04503-7].