Noise in a Point-Contact DC SQUID.

Measurements of five point-contact dc SQUIDs (Superconducting QUantum Interference Devices) are presented. The energy resolution and other parameters of these SQUIDs are examined and compared with the predictions of the Resistively Shunted Junction model. For these SQUIDs, the measured minimum energy resolution was approximately 2 times 10^{-30} J/Hz. Excess noise in the point contacts was found to limit the energy resolution of the SQUIDs. The typical parameters and the noise obtained for our point-contact Josephson junctions are compared with the results that have been reported by others. The relatively low critical currents of our point contacts and the effect that the resonant cavity of the point-contact dc SQUID had on the I-<{rm V}> characteristics give evidence that Joule heating was present in the junctions. The white voltage noise spectral density was found to have an approximately parabolic dependence on the average voltage < {rm V}> for bias currents I larger than the critical current {rm I_{o}}. While this parabolic dependence could have its origin in heating effects, it is found that the amplitude of the noise cannot be explained in terms of a heating model. Thus, another mechanism appears to be responsible for the excess noise which has not been identified at the present time. The low-frequency noise spectral density of the point contacts has also been measured and parameterized in terms of the equation S_{rm 1/f} = A^{2} (<{rm V}>/1 muV)^gamma /(f/1 Hz)^alpha, where alpha, gamma, and A are constants, <{rm V}> is the average voltage across the junction, and f is the frequency. We find 0.6 < alpha < 2.0, 1.4 < gamma < 2.6, 0.03 nV/Hz^{1/2} < A < 0.3 nV/Hz^{1/2 } for I ~ {rm I_{o}} and 0.001 nV/Hz^{1/2} < A < 0.05 nV/Hz^ {1/2} for I > {rm I_{o}}..