Contribution of Thermal Noise to the Line-Width of Josephson Radiation From Superconducting Point Contacts

The line-width of the Josephson oscillations of a voltage-biased superconducting point contact has been measured between 1.4°K and 8°K, with bias resistors R between 1.7 × 10−10 Ω and 2.6 × 10−5 Ω. Within the experimental accuracy the line-width is proportional to RT, and is consistent with the estimated theoretical value 8kTR/Φ02, where k is Boltzmann's constant and Φ0 is the flux quantum. Line-widths below 0.1 Hz have been observed at 4.2°K for R = 1.7 × 10−10 Ω, providing an experimental upper limit to other noise sources and indicating that this is useful as a voltmeter and thermometer below 10−16 V and 10−4°K.