CONTRIBUTION OF THERMAL NOISE TO THE LINE-WIDTH OF JOSEPHSON RADIATION FROM SUPERCONDUCTING POINT CONTACTS

Abstract

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.