Radio frequency measurements on a superconducting ring containing a single weak link

An accurate measurement of the pair quasiparticle interference term of the Josephson equations was made. These experiments were performed using a standard RF SQUID circuit consisting of a niobium point contact weak link incorporated into a superconducting ring which was coupled to a tank circuit. A toroidal configuration was used for the superconducting ring in order both to minimize its inductance and to shield it from extraneous fields. The tank circuit was driven by an RF oscillator near its resonant frequency approximately 30 Mhz.