Minimum supercurrent for rf SQUID operation

Theory and observation on rf biased SQUIDs in the regime Isub(c)> phi0/2πL where phi0 is the magnetic flux quantum and L is the inductance of the SQUID are in good agreement. Recent studies have demonstrated that SQUIDs operating in this regime have reached a fundamental noise limit derived by Kurkijaervi and Kurkijaervi and Webb. Since the ultimate performance of any system incorporating a SQUID is limited by thermal noise, it is important to determine whether it is possible to reduce this noise by operating in the regime Isub(c)< phi0/2πL. In this paper a determination of the minimum supercurrent an rf biased thin film SQUID may have and still operate is reported. (Auth.)