Limit characteristics of two-junction magnetic flux detector

The sensitivity of a two-Josephson-junction interferometer as a magnetic flux detector (dc SQUID) is estimated. The minimum value of the basic SQUID characteristic-noise equivalent temperature δT = (δΦ)²/kBL appears to be as low as for the best rf SQUID mode:(\deltaT)_{\min}/(\Deltaf)\cong 3 T/f_{o}. HereTis an operating temperature, fois a characteristic frequency of the Josephson junctions and\Deltafis a post-detector bandwidth. For a typical value of SQUID inductance L this limit corresponds to flux sensitivity of the order of 10^-7Φo/Hz^1/2.