Sensitivity enhancement of Quantum Design dc superconducting quantum interference devices in two-stage configuration

The energy sensitivity of a direct current (dc) superconducting quantum int erference device (SQUID) can be improved if it is operated in a two-stage c onfiguration. Employing this technique, a commercial dc SQUID system was mo dified and made competitive with other sensors especially designed for very low noise applications. We report the noise measurements performed in the temperature range 4.2 K-25 mK. At 4.2 K, the coupled energy sensitivity obt ained with the two-stage dc SQUID was approximately ten times better than w ith a conventional readout electronics. The noise energy decreases linearly until approximately 300 mK, in good agreement with theoretical previsions. At lower temperature the hot-electron effect produces a saturation and the best energy sensitivity measured with open input coil is 35 h. (C) 2001 Am erican Institute of Physics.