Digital SQUIDs based on Smart DROS

A Double Relaxation Oscillation SQUID (DROS) with the complete flux locked loop (FLL) circuitry on one single chip, a Smart DROS, has been realized. A superconducting storage loop interrupted by two write gates to add or extract flux quanta acts as an integrator that supplies the feedback flux to the DROS. The pulsed output of the Smart DROS enables the readout with a digital counter at room temperature. In this digital readout scheme, the number of pulses corresponds to the amount of applied flux. In principle, very large bandwidths together with high slew rates up to 108 Φ0/s can be achieved. We measured output voltage pulses and the corresponding power spectra of a 100 MHz prototype of the Smart DROS, as a first step towards a digital readout mode.