Low noise operation of a DC SQUID in a large β regime and its application to the design of a high Tc SQUID

The noise characteristics of a large-β DC SQUID (superconducting quantum interference device) have been studied experimentally in the case where the SQUID inductance is resistively shunted. Energy resolutions of the SQUIDs, ε, have been measured for various β values up to β=19. It is shown experimentally that the energy resolution of these large-β SQUIDs is as small as that of the conventional SQUID with β=1. For example, an energy resolution of 25h was obtained for β=12 (L= 340 pH and I₀=37 μA), where h is Planck´s constant. The experimental results agree quantitatively with theoretical ones. It is also shown that the present resistivity shunted inductance scheme is very useful in developing SQUIDs operating at T=77 K with high-T_c superconductors, where the value of β will necessarily be large in order to avoid noise effects. Numerical simulations of the performance of the high-T_c SQUID are presented