YBaCuO input coils with low T_c and high T_c SQUIDs

We have efficiently coupled single-level YBaCuO (YBCO) input coils to superconducting quantum interference devices (SQUIDs) using coils and SQUIDs made on separate substrates and sandwiched together. At 4.2 K, the coupling efficiency α to a low noise Nb-based SQUID was ∼0.76, with no increase in flux noise. At 77 K, the coupling efficiency of a YBCO coil to a TlBaCaCuO SQUID was ∼0.81. The magnetic field sensitivity of the coupled high T c SQUID at 77 K was 9 pT/√Hz at 1000 Hz, a five times improvement compared to the bare SQUID.