YBaCuO input coils with low T c and high T c SQUIDs

Abstract
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 Tc SQUID at 77 K was 9 pT/√Hz at 1000 Hz, a five times improvement compared to the bare SQUID.