Prospects for high temperature SQUIDS in instrumentation

A SQUID (Superconducting Quantum Interference Device) is a detector of magnetic flux with an unrivalled sensitivity. In unit bandwidth a state-of-the-art system operating at 4.2 K can detect a flux of 1μΦ₀ (where Φ₀ is the flux quantum, equal to 2.07×10^-15 Wb), or it can be configured to detect magnetic field in the femtotesla range, or currents down to 0.05 pA, or voltages as low as 1 pV. This paper reviews the prospects in these roles for SQUIDs constructed from high T_c materials such as YBaCuO. The issues considered include the expected and currently demonstrated sensitivity of SQUIDs operating at 77 K, and the fabrication problems presented by the new materials. These are assessed after first outlining the well-established technology for low-temperature SQUIDs