Improved self-shielded d.c.-SQUID

We report on the design, fabrication and performance of a thin-film multiloop d.c.-SQUID, built in such a way to realize an efficient shielding against uniform ambient fields. The multiloop geometry is a design which allows one to reduce the SQUID inductance. This is an important item to improve the device sensitivity towards the quantum limit, but it has the drawback of a lower coupling coefficient with the input coil, compared to other geometries such as the washer one. In this new design, we improve the design of the input coil to overcome the problem of the coupling and arrange the SQUID loops in a gradiometric configuration to obtain self-shielding of the device.