Ultra-low-noise tunnel junction dc SQUID with a tightly coupled planar input coil

Abstract

We present a new superconducting quantum interference device (SQUID) that combines ultra‐low‐noise performance with tight coupling to a useable input coil. The inductive loop of the SQUID consists of a superconducting square washer that is slit in a low inductance fashion from its central hole to an outside edge where two Josephson tunnel junctions are incorporated. The inductance of the SQUID is about 90 pH, and each junction has a critical current of 15 μA and a resistive shunt of 4.5 Ω. The 190‐nH input coil is a planar spiral of 50 turns situated immediately above the washer and coupled to the SQUID with a coupling constant k² of 0.86. The SQUID has a minimum coupled energy sensitivity of 19h at 4.2 K and 8h at 1.5 K, in good agreement with theory. The crossover between white and low frequency noise occurs at ∼250 Hz.