Very low noise, tightly coupled, dc SQUID amplifiers

• Published in: Applied physics letters Vol. 49; no. 17; pp. 1118 - 1120
• Main Authors: MUHLFELDER, B, BEALL, J. A, CROMAR, M. W, ONO, R. H
• Format: Journal Article
• Language: English
• Published: Melville, NY American Institute of Physics 27-10-1986
• Subjects: Applied sciences; Electronics; Exact sciences and technology; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; Superconducting devices; Sensitivity; Direct current; Superconducting quantum interferometer device; Superconductivity; Strong coupling; Amplifier; Background noise; Niobium; Thin film
• ISSN: 0003-6951; 1077-3118
• DOI: 10.1063/1.97440

We have fabricated and tested thin film, niobium edge junction, double transformer, dc superconducting quantum interference devices (SQUID’s) that were stable under room-temperature storage and thermal cycling and that had very good noise performance. The input inductance, approximately 1.7 μH, was large enough to facilitate good matching to many experiments. When the SQUID was operated as a small-signal amplifier, the minimum detectable energy per unit bandwidth (Se) was 5×10−33 J/Hz at 100 kHz, referred to the SQUID loop (uncoupled). The minimum detectable energy per unit bandwidth was 1.8×10−31 J/Hz at 100 kHz, referred to the input coil. The SQUID’s had good characteristics for flux-locked operation since the small signal Se was low over a substantial range of bias current and magnetic flux. For operation in a flux-locked feedback circuit, Se was 6×10−32 J/Hz at 1 kHz.