Superconducting quantum interference device as a near-quantum-limited amplifier at 0.5 GHz

A dc superconducting quantum interference device (SQUID) with a resonant mi crostrip input is operated as an amplifier at temperatures down to 20 mK. A second SQUID is used as a postamplifier. Below about 100 mK, the noise tem perature is 52 +/- 20 mK at 538 MHz, estimated from measurements of signal- to-noise ratio, and 47 +/- 10 mK at 519 MHz, estimated from the noise gener ated by a resonant circuit coupled to the input. The quantum-limited noise temperatures are 26 and 25 mK, respectively. The measured noise temperature is limited by hot electrons generated by the bias current. (C) 2001 Americ an Institute of Physics.