Superconducting microstrip amplifiers with sub-Kelvin noise temperature near 4 GHz

We present measurements of an amplifier operating at 3.8 GHz with 150 MHz of bandwidth based on the microstrip input-coil resonance of a dc superconducting quantum interference device (SQUID) with submicron Josephson junctions. The noise temperature is measured using two methods: comparing the signal-to-noise ratio of the system with and without the SQUID in the amplifier chain, and using a modified Y-factor technique where calibrated narrowband noise is mixed up to the SQUID amplifier operating frequency. With the SQUID cooled to 0.35 K we observe a minimum system noise temperature of 0.55 $pm~0.13$ K, dominated by the contribution from the SQUID amplifier.

Microstrip superconducting quantum interference device amplifiers with submicron Josephson junctions: enhanced gain at gigahertz frequencies

We present measurements of an amplifier based on a dc superconducting quantum interference device (SQUID) with submicron Al-AlOx-Al Josephson junctions. The small junction size reduces their self-capacitance and allows for the use of relatively large resistive shunts while maintaining nonhysteretic operation. This leads to an enhancement of the SQUID transfer function compared to SQUIDs with micron-scale junctions. The device layout is modified from that of a conventional SQUID to allow for coupling signals into the amplifier with a substantial mutual inductance for a relatively short microstrip coil. Measurements at 310 mK exhibit gain of 32 dB at 1.55 GHz.