Characterizing cryogenic amplifiers with a matched temperature-variable noise source

TL;DR

This paper introduces a cryogenic microwave noise source with adjustable temperature, enabling precise noise measurements of amplifiers at millikelvin temperatures for quantum computing applications.

Contribution

A novel, temperature-variable cryogenic noise source that allows accurate noise characterization of microwave amplifiers without significant heating effects.

Findings

Achieved system noise temperatures as low as 680 mK at 5.7 GHz.

Demonstrated the noise source's effectiveness in amplifier noise measurements.

Validated the noise source's application in solid-state qubit readout validation.

Abstract

We present a cryogenic microwave noise source with a characteristic impedance of 50 $\Omega$, which can be installed in a coaxial line of a cryostat. The bath temperature of the noise source is continuously variable between 0.1 K and 5 K without causing significant back-action heating on the sample space. As a proof-of-concept experiment, we perform Y-factor measurements of an amplifier cascade that includes a traveling wave parametric amplifier and a commercial high electron mobility transistor amplifier. We observe system noise temperatures as low as $680^{+20}_{-200}$ mK at 5.7 GHz corresponding to $1.5^{+0.1}_{-0.7}$ excess photons. The system we present has immediate applications in the validation of solid-state qubit readout lines.