A Cryogenic Variable Temperature Microwave Noise Source

We present a cryogenic microwave noise source with a characteristic impedance of 50 Ω designed to be installed in a coaxial line of a cryostat [1]. The bath temperature of the noise source is continuously variable between 0.1 K and 5 K without causing significant heating on the sample space. As a proof-of-concept experiment, we performed Y-factor measurements of an amplifier cascade that includes a traveling wave parametric amplifier and a commercial high electron mobility transistor amplifier. We observed 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 device we present has immediate applications in the characterization of solid-state qubit readout lines.
References: [1] S. Simbierowicz et al., arXiv:2009.03010 [quant-ph] (2020).