Dispersive Thermometry with a Josephson Junction*

We have constructed a "Josephson bolometer" that detects weak electrical fluctuations and indicates the strength of these fluctuations via a shift in the frequency of a microwave resonator [1]. With this functionality, we demonstrate non-local primary thermometry by detecting the noise of a remote resistor. Importantly, even though this mode of operation resembles that of a noise thermometer, our device indicates the sensed temperature directly through a change in the phase of a microwave probe signal instead of relying on rectification with external electronics. Using a standard HEMT preamplifier, we achieve a noise-equivalent temperature of < 10 μK/√Hz at 50 mK with a power dissipation below 1 fW.

In addition, we combine DC biasing of the junction with microwave probing for a detailed study of the nonlinear junction-environment interactions. We observe strong modulation of the internal quality factor of the resonator, indicating stimulated microwave emission by the junction. All features are accurately described by a theoretical model with few free parameters.

[1] Phys. Rev. Applied 6, 024005 (2016).