Dispersive thermometry with a Josephson junction coupled to a resonator

TL;DR

This paper introduces a dispersive thermometry technique using a Josephson junction in a resonator, enabling precise temperature measurements from 300 mK to below 100 mK with high bandwidth and low noise.

Contribution

It presents a novel thermometry method based on phase diffusion in a Josephson junction coupled to a resonator, with a comprehensive theoretical model based on dynamical Coulomb blockade.

Findings

Achieved primary thermometry from 300 mK to below 100 mK.

Demonstrated high-bandwidth operation at 7.5 MHz.

Attained a noise-equivalent temperature below 10 μK/√Hz.

Abstract

We have embedded a small Josephson junction in a microwave resonator that allows simultaneous dc biasing and dispersive readout. Thermal fluctuations drive the junction into phase diffusion and induce a temperature-dependent shift in the resonance frequency. By sensing the thermal noise of a remote resistor in this manner, we demonstrate primary thermometry in the range from 300 mK to below 100 mK, and high-bandwidth (7.5 MHz) operation with a noise-equivalent temperature of better than 10 $\mathrm{\mu K/\sqrt{Hz}}$. At a finite bias voltage close to a Fiske resonance, amplification of the microwave probe signal is observed. We develop an accurate theoretical model of our device based on the theory of dynamical Coulomb blockade.