High Frequency Quantum Admittance and Noise Measurement with an On-chip Resonant Circuit

TL;DR

This paper demonstrates a method to measure high-frequency admittance and noise in a superconducting Josephson junction using an on-chip resonant circuit, revealing frequency-dependent quantum effects and zero-point fluctuations.

Contribution

It introduces a novel on-chip resonant circuit technique to probe high-frequency properties and noise in Josephson junctions, including emission and absorption noise at quantum levels.

Findings

Frequency-dependent singularities in admittance linked to superconducting density of states

Observation of zero-point fluctuation related absorption noise at low temperatures

Detection of emission noise at higher temperatures in the quantum regime

Abstract

By coupling a quantum detector, a superconductor-insulator-superconductor junction, to a Josephson junction \textit{via} a resonant circuit we probe the high frequency properties, namely the ac complex admittance and the current fluctuations of the Josephson junction at the resonant frequencies. The admittance components show frequency dependent singularities related to the superconducting density of state while the noise exhibits a strong frequency dependence, consistent with theoretical predictions. The circuit also allows to probe separately the emission and absorption noise in the quantum regime of the superconducting resonant circuit at equilibrium. At low temperature the resonant circuit exhibits only absorption noise related to zero point fluctuations, whereas at higher temperature emission noise is also present.