Emission and absorption asymmetry in the quantum noise of a Josephson junction

TL;DR

This study measures quantum current fluctuations in mesoscopic devices, revealing a strong asymmetry between emission and absorption in the noise spectrum of a Josephson junction.

Contribution

It demonstrates the separate detection of emission and absorption noise components and uncovers asymmetry in current fluctuations of a Josephson junction at high frequencies.

Findings

Negative and positive frequency noise parts are detected separately.

Excess current fluctuations show strong emission-absorption asymmetry.

Method uses quasiparticle photon-assisted tunneling in a superconductor-insulator-superconductor junction.

Abstract

We measure current fluctuations of mesoscopic devices in the quantum regime, when the frequency is of the order of or higher than the applied voltage or temperature. Detection is designed to probe separately the absorption and emission contributions of current fluctuations, i.e. the positive and negative frequencies of the Fourier transformed nonsymmetrized noise correlator. It relies on measuring the quasiparticles photon assisted tunneling current across a superconductor-insulator-superconductor junction (the detector junction) caused by the excess current fluctuations generated by quasiparticles tunneling across a Josephson junction (the source junction). We demonstrate unambiguously that the negative and positive frequency parts of the nonsymmetrized noise correlator are separately detected and that the excess current fluctuations of a voltage biased Josephson junction present a strong asymmetry between emission and absorption.