Cavity photons as a probe for charge relaxation resistance and photon emission in a quantum dot coupled to normal and superconducting continua

TL;DR

This paper demonstrates how microwave cavities can probe charge transfer dynamics in a quantum dot coupled to normal and superconducting contacts, revealing photon dissipation, emission, and the effects of biasing.

Contribution

It introduces a combined experimental and theoretical approach to study charge relaxation and photon emission in a quantum dot system with fermionic continua.

Findings

Universal photon dissipation governed by charge relaxation.

Modification of photon dissipation from capacitive to inductive regimes.

Photon emission due to inelastic tunneling at superconducting BCS peaks.

Abstract

Microwave cavities have been widely used to investigate the behavior of closed few-level systems. Here, we show that they also represent a powerful probe for the dynamics of charge transfer between a discrete electronic level and fermionic continua. We have combined experiment and theory for a carbon nanotube quantum dot coupled to normal metal and superconducting contacts. In equilibrium conditions, where our device behaves as an effective quantum dot-normal metal junction, we approach a universal photon dissipation regime governed by a quantum charge relaxation effect. We observe how photon dissipation is modified when the dot admittance turns from capacitive to inductive. When the fermionic reservoirs are voltage biased, the dot can even cause photon emission due to inelastic tunneling to/from a Bardeen-Cooper-Schrieffer peak in the density of states of the superconducting contact. We can model these numerous effects quantitatively in terms of the charge susceptibility of the quantum dot circuit. This validates an approach that could be used to study a wide class of mesoscopic QED devices.