Cooper pair tunnelling and quasiparticle poisoning in a galvanically isolated superconducting double dot

TL;DR

This study investigates a galvanically isolated superconducting double dot, revealing increased quasiparticle poisoning rates and emphasizing the role of quasiparticle exchange with leads as a key relaxation process.

Contribution

It demonstrates the effects of galvanic isolation on quasiparticle poisoning and models the device's response to understand relaxation mechanisms.

Findings

Quasiparticle poisoning rate is higher in isolated double dots.

Quasiparticle exchange with leads is a major relaxation pathway.

Device energy states are characterized using high frequency reflectometry.

Abstract

We increase the isolation of a superconducting double dot from its environment by galvanically isolating it from any electrodes. We probe it using high frequency reflectometry techniques, find 2e-periodic behaviour, and characterise the energy structure of its charge states. By modelling the response of the device, we determine the quasiparticle poisoning rate to be significantly worse than that of a unisolated double dot, and conclude that quasiparticle exchange between the dot and the leads is an important relaxation mechanism.