Proximity effect in normal-metal quasiparticle traps

TL;DR

This paper investigates how the proximity effect influences the efficiency of normal-metal quasiparticle traps in superconducting qubits, finding an optimal trap placement that minimizes qubit relaxation caused by quasiparticles.

Contribution

It provides a detailed analysis of the proximity effect in NS bilayers and extends this to optimize trap placement in superconducting qubits.

Findings

Optimal trap-junction distance is 4 to 20 coherence lengths.

Proximity effect can be mitigated by appropriate trap placement.

Novel insights into the density of states in NS bilayers.

Abstract

In many superconducting devices, including qubits, quasiparticle excitations are detrimental. A normal metal ($N$) in contact with a superconductor ($S$) can trap these excitations; therefore such a trap can potentially improve the devices performances. The two materials influence each other, a phenomenon known as proximity effect which has drawn attention since the '60s. Here we study whether this mutual influence places a limitation on the possible performance improvement in superconducting qubits. We first revisit the proximity effect in uniform $NS$ bilayers; despite the long history of this problem, we present novel findings for the density of states. We then extend our results to describe a non-uniform system in the vicinity of a trap edge. Using these results together with a phenomenological model for the suppression of the quasiparticle density due to the trap, we find in a transmon qubit an optimum trap-junction distance at which the qubit relaxation rate is minimized. This optimum distance, of the order of 4 to 20 coherence lengths, originates from the competition between proximity effect and quasiparticle density suppression. We conclude that the harmful influence of the proximity effect can be avoided so long as the trap is farther away from the junction than this optimum.