Normal-metal quasiparticle traps for superconducting qubits

TL;DR

This paper develops and experimentally validates a model for how small normal-metal traps can effectively evacuate quasiparticles from superconducting qubits, improving their coherence by reducing quasiparticle presence.

Contribution

It introduces a theoretical and experimental model for quasiparticle traps in superconducting qubits, quantifying how trap size influences quasiparticle evacuation times.

Findings

Larger traps decrease quasiparticle evacuation time.

Evacuation time saturates at a limit set by diffusion and geometry.

Characteristic trap size can optimize quasiparticle removal.

Abstract

The presence of quasiparticles in superconducting qubits emerges as an intrinsic constraint on their coherence. While it is difficult to prevent the generation of quasiparticles, keeping them away from active elements of the qubit provides a viable way of improving the device performance. Here we develop theoretically and validate experimentally a model for the effect of a single small trap on the dynamics of the excess quasiparticles injected in a transmon-type qubit. The model allows one to evaluate the time it takes to evacuate the injected quasiparticles from the transmon as a function of trap parameters. With the increase of the trap size, this time decreases monotonically, saturating at the level determined by the quasiparticles diffusion constant and the qubit geometry. We determine the characteristic trap size needed for the relaxation time to approach that saturation value.