Fragility of multi-junction flux qubits against quasiparticle tunneling

TL;DR

This paper investigates how quasiparticle tunneling affects decoherence in superconducting flux qubits, considering deviations from equilibrium BCS theory, and provides decay time estimates consistent with experimental data.

Contribution

It models quasiparticle tunneling processes beyond equilibrium BCS theory and predicts decoherence rates aligning with observed qubit decay times.

Findings

Decay times match experimental measurements for Aluminum flux qubits.

Decay times are consistent with observed data for Niobium flux qubits.

Quasiparticle density significantly influences qubit decoherence.

Abstract

We study decoherence in superconducting qubits due to quasiparticle tunneling which is enhanced by two known deviations from the equilibrium BCS theory. The first process corresponds to tunneling of an already existing quasiparticle across the junction. The quasiparticle density is increased, e.g., because of an effective quasiparticle doping of the system. The second process is quasiparticle tunneling by breaking of a Cooper pair. This can happen at typical energies of superconducting qubits if there is an extended quasiparticle density inside the gap. We calculate the induced energy decay and pure dephasing rates in typical qubit designs. Assuming the lowest reported value of the non-equilibrium quasiparticle density in Aluminum, we find for the persistent-current flux qubit decay times of the order of recent measurements. Using the typical sub-gap density of states in Niobium we also reproduce observed decay times in the corresponding Niobium flux qubits.