Relaxation and frequency shifts induced by quasiparticles in superconducting qubits

TL;DR

This paper develops a comprehensive theory describing how quasiparticles cause relaxation and frequency shifts in superconducting qubits, with implications for understanding decoherence mechanisms and improving qubit design.

Contribution

It introduces a general theoretical framework for qubit decay rates due to quasiparticles, applicable to various superconducting qubit types and quasiparticle states.

Findings

Decay rate estimates for thermal and non-equilibrium quasiparticles.

Proposed measurement method using split transmon to probe quasiparticle distributions.

Derived expressions for qubit frequency shifts caused by quasiparticles.

Abstract

As low-loss non-linear elements, Josephson junctions are the building blocks of superconducting qubits. The interaction of the qubit degree of freedom with the quasiparticles tunneling through the junction represent an intrinsic relaxation mechanism. We develop a general theory for the qubit decay rate induced by quasiparticles, and we study its dependence on the magnetic flux used to tune the qubit properties in devices such as the phase and flux qubits, the split transmon, and the fluxonium. Our estimates for the decay rate apply to both thermal equilibrium and non-equilibrium quasiparticles. We propose measuring the rate in a split transmon to obtain information on the possible non-equilibrium quasiparticle distribution. We also derive expressions for the shift in qubit frequency in the presence of quasiparticles.