Rate of tunneling nonequilibrium quasiparticles in superconducting qubits

TL;DR

This paper presents a phenomenological model for the tunneling rate of nonequilibrium quasiparticles in superconducting qubits, revealing non-monotonic temperature dependence and implications for qubit coherence.

Contribution

It introduces a new model linking nonequilibrium quasiparticle tunneling to temperature-dependent chemical potential shifts, explaining relaxation behavior in superconducting qubits.

Findings

Non-monotonic relaxation rate as a function of temperature.

Lowest tunneling rates occur near the onset temperature.

Quasiparticle tunneling influences qubit transition probabilities.

Abstract

In superconducting qubits the lifetime of quantum states cannot be prolonged arbitrarily by decreasing temperature. At low temperature quasiparticles tunneling between electromagnetic environment and superconducting islands takes the condensate state out of equilibrium due to charge imbalance. We obtain the tunneling rate from a phenomenological model of non-equilibrium, where nonequilibrium quasiparticle tunnelling stimulates a temperature-dependent chemical potential shift in superconductor. As a result we obtain a non-monotonic behavior for relaxation rate as function of temperature. Depending on the fabrication parameters for some qubits the lowest tunneling rate of nonequilibrium quasiparticles can take place only near the onset temperature below which nonequilibrium quasiparticles dominate over equilibrium one. Our theory also indicates that such tunnelings can influence the probability of transitions in qubits through a coupling to the zero-point energy of phase fluctuations.