Modeling of quasiparticle-induced excitations of a Josephson charge-phase qubit

TL;DR

This paper investigates how non-equilibrium quasiparticles in aluminum charge-phase qubits cause stochastic excitations, affecting qubit states, with a numerical model matching experimental impedance measurements at 80 MHz.

Contribution

It provides a detailed numerical model of quasiparticle-induced excitations in a charge-phase qubit and compares it with experimental impedance data.

Findings

Quasiparticles cause stochastic excitations of the qubit.

The numerical model accurately reproduces impedance measurements.

Quasiparticle tunneling leads to a mixed qubit state.

Abstract

We have analyzed quasiparticle transitions in an Al charge-phase qubit inducing a dynamic change of the qubit states. The time-averaged mixed state is related to the strong coupling of the qubit to an ensemble of non-equilibrium quasiparticles in the leads. Such quasiparticles tunnel stochastically on and off the island and can excite the qubit. Continuous monitoring of the qubit impedance at a frequency of 80 MHz shows the admixture of the excited state. We present a numerical description of these cyclic transitions and compare it with our experimental data.