Quasiparticle Absorption Limits Microwave Qubit Fidelity

A single quasiparticle tunneling across a superconducting qubit's Josephson junction can simultaneously absorb a qubit excitation and photons from the microwave drive, according to a new theory developed by Mykola Kishmar of Columbia University, Pavel D. Kurilovich and Thomas Connolly of Yale University, and Andrey Klots, Igor L. Aleiner, and Vladislav D. Kurilovich of Google Quantum AI. Even at cryogenic temperatures, which are two orders of magnitude smaller than the superconducting energy gap, an appreciable fraction of Cooper pairs remain broken, contributing to these quasiparticle excitations. The work establishes a fundamental limitation on the fidelity of microwave qubit…