Hot non-equilibrium quasiparticles in transmon qubits

Understand

Non-equilibrium quasiparticle excitations degrade the performance of a variety of superconducting circuits.

• Understanding the energy distribution of these quasiparticles will yield insight into their generation mechanisms, the limitations they impose on superconducting devices, and how to efficiently mitigate quasiparticle-induced qubit decoherence.
• To probe this energy distribution, we systematically correlate qubit relaxation and excitation with charge-parity switches in an offset-charge-sensitive transmon qubit, and find that quasiparticle-induced excitation events are the dominant mechanism behind the residual excited-state population in our samples.
• By itself, the observed quasiparticle distribution would limit $T_1$ to $\approx200~\mu\mathrm{s}$, which indicates that quasiparticle loss in our devices is on equal footing with all other loss mechanisms.