Direct Dispersive Monitoring of Charge Parity in Offset-Charge-Sensitive Transmons

Improving the coherence of superconducting qubits is a crucial step toward the goal of fault-tolerant quantum processors based on them. An ongoing experimental challenge is to diagnose and mitigate the dominant decoherence mechanisms. In this work, the authors demonstrate a method of probing the decoherence induced by nonequilibrium superconducting quasiparticles, and show that improved filtering of quasiparticle-generating radiation can improve the energy-relaxation time T₁ of superconducting qubits, reaching an average of 200 μs in the measured device. Furthermore, the measurement technique is relevant for ultralow-noise sensing in general.