Controlled mitigation of quasiparticle tunneling in a 3D transmon

Non-equilibrium quasiparticle tunneling determines an upper bound for the coherence time of superconductor-based qubits. An important source of quasiparticles and quasiparticle tunneling is photons with energy above twice the superconducting gap. By using a charge-parity-sensitive 3D transmon qubit, we directly measure the quasiparticle tunneling rate, and we evaluate non-ambiguously the effectiveness of microwave line filters and radiation shields for reducing the incident flux of high-frequency photons on the transmon. We find that its quasiparticle tunneling rate can be reduced below 1/(100 ms) with a combination of  filtering and shielding measures.