Theory of multi-photon drive-induced quasiparticle tunneling in superconducting qubits

Modern superconducting qubits are controlled and read out with microwave drives. At the same time, a key operating principle of these devices is that the conduction electrons remain in the superconducting condensate. When this condition is not completely satisfied, the resulting electronic quasiparticles (QPs) can cause qubit decoherence when they tunnel across the Josephson junctions in the qubit circuit. In this work, we theoretically investigate how the microwave drives critical for control and readout can generate QPs, leading to qubit errors. We specifically consider strongly driven superconducting circuits and use Floquet theory to describe the driven dynamics exactly at all drive strengths. We find that multi-photon absorption processes can lead to photon-assisted QP generation and tunneling at large drive amplitudes, and we perform numerical simulations to quantify the rates of QP-induced decoherence for various driven problems of interest, such as for gates, Floquet qubits, and superconducting circuit readout. Our work is closely related to the problem of measurement-induced state transitions and helps establish bounds on the maximum driving strengths possible in superconducting qubits.