Dynamics of Non-equilibrium Quasiparticles in Transmon Qubits with Different Shunting Capacitors

Non-equilibrium quasiparticles (QPs) in superconducting qubits are a source of energy relaxation and dephasing. To mitigate this source of decoherence, an understanding of both the generation and trapping/recombination mechanisms of QPs in qubits is critical. Two experiments were conducted that shed light on these two questions. In the first experiment, the rates of QP tunneling across the Josephson junction were measured by monitoring telegraphic changes between the two |0> to |1> transition frequencies of transmon qubits sensitive to single electron offset charges. In the second experiment, QPs were generated at the Josephson junction of the qubit by applying a large amplitude cavity microwave pulse. The quasiparticles generated, results in a decrease in the energy relaxation time (T₁) of the qubit. We extract the rate of recombination and trapping of QPs by measuring T₁ after different delays at the end of the QP generation pulse. We report on the formation of QPs and their recovery in transmon qubits with different shunting capacitor designs (i.e. an x-mon and floating two-pads) and using Al and Ta for the shunting capacitor.