Quantum-circuit refrigerator for reset of superconducting qubits*

Quantum-circuit refrigerator (QCR) [1] is an active on-chip component which can change the dissipation rate in superconducting microwave devices in-situ by orders of magnitude. The dissipation channels can be turned on and off by applying a dc or rf voltage, or both [2]. Such an additional energy input together with an energy quantum from the refrigerated quantum circuit promotes photon-assisted quasiparticle tunneling through a normal-metal–insulator–superconductor junction. Previously, we have experimentally demonstrated that a QCR can change the quality factor of a superconducting microwave resonator by several orders of magnitude also giving rise to an effective Lamb shift of the resonator frequency [3] and that the dissipation can be turned on or off in a few nanoseconds [4]. We have also demonstrated that a superconducting qubit can be reset with a QCR from 100% to a few percent population in less than 100 ns. However, we observed non-exponential decay of the population rendering the optimization of the qubit reset involved. Here, we introduce a single-junction QCR [5] which simplifies the optimal control pulses, removes non-idealities related to junction asymmetry, increases the physical distance between the QCR and the qubit, and provides opportunities for optimizing the coupling strength for inimal spurious dissipation. We report on the first experiments of the single-junction QCR coupled to a transmon qubit and provide evidence supporting these claims.