  We present here our recent results on qubit reset scheme based on a
quantum-circuit refrigerator (QCR). In particular, we use the photon-assisted
quasiparticle tunneling through a
superconductor--insulator--normal-metal--insulator--superconductor junction to
controllably decrease the energy relaxation time of the qubit during the QCR
operation. In our experiment, we use a transmon qubit with dispersive readout.
The QCR is capacitively coupled to the qubit through its normal-metal island.
We employ rapid, square-shaped QCR control voltage pulses with durations in the
range of 2--350 ns and a variety of amplitudes to optimize the reset time and
fidelity. Consequently, we reach a qubit ground-state probability of roughly
97% with 80-ns pulses starting from the first excited state. The qubit state
probability is extracted from averaged readout signal, where the calibration is
based of the Rabi oscillations, thus not distinguishing the residual thermal
population of the qubit.
