Recently, great progress has been made in the field of ultrasensitive microwave detectors, reaching even the threshold for utilization in circuit quantum electrodynamics (cQED). However, these cryogenic sensors lack the ability to perform broad-band metrologically traceable power absorption measurements, which limits their scope of applications. Here, we demonstrate such measurements using an ultralow-noise nanobolometer supplemented by an additional direct-current (dc) input. The tracing of the absorbed power relies on comparing the response of the bolometer between radio frequency (rf) and dc heating powers traced through the Josephson voltage and quantum Hall resistance. To illustrate this technique, we demonstrate a fast calibration process of an attenuated input line over more than nine octaves of bandwidth with an rf heating power of −114 dBm and uncertainty down to 0.33 dB.

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 on Rabi oscillations, thus not distinguishing the residual thermal population of the qubit.