  Recent advancements in circuit quantum electrodynamics have enabled precise
manipulation and detection of the single energy quantum in quantum systems. A
quantum circuit refrigerator (QCR) is capable of electrically cooling the
excited population of quantum systems, such as superconducting resonators and
qubits, through photon-assisted tunneling of quasi-particles within a
superconductor-insulator-normal metal junction. In this study, we demonstrated
instantaneous QCR in the quantum regime. We performed the time-resolved
measurement of the QCR-induced cooling of photon number inside the
superconducting resonator by harnessing a qubit as a photon detector. From the
enhanced photon loss rate of the resonator estimated from the amount of the AC
Stark shift, the QCR was shown to have a cooling power of approximately 300 aW.
Furthermore, even below the single energy quantum, the QCR can reduce the
number of photons inside the resonator with 100 ns pulse from thermal
equilibrium. Numerical calculations based on the Lindblad master equation
successfully reproduced these experimental results.
