Probing fast quantum circuit refrigeration in the quantum regime

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