Shuji Nakamura

Probing instantaneous quantum circuit refrigeration in the quantum regime

  1. Shuji Nakamura,
  2. Teruaki Yoshioka,
  3. Sergei Lemziakov,
  4. Dmitrii Lvov,
  5. Hiroto Mukai,
  6. Akiyoshi Tomonaga,
  7. Shintaro Takada,
  8. Yuma Okazaki,
  9. Nobu-Hisa Kaneko,
  10. Jukka Pekola,
  11. and Jaw-Shen Tsa
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.
arxiv pdf

Active Initialization Experiment of Superconducting Qubit Using Quantum-circuit Refrigerator

  1. Teruaki Yoshioka,
  2. Hiroto Mukai,
  3. Akiyoshi Tomonaga,
  4. Shintaro Takada,
  5. Yuma Okazaki,
  6. Nobu-Hisa Kaneko,
  7. Shuji Nakamura,
  8. and Jaw-Shen Tsai
The initialization of superconducting qubits is one of the essential techniques for the realization of quantum computation. In previous research, initialization above 99% fidelity
has been achieved at 280 ns. Here, we demonstrate the rapid initialization of a superconducting qubit with a quantum-circuit refrigerator (QCR). Photon-assisted tunneling of quasiparticles in the QCR can temporally increase the relaxation time of photons inside the resonator and helps release energy from the qubit to the environment. Experiments using this protocol have shown that 99\% of initialization time is reduced to 180 ns. This initialization time depends strongly on the relaxation rate of the resonator, and faster initialization is possible by reducing the resistance of the QCR, which limits the ON/OFF ratio, and by strengthening the coupling between the QCR and the resonator.
arxiv pdf