Jukka Pekola

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

Dicke superradiant enhancement of the heat current in circuit QED

  1. Gian Marcello Andolina,
  2. Paolo Andrea Erdman,
  3. Frank Noé,
  4. Jukka Pekola,
  5. and Marco Schirò
Collective effects, such as Dicke superradiant emission, can enhance the performance of a quantum device. Here, we study the heat current flowing between a cold and a hot bath through
an ensemble of N qubits, which are collectively coupled to the thermal baths. We find a regime where the collective coupling leads to a quadratic scaling of the heat current with N in a finite-size scenario. Conversely, when approaching the thermodynamic limit, we prove that the collective scenario exhibits a parametric enhancement over the non-collective case. We then consider the presence of a third uncontrolled {\it parasitic} bath, interacting locally with each qubit, that models unavoidable couplings to the external environment. Despite having a non-perturbative effect on the steady-state currents, we show that the collective enhancement is robust to such an addition. Finally, we discuss the feasibility of realizing such a Dicke heat valve with superconducting circuits. Our findings indicate that in a minimal realistic experimental setting with two superconducting qubits, the collective advantage offers an enhancement of approximately 10% compared to the non-collective scenario.
arxiv pdf