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Physical Review Applied

Probing fast quantum circuit refrigeration in the quantum regime

Shuji Nakamura, Teruaki Yoshioka, S. A. Lemzyakov, Dmitrii Lvov, Hiroto Mukai, Akiyoshi Tomonaga, Shintaro Takada, Yuma Okazaki, Nobu‐Hisa Kaneko, Jukka P Pekola, Jaw-Shen Tsai

Published Jan 10, 2025• 2 citations
Publisher

Abstract

Photon-absorption-based quantum circuit refrigeration is crucial to developing superconducting quantum computers, particularly for initializing qubits. However, precise and rapid measurement of residual photons in the circuit immediately after absorption has been a significant technical challenge. This study employs the ac Stark shift of superconducting qubits to detect sub-photon-level energies in the circuit, confirming the effectiveness of the fast refrigeration even in the quantum regime. These results will facilitate the development of more efficient quantum refrigeration devices and faster, high-fidelity qubit initialization.

DOI
10.1103/physrevapplied.23.l011003

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Photon-absorption-based quantum circuit refrigeration is crucial to developing superconducting quantum computers, particularly for initializing qubits. However, precise and rapid measurement of residual photons in the circuit immediately after absorption has been a significant technical challenge. This study employs the ac Stark shift of superconducting qubits to detect sub-photon-level energies in the circuit, confirming the effectiveness of the fast refrigeration even in the quantum regime. These results will facilitate the development of more efficient quantum refrigeration devices and faster, high-fidelity qubit initialization.
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.
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 200 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.
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.
Superconducting microwave circuits are promising candidates for a variety of different applications, ranging from extremely sensitive photon detectors to large-scale quantum computer. Thus, they may revolutionize our everyday lives in the future through substantial technological development. Superconducting quantum circuits typically operate at low power levels and consist of linear and nonlinear resonators. In order to enhance their operation fidelity, they need to be cooled efficiently, and the dissipation of the circuit needs to be well engineered. In this chapter, we describe a refrigerator for electric quantum circuits based on photon-assisted electron tunneling in hybrid superconductor–insulator–normal-metal junctions. Furthermore, such junctions can also serve as incoherent microwave sources. Importantly, the photon absorption and emission rates can be tuned over several orders of magnitude. In addition, these junctions are used for refrigeration and heating of the normal-metal electrons as well as for probing the electron temperature. Here, we present their operation principle and provide insight to their potential applications as well as briefly describe fabrication methods.
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.
Quantum technology promises revolutionizing applications in information processing, communications, sensing and modelling. However, efficient on-demand cooling of the functional quantum degrees of freedom remains challenging in many solid-state implementations, such as superconducting circuits. Here we demonstrate direct cooling of a superconducting resonator mode using voltage-controllable electron tunnelling in a nanoscale refrigerator. This result is revealed by a decreased electron temperature at a resonator-coupled probe resistor, even for an elevated electron temperature at the refrigerator. Our conclusions are verified by control experiments and by a good quantitative agreement between theory and experimental observations at various operation voltages and bath temperatures. In the future, we aim to remove spurious dissipation introduced by our refrigerator and to decrease the operational temperature. Such an ideal quantum-circuit refrigerator has potential applications in the initialization of quantum electric devices. In the superconducting quantum computer, for example, fast and accurate reset of the quantum memory is needed.