Fast Unconditional Initialization for Superconducting Qubit Using Quantum-Circuit Refrigerator

Vasilii Sevriuk, Jani Tuorila, Johannes Heinsoo +12 more

Timm Mörstedt, Giacomo Catto, A.J. Viitanen +13 more

Shuji Nakamura, Teruaki Yoshioka, S. A. Lemzyakov +8 more

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.

T. Yoshioka, J. S. Tsai

In a quantum computation scheme, such as recycling a qubit, the initialization of the qubit is always required. At present, the initialization of the qubit is one of the bottlenecks in such computation, and high fidelity and fast initialization still remain important research subjects. In this study, by coupling a SINIS (superconductor–insulator–normal metal–insulator–superconductor) junction to a coupled system consisting of a qubit and a resonator, a photon is removed from the resonator by single-photon-assisted tunneling, and the effective relaxation rate of the resonator is increased. In addition, by applying an |e,0⟩−|f,0⟩ drive pulse and an |f,0⟩−|g,1⟩ drive pulse to the qubit, the energy of the qubit is rapidly transferred to the resonator to realize high-speed initialization. The present simulation shows that the qubit can be initialized with 99% fidelity in 80 ns and 99.9% in 137 ns when the electron temperature of the normal metal of the SINIS junction is 10 mK. The simulation also shows that the relaxation time of the qubit is about 13% longer when the resonator is interposed between the SINIS junction and the qubit than when the SINIS junction is directly coupled to the qubit.

Yoshioka, Teruaki, Hiroto Mukai, Akiyoshi Tomonaga +5 more

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.

Jan Goetz, András Gunyhó, Matti Silveri +3 more