To provide a circuit system using SINIS capable of simultaneously initializing a coupling system of a superconducting qubit and a resonator, and its operation method.
There are many calculations that require initialization. Basically, in a scheme that recycles a qubit, an initialization operation is involved, and if we wait for the natural relaxation of the qubit, that becomes a bottleneck and high-speed calculation cannot be performed.
In the previous research, there is an experimental result that it takes 280 ns for the initialization process to reach 99% fidelity, but taking the surface code as an example, one cycle of the error detection scheme is about 200 ns, and the fidelity of initialization is required to be more than 99%, which is still less than the requirement. In this experiment, the Q value of the resonator coupled to the qubit is decreased, and the initialization speed is increased by applying a drive pulse, but if the Q value of the resonator is decreased, the reading takes time.
In order to perform initialization and readout efficiently using the same resonator, the Q value of the resonator needs to be variable. The SINIS junction is used to realize it. SINIS is a nanosized junction of superconductor (S) -insulator (I) -normal-metal (N) -insulator (I) -superconductor (S), which can extract energy from a coupled quantum system by applying a DC bias. The coupling strength can be varied by an applied bias, and the Q value of the resonator can be varied by coupling SINIS to the resonator, so that a general use can be made in which read out is performed at a high Q value and initialization is performed at a low Q value. In the calculation of the initialization process using this method, 99% fidelity was obtained in about 80 ns.

Keywords: superconducting qubit, SINIS