Abstract Body

We have successfully demonstrated an increase of transfer efficiency in signal transfer circuits (STCs). Conventional STCs are composed of an input coil and an output coil such as a combination of a pickup coil and an input coil of a SQUID magnetometer. We placed a π-junction in parallel with the two coils in the STC studied here (hereafter, π-STC). In other words, π-STC is composed of two single-junction SQUIDs sharing a π-junction (π-SQUID). The synthesized inductance of the inductance of the input coil and that of the output coil needs to satisfy the condition that provides a non-hysteretic characteristic between internal and external magnetic flux for both single-junction SQUIDs.

It is well known that a conventional Josephson junction behaves as an inductor. Also, a π-junction behaves as an inductor, but the inductance value becomes negative because of the negative current-phase relationship (CPR). When we form a π-SQUID under the non-hysteretic characteristic and provide a loop inductance of the π-SQUID with a dc current or dc magnetic flux from external, the negative inductance induces a circulating current in the direction that the current flowing on the loop inductance is enhanced. By using this effect, the transfer efficiency of the π-STCs can be high compared to that of the conventional STCs made up of only input and output coils.

We have already confirmed the enhanced self-inductance of π-SQUIDs caused by the negative inductance experimentally [1]. In this study, we report experimental and numerical analyses of the transfer efficiencies of the π-STCs with shared π-junctions with different critical currents. In the actual experiments, we replace a π-junction with a serial connection of a conventional junction (0-junction) and a π-junction, forming a 0-π SQUID. The critical current of the π-junction with a Nb-PdNi-Nb sandwich structure is around 2 mA, which is large enough for the π-junction to act just as a π-phase shifter. The critical currents of the Nb-based 0-junctions are designed to be 50 and 60 µA.

The experimentally obtained transfer efficiency of the π-STCs are 3 to 10 times larger than that of the conventional STC. The transfer efficiency increases with an increase of the critical current of the 0-junctions under satisfying the above-mentioned condition. These characteristics agree with the numerical analysis. The increased transfer efficiency enables improved coupling between qubits in a quantum annealer or an improved coupling coefficient of a transformer used in current-recycling circuits.

References

[1] M. Higashi et al., IEEE Trans. Appl. Supercond., vol 33, p. 1600805, 2023.

Acknowledgment

This work was supported by JSPS KAKENHI under Grants JP23H05447, JP18H05211, and JP22H01548JST and JST-CREST under Grant JPMJCR20C5. The circuits were partly fabricated at Qufab, AIST.