EDP1-3
Improved operating current margin of the Josephson voltage standard by supplying the bias current with a long Josephson junction
13:30-15:00 Dec.3
*Hirotake Yamamori, Daiki Matsumaru, Michitaka Maruyama, Chiharu Urano and Nobu-Hisa Kaneko
National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8563 Japan
The quantum voltage standard realizes D/A conversion by many Josephson junctions in series, and the junction array is implemented by folding it many times on the chip. The superconducting current concentrates on the inner circumference of the wiring, and the apparent critical current of the Josephson junction decreases due to the influence of the asymmetrical current bias. At first glance, this decrease in current is difficult to distinguish from that due to magnetic flux trapping. Unfortunately, this reduces the width of the Shapiro step and reduces the operating current margin of the D/A conversion. This problem becomes even more severe for superconducting materials with higher critical temperature such as NbN, which have a large magnetic penetration depth.
To prevent this current concentration, several ways have been proposed, such as thickening the film thickness in the current concentration area [1] or making the inner circumference jagged so that its physical length is equal to the outer circumference [2]. We propose a more effective method that is easier to implement. By placing a junction with a critical current larger than the bias current in the current concentration area, this junction is affected by the current concentration in the wiring, but since the critical current is sufficiently larger than the current flowing in the circuit, it functions as a superconducting wiring. Figure 1 shows the numerically calculated current distribution of a long Josephson junction with asymmetric 90-degree cross bias current, which shows that the current is concentrated on the short side, providing a symmetric current to the Josephson junction array. Figure 2 experimentally shows that the operating current margin is significantly improved, which suggests the junction array with the long Josephson junction is effective in preventing the effect of current concentration due to bending of the wiring.
[1] J.M. Xiong at al., SuST, 35, p.055015 (2022).
[2] H. Kim at al., IEEE TMTT, 57, pp.442-448 (2009).
This research was supported by the New Energy and Industrial Technology Development Organization (NEDO) under project “JPNP19005.”
Keywords: Josephson Voltage standard, Josephson junction array