Elemental technologies to develop single photon detectors having high temperature, high speed and high sensitivity operation

研究了高温超导纳米线作为量子密码通信的关键器件“单光子探测器”的基本技术。用YBa_2Cu_3O_y（YBCO）进行了实验。结果表明，采用250圈蜿蜒的结构，可以在100μm×100μm正方形中构建200纳米宽、25毫米长的纳米线。我们研究了在等离子体刻蚀过程中如何抑制纳米线的超导性退化。液氮冷却到样品级可以有效地防止等离子体蚀刻离子轰击的降解。结果表明，在宽200 nm、长1 μm的纳米线上，可以观察到超导电流。我们还研究了YBCO和单晶衬底之间异质外延表面的晶格不匹配。厚度仅为20 nm的YBCO薄膜的临界温度（T_c）大于80 K。另一方面，从80 nm到10 nm，研究了T_c对YBCO厚度的依赖关系。这为实现有效厚度小于10nm的超薄高温超导纳米线提供了可能。

We studied elemental technologies to develop HTS(high temperature superconducting) nano-wire as a key device "single photon detector" for the quantum cryptographic communication. Experiments were carried out using YBa_2Cu_3O_y(YBCO). It was shown that a 200-nm-wide and 25-mm-long nano-wire could be constructed in a 100μm×100μm square by using a 250-turns-meandering configuration. We examined how to suppress superconductivity degradation in the nano-wire during plasma etching processes. Liquid N2 cooling to a sample stage was effective to prevent the degradation from ion bombardments of plasma etching. As a result, superconducting current was observed for a 200-nm-wide and 1-μm-long nano-wire. We also investigated lattice mismatching at a hetero-epitaxial surface between YBCO and single crystal substrate. Then, YBCO thin film with only 20 nm thickness showed critical temperature(T_c) over 80 K. On the other hand, a T_c dependence on the YBCO thickness was examined from 80 nm to 10 nm. It produced a possibility to realize very thin HTS nano-wire with effective thickness less than 10 nm.