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关键器件“单光子探测器”的基本技术。实验采用YBa_2Cu_3O_y（YBCO）材料。结果表明，采用250匝的曲折结构，可以在100μm×100μm的正方形内构建出宽200 nm、长25 mm的纳米线。我们研究了如何在等离子体蚀刻过程中抑制纳米线的超导性退化。液氮冷却样品台是有效的，以防止从等离子体蚀刻的离子轰击退化。结果，在200 nm宽、1 μ m长的纳米线上观察到超导电流。我们还研究了YBCO和单晶衬底之间的异质外延表面的晶格失配。厚度仅为20 nm的YBCO薄膜的临界温度（Tc）超过80 K。另一方面，在80 ~ 10 nm范围内，研究了YBCO薄膜的T_c随厚度的变化。这为实现有效厚度小于10 nm的极细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.