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

我们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YBa2Cu3Oy(YBCO)进行了实验研究。结果表明，在100μm×100μm的正方形中，采用250圈的弯曲构型可以构建出一条宽200 nm、长25 mm的纳米线。我们研究了如何在等离子体刻蚀过程中抑制纳米线中的超导电性退化。液氮冷却到样品阶段可以有效地防止等离子体刻蚀过程中离子轰击引起的退化。结果表明，在200 nm宽、1μm长的纳米线上观察到了超导电流。我们还研究了YBCO与单晶衬底异质外延表面的晶格失配。结果表明，厚度仅为20 nm的YBCO薄膜的临界温度(T_C)在80K以上，并在80~10 nm范围内研究了薄膜的T_c与厚度的关系。这为实现有效厚度小于10 nm的超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.