Optical Interface for Superconducting Digital Circuits using Low temperature Photon Detectors

点击翻译按钮获取中文摘要

We investigated superconducting NbN nanowire as a photo switch for generating single flux quantum (SFQ) pulses.Epitaxially grown NbN thin films were prepared on MgO (100) substrates by a dc magnetron sputtering method using gas mixture of Ar + N_2, without substrate heating. NbN thin films showed Tc of 14K for 8 nm, and 10K for 4 nm.NbN nanowires were patterned using a electron direct drawing method with 200 nm thick electron resist. To increasing photo-sensitive regions, we fabricated 50 x 50 μm^2 meander patterns with NbN nanowires. Line width of nanowires was varied from 200 nm to 500 nm. Critical current density of NbN nanowires was about 3x10^6A/cm^2 at 4.2 K.We irradiated 850-nm laser pulses via 50-mm multi-mode optical fibers. Repetition frequency and pulse width could be controlled by a pulse pattern generator. The maximum laser pulse power was about 1 mW. Using 200-nm-wide NbN nanowires, one third of critical current was suppressed by 1 mW CW laser irradiations. High speed responses were observed for laser pulses with repetition frequency of 1MHz and pulse width of 2 ns.On the other hand, we designed an SFQ generation circuit using Josephson transmission lines assuming NbN nanowire switch generates 60 μA current pulses. The designed circuit could generate SFQ pulses by 10 μA, 25 GHz current pulses. Dc bias margin of the circuit was +- 15% for 60 μA current pulses.