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We have established the new method of fabricating high-c YBa 2 Cu 3 O 7 x (YBCO) nanobridges with high reproducibility. Nanobridges ranging in 30-400 nm wide were formed by using electron beam lithography. An ultra thin film of 2-3 nm-thick insulative YBCO was deposited after nanobridge formation for recovering the damaged films occurred in the process.The critical current density c was approximately 620 MA cm 2 at 4.2 K in a width of 30 nm bridges, and increased by about two orders of magnitude in the different widths of c . The critical temperature c was 87 K, about the same as c of the film. The current-voltage characteristics showed a different curve from the conventional flux-flow type as the widths have been narrowed. Nanobridges developed in this study are expected to the applications for nano-SQUIDs or the optical input/output interfaces in single-flux-quantum circuits.

We have successfully obtained asymmetrical current–voltage (I–V) characteristics in YBa2Cu3O7-x (YBCO) nanobridges patterned with asymmetrical geometry. These asymmetrical nanobridges (ANBs) are shown to control vortices one at a time. The critical current I c, of 200-nm-wide and 100-nm-thick ANB changes by a maximum of 9% when a magnetic field of 2.4 mT is applied. The I c variation ΔI c increases monotonically with increased magnetic field from -2.2 to 2.4 mT. The asymmetrical I–V characteristics obtained in our experiment are due to the restriction of vortex motion by the asymmetrical surface barrier (the so-called vortex ratchet effect). The width and thickness dependencies of the I–V asymmetry are also studied, and indicate that the bias current and the Meissner screening current, determined by the relative width compared to the effective penetration depth of the YBCO film, dominates the I–V asymmetry characteristics.

SUMMARYWe report the energy-efficient optical input interface using NbN superconducting nanowire-based optical-to-electrical (SN-OE) converters for a single-flux-quantum (SFQ) data processing system. The SN-OE converters with small active areas ranging from 1 × 1 to 10 × 10 µm 2 were fabricated to improve the recovery time by reducing the kinetic inductance of the nanowire. The SN-OE with the smallest area of 1 × 1 µm 2 showed the recovery time of around 0.3 ns, while its detection efficiency for a single photon was reduced below 0.1% due to insufficient coupling efficiency with a single-mode optical fiber. However, the optical power dependence of the error rate of this device showed that the required optical power to achieve the error rate below 10 −12 at 10 GHz operation is as large as 70 µW, which is still one order of magnitude lower than semiconductor photo diodes. We also demonstrated the operation of the SN-OE converters combined with the SFQ readout circuit and confirmed the operating speed up to 77 MHz.

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