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IEEE International Superconductive Electronics Conference (ISEC)

Error-Free Operation of Superconducting Nanowire Optical-to-Electrical Converter Connected with SFQ Circuits

Kemmei Kajino, Shigehito Miki, Taro Yamashita, Hirotaka Terai

Published Jul 1, 2015• 0 citations
Publisher

Abstract

We report on the demonstration of the error-free operation of optical-to-electrical (O/E) signal converter based on superconducting nanowires with single-flux-quantum circuits. Optical input pulses were generated by the pulse pattern generator and the optical modulator, and the error rate of the output signals was measured. For a 300-nm-wide device with active area of 15×15 μm2, correct O/E conversion was confirmed with the error-rate of less than 10-6at the maximum repetition frequency of 120 MHz. The optical energy consumption per pulse was one order of magnitude lower than those of conventional semiconductor photodiodes.

DOI
10.1109/isec.2015.7383456

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We report on the demonstration of the error-free operation of optical-to-electrical (O/E) signal converter based on superconducting nanowires with single-flux-quantum circuits. Optical input pulses were generated by the pulse pattern generator and the optical modulator, and the error rate of the output signals was measured. For a 300-nm-wide device with active area of 15×15 μm2, correct O/E conversion was confirmed with the error-rate of less than 10-6at the maximum repetition frequency of 120 MHz. The optical energy consumption per pulse was one order of magnitude lower than those of conventional semiconductor photodiodes.
We 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 μm2 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 μm2 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.
We demonstrate the transfer of single photon triggered electrical pulses from a superconducting nanowire single photon detector (SNSPD) to a single flux quantum (SFQ) pulse. We describe design and test of a digital SFQ based SNSPD readout circuit and demonstrate its correct operation. Both circuits (SNSPD and SFQ) operate under the same cryogenic conditions and are directly connected by wire bonds. A future integration of the present multi-chip configuration seems feasible because both fabrication process and materials are very similar. In contrast to commonly used semiconductor amplifiers, SFQ circuits combine very low power dissipation (a few microwatts) with very high operation speed, thus enabling count-rates of several gigahertz. The SFQ interface circuit simplifies the SNSPD readout and enables large numbers of detectors for future compact multi-pixel systems with single photon counting resolution. The demonstrated circuit has great potential for scaling the present interface solution to 1,000 detectors by using a single SFQ chip.
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 13.6 K for 8 nm, and 10 K for 4 nm. NbN nanowires were patterned using a electron beam lithography with 200 nm thick electron beam resist. To increasing photo-sensitive regions, we fabricated 50 × 50 μm 2 meander patterns with NbN nanowires. Line width of nanowires was varied from 150 nm to 500 nm. Critical current density of NbN nanowires was about 3 × 10 6 A/cm 2 at 4.2 K. We irradiated 850-nm laser pulses via 50-μm 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 150-nm-wide NbN nanowires, half of critical current was suppressed by 1 mW CW laser irradiations. High speed responses were observed for laser pulses with repetition frequency of 1 MHz and pulse width of 2 ns. Besides the NbN nanowire photo switches, we designed an SFQ interface circuit based on Josephson transmission lines. The designed circuit could generate SFQ pulses by input current of 10 μA, 25 GHz current pulses. Dc bias margin of the circuit was ±15% for 60 μA current pulses.
Recently we designed and successfully tested a prototype of a superconducting digital-to-analog converter (DAC) based on processing single flux quantum (SFQ) pulses. Two unique features make the device very attractive for metrology applications: (i) The generation of output voltage is based on the fundamentally accurate Josephson frequency-to-voltage relationship. (ii) The device does not require any external signals with frequencies higher than 100 MHz. It is expected that the DAC will be used as a key component of novel standards and calibrators of ac voltage with frequency up to 1 MHz. The implemented prototype has low magnitude of output voltage (about 10 mV) which is far lower than the 1 to 10 V target magnitude.
We developed an optical input module with a superconducting single-flux-quantum (SFQ) circuit for application of ultrafast photonic networks in the near future. The optical input module, consisting of a built-in photodiode (PD) with a coplanar waveguide transmission line, was fabricated on an InP substrate and flip-chip bonded with the SFQ circuit on a Si substrate. The fabricated PD showed photosensitivities of more than 0.2 A/W for the wavelength range 1480 to 1530 nm at 4.2 K. SFQ pulses were generated by less than 1 mW optical pulse input.