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IEICE Transactions on Electronics

Low-Energy Optical-to-Electrical Converters Based on Superconducting Nanowire for Single-Flux-Quantum Circuits

Kemmei Kajino, Shigehito Miki, Taro Yamashita, Hirotaka Terai

Published Jan 1, 2015• 1 citation
Publisher

Abstract

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.

DOI
10.1587/transele.e98.c.227

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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 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 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.
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.
Superconducting single-photon detectors (SSPDs) based on NbN nanowires possess properties such as high counting rate, low dark count rate and small timing jitter, which are promising characteristics for a quantum key distribution system. An SSPD array has recently attracted attention, because it can provide a higher counting rate, and spatial and photon-number resolutions. We propose readout electronics using single-flux-quantum (SFQ) circuits to process a large number of output signals from an SSPD array. We designed and tested an interface circuit between an SSPD and SFQ circuit, and successfully observed SFQ pulse generation for an input pulse with a current amplitude of 17.4 ~ 36.4 muA , which is comparable with an output current amplitude of the SSPD. The interface circuit was also operational for an input pulse duration of 1.6 ns, which is shorter than the typical recovery time of the SSPD.
We develop single-photon detectors comprising single-mode fiber-coupled superconducting nanowires, with high system detection efficiencies at a wavelength of 940 nm. The detector comprises a 6.5-nm-thick, 110-nm-wide NbN nanowire meander fabricated onto a Si substrate with a distributed Bragg reflector for enhancing the optical absorptance. We demonstrate that, via the design of a low filling factor (1/3) and active area (Φ = 10 μm), the system reaches a detection efficiency of ∼60% with a dark count rate of 10 Hz, a recovery time <12 ns, and a timing jitter of ∼50 ps.
Superconducting nanowire single-photon detectors (SNSPDs) with series-parallel meander-type configurations are demonstrated to have photon-number-resolving capabilities and are expected to be used for an evaluation system of single-photon generators and/or an energy-resolved fast detector for a new-generation time-of-flight mass spectrometry. In this paper, we present the design, fabrication, and confirmation of correct operations of two kinds of single-flux-quantum (SFQ) digital readout circuits for an SNSPD with a series-parallel meander-type configuration. Assuming a six-parallel meander-type configuration, we designed: a six-level current discriminator using six different magnetically coupled DC/SFQ converter circuits; and a Δ-type A/D converter using a quasi-one-junction SQUID (QOS) comparator with an SFQ feedback loop. For the design of the magnetically coupled inputs, we used a 3-D inductance extraction program FastHenry and the designed circuits were fabricated using a HYPRES 4.5-kA/cm2Nb process and a National Institute of Advanced Industrial Science and Technology (AIST) standard 2.5-kA/cm2Nb process.