Invited: SFQ technology developments in NEDO next-generation high-efficiency network device project (超伝導エレクトロニクス)

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Increase of Internet traffic in the future will require dramatic improvements in basic network functionality. However, simply expanding currently available technology could lead to a significant increase in the energy consumption of a network as a whole. Energy efficiency in router and switching system is defined as function of installed components or units to the actual throughput. Innovative network devices that incorporate advanced optical technology hold promise for reducing energy consumption while enhancing performance. Development of Next-generation High-efficiency Network Device Technology is a project from 2007 to 2011, launched by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The NEDO project is devoted to developments of optical/electronic device technologies and their integration/packaging as well as system demonstrations of large-scale edge routers and ultra high-speed local area networks are being developed. Optical, semiconductor and superconductor devices are included in the project to accomplish the ends. Superconductive Single-Flux Quantum (SFQ) device is shown to operate at a more than 100 GHz clock frequency with less than 1μW power consumption per gate. It is attractive to realize extremely huge router with low power dissipation. In the NEDO project, SRL/ISTEC is investigating some significant technologies for implementing large-scale SFQ systems, such as optical I/O, current recycle and cryo-packaging. Besides the investigations, a small-scale application using the SFQ device is developing. The system is a superconductive real-time oscilloscope (SRO) which measures high-speed waveform regardless of its periodicity. The SRO is indispensable for future network monitoring, for example performance monitor for 100G Earth, operating analyses for photonic devices and transient monitor for optical switches. Moreover, it is promising as a monitor for digital coherent communication. The key element in the SRO is a flush-type Analog to Digital Converter (ADC). There are some advantages of SFQ circuits for implement the ADC than semiconductor devices. First, it has a quite high-speed nature. Second, a superconductive quantum interference device (SQUID), which has almost the same configuration as an SFQ loop, has periodic input current characteristics, which results in a smaller number of comparators in the ADC. For instance, an n-bits SFQ ADC can be constructed with n comparators. On the other hand, a semiconductor device requires 2^n-1 comparators to implement the n-bits ADC. In the case of 5 bits, an SFQ ADC needs at least 5 comparators but a semiconductor ADC requires 31. Third, it is easy to input an optical signal to SFQ circuits because an SFQ circuit is a low-impedance current-driven device. Fourth, SFQ circuit can carry out high-speed processing for output data from the ADC. A quasi-one-junction SQUID (QOS) has been used as a comparator in the ADC. The conventional comparator has the drawbacks of non-linearity and asymmetry. We proposed a new design comparator, called complementary comparator, that can solve these problems. We investigated the performance of the comparator using computer simulations. The sampling clock and input bandwidth of the ADC were improved by increasing J_c. By introducing the new comparator design and increasing the J_c to 40kA/cm^2, we improved the sampling clock to more than 150GS/s without any interleaves. This sampling clock enables us to achieve a real-time oscilloscope that can monitor 40-Gb/s digital waveforms with high accuracy.
2008-10-23

Invited: SFQ technology developments in NEDO next-generation high-efficiency network device project (超伝導エレクトロニクス)

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