The 100GHz second generation

Superconductors have been tantalising researchers with their promise of high speed at low power since the late 1960s.

But, until recently, difficulties with the fabrication process and a cumbersome logic family have kept the technology from growing into a large-scale commercial success.

But the Institute of Electrical and Electronics Engineers report on a new manufacturing process that is rugged and reliable, and a new logic family that is faster and more compact than its predecessor and promises to make commercial systems feasible within the next five years.

Superconductors currently run on niobium, which has no electrical resistance at temperatures near absolute zero, and should ideally operate below 9K.

In addition to zero electrical resistance, the new logic family relies on another property of superconductors: within a closed loop, magnetic flux can exist only in quantified amounts that are multiples of magnetic flux quantum.

In rapid single flux quantum (RSFQ) logic, the presence or absence of a magnetic flux quanta represents information bits. The quanta are sent from one logic device to another along virtually dispersionless superconducting transmission lines.

Performance of RSFQ logic is said to be spectacular. It can operate in complex circuits at clock frequencies beyond 100 GHz; and once researchers succeed in building superconductor ICs with the same line widths used in semiconductor manufacturing, performance should be even more impressive.

Operating speeds of more than 750 GHz have already been achieved experimentally for RSFQ toggle flip-flops having gates of 0.25 micrometer–roughly the size of their semiconductor counterparts.

The first RSFQ products will probably leverage superconductor superiority in performing high-speed and high-accuracy analogue-to-digital conversion.

Given adequate resources, the next few years are likely to see the first superconductor digital RF modules for both wireless communications base stations and high-performance instrumentation.

Other potential applications include petaflops computing platforms and high-throughput and high-density network switches.

In short, thanks to two key features–speed conjoined to low power, and analogue-to-digital quantum accuracy–RSFQ circuits should extend digital power and flexibility directly into the RF and microwave domains.