Anna Y. Herr

Ultra-low-power superconductor logic

We have developed a new superconducting digital technology, Reciprocal Quantum Logic, that uses ac power carried on a transmission line, which also serves as a clock. Using simple experiments, we have demonstrated zero static power dissipation, thermally limited dynamic power dissipation, high clock stability, high operating margins, and a low bit-error rate. These features indicate that the technology is scalable to far more complex circuits at a significant level of integration. On the system level, Reciprocal Quantum Logic combines the high speed and low-power signal levels of single-flux-quantum signals with the design methodology of semiconductor digital logic, including low static power dissipation, low latency combinational logic, and efficient device count.

An 8-bit carry look-ahead adder with 150 ps latency and sub-microwatt power dissipation at 10 GHz

Reciprocal quantum logic combines the speed and power-efficiency of single-flux quantum superconductor devices with design features that are similar to CMOS. We have demonstrated an 8-bit carry look-ahead adder in the technology using combinational gates with fanout of four and non-local interconnect. Measured power dissipation of the fully active circuit is only 510 nW at 6.2 GHz. Latency is only 150 ps at a clock rate of 10 GHz.

Integrated Power Divider for Superconducting Digital Circuits

We present design, analysis and test of a superconducting microwave power divider for a new superconducting Reciprocal Quantum Logic (RQL). The RQL logic family, using combination of AC power and SFQ data encoding, allows scalable superconducting digital circuits with zero static power dissipation. The Wilkinson 1:8 power splitter/combiner based on λ/4 resonators has been analyzed for geometric series and maximum flat response. Simulated maximum flat response gives one octave of bandwidth with non-uniformity of the bias current distribution within ±10%. This result is valid with up to 40 ps of electrical length mismatch in the power lines that is well within the requirements for complex circuits. We have experimentally confirmed correct operation of the divider/combiner in a frequency band 0-12 GHz in even mode.

Digital circuits using self-shunted Nb/NbxSi1-x/Nb Josephson junctions

Superconducting digital circuits based on Josephson junctions with amorphous niobium-silicon (a-NbSi) barriers have been designed, fabricated, and tested. Single-flux-quantum (SFQ) shift registers operated with ±30% bias margins, confirming junction reproducibility and uniformity. Static digital dividers operated up to 165 GHz for a single value of bias current, which was only marginally slower than circuits fabricated with externally shunted AlOx-barrier junctions having a comparable critical current density of 4.5 kA/cm2. In comparison, self-shunted a-NbSi junctions enabled a doubling in circuit density. This and their relatively thick 10 nm barriers could increase the yield of complex SFQ circuits.