Reciprocal Quantum Logic Compatible SFQ-to-CMOS Amplifiers for High-Speed Data Transmission

We have developed an amplifier for use in transmitting high-speed reciprocal quantum logic signals to CMOS digital circuitry at room temperature. The amplifier uses an integrated digital latch to convert the normally return-to-zero (RZ) format of a single flux quantum (SFQ) signal to non-return-to-zero (NRZ) format with a simple and space-efficient design. The NRZ format provides more signal power per unit bandwidth than RZ, and is easier to interface with CMOS circuitry. The output voltage is provided by a series of inductively isolated asymmetric DC superconducting quantum interference devices (SQUIDs). The power efficiency is estimated to be 32±9%. The DC SQUIDs are designed in-line with a 50 Ohm transmission line to reduce high frequency oscillation and improve output signal integrity. Experimentally, the output amplifier demonstrated a bit error rate of better than 9.5E-8 at 1 Gb/s; extrapolations to the optimal operating point project a bit error rate of 1E-37.