S-PAM: Superconductor-Semiconductor Interface Circuit with Pulse-Amplitude Modulation

Superconductor-semiconductor interface circuits are integral part of connecting cryogenic classical and quantum computing systems with the room temperature electronics. Due to the cooling power constraints of cryogenic setups, the number of data transmission channels between temperature stages is often limited by the heat load of cables and connectors. As a result, a high data rate (throughput) per channel is desired for large-scale computing systems. The state-of-the-art superconductor-semiconductor interface circuits use a binary data format. In this work, a novel interface circuit is proposed that produces signals with pulse-amplitude modulation (PAM). As a case study, a PAM-4 SQUID (superconducting quantum interference device) stack interface circuit is developed. With four output voltage levels, the effective data rate can be doubled while keeping the same operating frequency. Alternatively, the number of cryogenic cables can be reduced by two times as compared to the conventional binary data links.