Superconducting Classical Circuits for Quantum Computing Readout and Control

Superconducting Josephson-junction based classical circuits present an attractive technology option for the implementation of the classical control infrastructure for quantum processors including qubit control, measurement, and error correction functions. Conventionally, all classical control operations are performed at room temperature. Such system configuration is difficult to scale and it introduces a long latency in quantum operations. For scalable low-latency classical control infrastructure, the first layer of control electronics should located as close to the quantum chips as possible. Superconducting Josephson circuits including single flux quantum (SFQ) circuits can be the technology of choice due to its low power (10-19 Joule to 10-21 Joule to per switching at 4K and 20-30 mK, respectively) producing minimal classical backaction on the qubit. The ability to operate at very high speed (tens of gigahertz clock) opens a way for digitizing and fast processing qubit output data for error correction and generation of qubit control signals.  Superconducting digital, mixed-signal, and analog circuits reached a relative maturity for classical applications including data and signal processing, analog-to-digital conversion (ADC), and RF sensing and amplification. In particular, some ADCs and digital receivers reached a commercial significance. However, the control infrastructure for quantum processor requires different design criteria compare to conventional applications. Among a variety of different superconducting ADCs developed to date, only a few can be applied for digitizing the qubit measurement data.  In this talk, different superconducting solutions for qubit readout, control and error correction will be reviewed and compared. For qubit readout, two different options will be considered: a combination of superconducting parametric amplifiers and ADCs and an approach based on Josephson photomultiplier (JPM). For qubit control, a superconducting arbitrary wave generator (AWG) will be described and compared to the SFQ pulse train generator approach. For error correction, various low-power SFQ technologies including ERSFQ, eSFQ, reciprocal quantum logic (RQL), and adiabatic quantum flux parametron (AQFP) will be reviewed and compared. Finally, a superconducting classical-quantum system 3D integration approach spreading across multiple temperature stages will be discussed.