This article presents a two-times interleaved, loop-unrolled SAR analog-to-digital converter (ADC) operational from 300 down to 4.2 K. The 6-8-bit resolution and the sampling speed up to 1 GS/s are targeted at digitizing the multi-channel frequency-multiplexed input in a spin-qubit reflectometry readout for quantum computing. To optimize the circuit for the altered device behavior at cryogenic temperatures, a modified common-mode switching scheme is adopted as well as a flexible calibration. The design is implemented in 40-nm CMOS technology and achieves 36.2-dB signal to noise and distortion ratio (SNDR) for Nyquist input at 4.2 K while maintaining a Walden figure of merit (FOM textsubscript W) of 200 pJ/conv-step (for a 10.8-mW power consumption), including the clock receiver, and 15 pJ/conv-step (for a 0.8-mW power consumption) for just the core ADC. With these specifications, the ADC can support the simultaneous readout of 20 qubit channels with a power consumption of 0.5 mW/qubit, thus advancing toward the full integration of the cryogenic readout for future large-scale quantum processors.

Quantum computers (QCs) promise significant speedup for relevant computational problems that are intractable by classical computers. QCs process information stored in quantum bits (qubits) that must be typically cooled down to cryogenic temperatures. Since state-of-the-art QCs employ only a few qubits, those qubits can be driven and read out by room-temperature electronics connected to the cryogenic qubits by only a few wires. However, practical QCs will require more than thousands of qubits, making this approach impractical due to system complexity and reliability concerns. Although frequency multiplexing would reduce the interconnects to room temperature by fitting many qubit channels in the same physical interconnect, an excessive number of interconnects would still be required. An alternative, more scalable solution is a cryogenic electronic interface operating very close to the quantum processor to keep the whole control loop at cryogenic temperature, hence avoiding any high-speed interconnect to room temperature. This system must comprise drivers, readout circuits (LNAs, ADCs), and a digital controller to steer the quantum-algorithm execution [1]. While cryogenic CMOS (cryo-CMOS) wideband drivers and LNAs supporting qubit frequency multiplexing have been shown before [1] -[3], no wideband cryo-CMOS ADC has been demonstrated yet.