Superconducting nanowire single-photon detectors (SNSPDs) have emerged as leading cryogenic photon detectors, thanks to their high detection efficiency and low jitter. However, their large-scale integration remains limited by the wiring bottleneck between the cryogenic detectors and their room-temperature readout electronics. In applications such as color-center-based quantum computers (QCs), thousands of detectors may need to operate in parallel within a limited cryogenic cooling budget, thus asking for a scalable, low-power cryogenic electronic readout. To address these needs, this work introduces a cryogenic readout circuit directly wire-bonded to the SNSPD and using a high-impedance input to maximize the quality of the detector signal, thus relaxing the requirement of the cascaded amplifier and reducing its power consumption. An active quenching circuit is then adopted to ensure a reliable reset after the latching of the detector induced by such high input impedance. Implemented in 40-nm CMOS with an active area of <0.14 mm², the system achieves competitive performance at 0.1 K, delivering low timing jitter (<40 ps), high speed (dead time of ≈5 ns), and dark count rates (DCRs) below 1 Hz, while achieving a 5× reduction in power consumption (down to 20 μW) with respect to the cryogenic-readout state-of-the-art. Its ultralow-power operation and compact footprint make the proposed solution well-suited for integration within large-scale quantum-computing architectures.

This paper provides a comprehensive analysis of the DC and RF behavior of HBTs, spanning temperatures from 350 to 3.8 K. It underscores the necessity of detailed studies for the design of RF circuits for quantum computing, including LNAs, VCOs, and mixers, due to the absence of cryogenic models. The DC gain shows betas of 800 at room temperature (RT) and 3000 at 3.8 K. RF characterization indicates a maximum fT of 500 GHz at 3.8 K and 300 GHz at RT. The proposed figure-of-merit, (gm.fT/Ic), typically used in CMOS design, is explored across the temperature range. The study reveals a noise equivalent temperature of sub-1 K at 3.8 K with source matching. The noise behavior of Si/SiGe:C HBTs within 0.13 μ m BiCMOS technology is characterized over 293 to 4 K and 10 kHz to 12 GHz. The analysis shows a significant increase in the flicker noise coefficient, K, and corner frequency reduction at 4 K. The high frequency parameter fT reaches 500 GHz, demonstrating better performance compared to advanced CMOS nodes. This research supports the modeling of HBTs that are critical for circuits operating at cryogenic temperatures. These models are particularly beneficial for designing classical-to-quantum interfaces.

This paper presents a scalable cryogenic readout solution for Superconducting Nanowire Single-Photon Detectors (SNSPDs) tailored for the readout of color-center-based qubits. The readout circuit, wire-bonded directly to the SNSPD, utilizes high input impedance to boost the signal amplitude, hence reducing the power consumption, and active quenching to prevent the latching induced by the high impedance. Fabricated in 40-nm CMOS in a 0.14-mm ² active area, the proposed system demonstrates competitive performance at 0.1 K, featuring low jitter [<60 ps Full Width at Half Maximum (FWHM)], high speed (dead time ≈ 5 ns) and low dark count rate (<1 Hz), while dissipating only 20 μ W. Such an ultra-low power and compact area enables the readout integration within a large-scale colorcenter quantum computer.