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Cryo-CMOS Readout of Single-Photon Detectors for Color-Center Quantum Computers

G. Carboni, M. Li, M.C. van der Maas, Luc Enthoven, Lin Jin, Jad Benserhir, Jan Riegelmeyer, Carlos Errando-Herranz, Ryoichi Ishihara, Masoud Babaie, Fabio Sebastiano

Published Sep 8, 2025• 2 citations
Publisher

Abstract

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$0.14-\text{mm}^{2}$active area, the proposed system demonstrates competitive performance at 0.1 K, featuring low jitter [$<60 \text{ps}$Full Width at Half Maximum (FWHM)], high speed (dead time$\approx 5 \text{ns}$) and low dark count rate ($<1 \text{Hz}$), while dissipating only$20 \mu \mathrm{W}$. Such an ultra-low power and compact area enables the readout integration within a large-scale colorcenter quantum computer.

DOI
10.1109/esserc66193.2025.11213981

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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 a0.14−mm20.14-\text{mm}^{2}0.14−mm2active area, the proposed system demonstrates competitive performance at 0.1 K, featuring low jitter [<60ps<60 \text{ps}<60psFull Width at Half Maximum (FWHM)], high speed (dead time≈5ns\approx 5 \text{ns}≈5ns) and low dark count rate (<1Hz<1 \text{Hz}<1Hz), while dissipating only20μW20 \mu \mathrm{W}20μW. Such an ultra-low power and compact area enables the readout integration within a large-scale colorcenter quantum computer.
Optical quantum information processing, quantum key distribution (QKD), and linear optical quantum computing (LOQC) rely fundamentally on single-photon detectors capable of ultra-high system detection efficiency, low dark count rates, sub-nanosecond timing jitter, and high maximum count rates. Superconducting Nanowire Single-Photon Detectors (SNSPDs) have emerged as the gold-standard quantum light detection technology, outperforming semiconductor avalanche photodiodes (APDs) and transition-edge sensors (TES) across every major performance metric. By biasing ultrathin superconducting nanometric strips (e.g., NbN, WSi) close to their critical superconducting-to-normal transition temperature, the absorption of a single optical photon breaks Cooper pairs, creating a localized resistive hot spot that generates an electrical voltage pulse with near-unity quantum efficiency (>98%) and picosecond timing jitter (<10 ps). Integrating SNSPDs directly onto Quantum Photonic Integrated Circuits (QPICs) on silicon-on-insulator or lithium niobate substrates unlocks scalable, multi-channel quantum photonic processors. However, scaling SNSPDs from single-pixel devices to large multiplexed focal plane arrays introduces severe cryogenic and wiring bottlenecks: massive coaxial wiring heat leaks at millikelvin dilution refrigerator stages, complex radio-frequency (RF) readout multiplexing, spatial thermal quenching limits, and single-photon coupling losses across photonic interfaces. This paper delivers an exhaustive, mathematically formal technical investigation and empirical benchmark of SNSPD-integrated quantum photonic integrated circuits. We formalize hot-spot electrothermal hotspot formation equations, derive system detection efficiency (SDE) and timing jitter bounds, and model cryogenic CMOS readout multiplexing architectures. Furthermore, we construct an enterprise multi-physics co-simulation and cryogenic optical testbed evaluating 64-channel SNSPD arrays integrated with silicon nitride QPICs across high-rate QKD and boson sampling workloads. Our empirical findings demonstrate that our integrated NbN SNSPD arrays achieve a system detection efficiency of 99.4% at 1550 nm, a timing jitter of 6.2 ps, and a dark count rate below 10^-4 Hz while operating at 2.1 K, outperforming conventional semiconductor APDs by orders of magnitude. Finally, we formulate open challenges in cryo-CMOS multiplexer thermal dissipation, multi-channel array yield, and monolithic single-photon emitter integration.
Scalable superconducting nanowire single photon detector (SNSPDs) arrays require cryogenic digital circuits for multiplexing the output detection pulses. Among existing superconducting digital devices, superconducting nanowire cryotron (nTron) is a three-terminal device with an ultra-compact size, which is promising for large scale monolithic integration. In this report, in order to evaluate the potential and possibility of using nTrons for reading and digitizing SNSPD signals, we characterized the grey zone, speed, timing jitter and power dissipation of a proper designed nTron. With a DC bias on the gate, the nTron can be triggered by a few μA high and nanoseconds wide input signal, showing the nTron was capable of reading an SNSPD pulse at the same signal level. The timing jitter depended on the input signal level. For a 20 μA high and 5 ns wide input pulse, the timing jitter was 33.3 ps, while a typical SNSPD's jitter was around 50 ps. With removing the serial inductors and operating it in an AC bias mode. The nTron was demonstrated to be operated at a clock frequency of 615.4 MHz, which was faster than the maximum counting rate of a typical SNSPD. In additional, with a 50 Ω bias resistor and biased at 17.6 μA, the nTron had a total power dissipation of 19.7 nW. Although RSFQ circuits are faster than nTrons, for reading SNSPD or other detector arrays that demands less operation speed, our results suggest a digital circuit made from nTrons could be another promising alternative.
The superconducting nanowire single-photon detector (SNSPD) is a leading technology for quantum information science applications using photons, and is finding increasing uses in photon-starved classical imaging applications. Critical detector characteristics, such as timing resolution (jitter), reset time, and maximum count rate, are heavily influenced by the readout electronics that sense and amplify the photon detection signal. We describe a readout circuit for SNSPDs using commercial off-the-shelf amplifiers operating at cryogenic temperatures. Our design demonstrates a 35 ps timing resolution and a maximum count rate of over 2 × 107 counts per second, while maintaining <3 mW power consumption per channel, making it suitable for a multichannel readout.
Scalable, high speed data transfer between cryogenic (0.1-4 K) and room temperature environments is instrumental in a broad range of fields including quantum computing, superconducting electronics, single photon imaging and space-based communications. A promising approach to overcome the limitations of conventional wire-based readout is the use of optical fiber communication. Optical fiber presents a 100-1,000x lower heat load than conventional electrical wiring, relaxing the requirements for thermal anchoring, and is also immune to electromagnetic interference, which allows routing of sensitive signals with improved robustness to noise and crosstalk. Most importantly, optical fibers allow for very high bandwidth densities (in the Tbps/mm2^22 range) by carrying multiple signals through the same physical fiber (Wavelength Division Multiplexing, WDM). Here, we demonstrate for the first time optical readout of a superconducting nanowire single-photon detector (SNSPD) directly coupled to a CMOS photonic modulator, without the need for an interfacing device. By operating the modulator in the forward bias regime at a temperature of 3.6 K, we achieve very high modulation efficiency (1000-10,000 pm/V) and a low input impedance of 500 ΩΩΩ with a low power dissipation of 40 μμμW. This allows us to obtain optical modulation with the low, millivolt-level signal generated by the SNSPD. Optical communication is becoming the preferred I/O solution in modern, room temperature high performance systems, and this work demonstrates its suitability for scalable cryogenic readout, which could help achieve the full potential of superconducting technologies.
Abstract Scalable, low power, high speed data transfer between cryogenic (0.1–4 K) and room temperature environments is essential for the realization of practical, large-scale systems based on superconducting technologies. A promising approach to overcome the limitations of conventional wire-based readout is the use of optical fiber communication. Optical fiber presents a 100–1,000x lower heat load than conventional electrical wiring, relaxing the requirements for thermal anchoring, and is also immune to electromagnetic interference, which allows routing of sensitive signals with improved robustness to noise and crosstalk. Most importantly, optical fibers allow for very high bandwidth densities (in the Tbps/mm2 range) by carrying multiple signals through the same physical fiber (Wavelength Division Multiplexing, WDM). Here, we demonstrate for the first time optical readout of a superconducting nanowire single-photon detector (SNSPD) directly coupled to a CMOS photonic modulator, without the need for an interfacing device. By operating the modulator in the forward bias regime at a temperature of 3.6 K, we achieve very high modulation efficiency (1,000–10,000 pm/V) and a low input impedance of 500 Ω with a low power dissipation of 40 μW. This allows us to obtain optical modulation with the low, millivolt-level signal generated by the SNSPD.
We demonstrate the transfer of single photon triggered electrical pulses from a superconducting nanowire single photon detector (SNSPD) to a single flux quantum (SFQ) pulse. We describe design and test of a digital SFQ based SNSPD readout circuit and demonstrate its correct operation. Both circuits (SNSPD and SFQ) operate under the same cryogenic conditions and are directly connected by wire bonds. A future integration of the present multi-chip configuration seems feasible because both fabrication process and materials are very similar. In contrast to commonly used semiconductor amplifiers, SFQ circuits combine very low power dissipation (a few microwatts) with very high operation speed, thus enabling count-rates of several gigahertz. The SFQ interface circuit simplifies the SNSPD readout and enables large numbers of detectors for future compact multi-pixel systems with single photon counting resolution. The demonstrated circuit has great potential for scaling the present interface solution to 1,000 detectors by using a single SFQ chip.