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arXiv:2604.22101

Reconfigurable Superconducting Logic for On-Chip Photon Coincidence Detection

Gabriel Le Guay, Matteo Castellani, Reed A. Foster, Francesca Incalza, Alejandro Simon, Owen Medeiros, Phillip D. Keathley, Karl K. Berggren

Published Apr 23, 2026• 1 citation
arXivHTMLPDF

Abstract

Scaling photonic quantum-information platforms requires arrays of superconducting nanowire single-photon detectors (SNSPDs) for feedforward control, in which optical operations are conditioned on Bell-state measurements relying on photon-coincidence detections. On-chip superconducting cryotron electronics, performing logic on detector outputs and driving optical modulators, could reduce latency and room-temperature interconnect complexity for feedforward schemes. To date, no cryotron circuits designed for this purpose have been demonstrated. We demonstrate a bias-programmable logic gate based on three nanocryotrons (nTrons) that implements selectable AND (coincidence), XOR (odd-parity), and OR functions. It operates on two electrical pulses at 4.2 K, with bit-error rates below $10^{-3}$, bias margins up to $\pm21.7\%$, and operation extending to 25 MHz over narrower bias windows. It performs coincidence and odd-parity detection on two SNSPDs' outputs with bit-error rates below $3.2 \times 10^{-2}$. As proof-of-concept, we show that nTrons can drive capacitive loads up to 1.15 V, potentially enabling compatibility with electro-optic modulators in feedforward schemes.

DOI
10.48550/arxiv.2604.22101

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Single-photon coincidence counters are essential components in integrated quantum photonics, enabling efficient logic discrimination and real-time error correction at the chip level. However, monolithic integration at cryogenic temperature remains challenging. Here, we demonstrate a coincidence counter based on superconducting nanowire cryotrons (nTron). The circuit comprises five nTron devices, including delay gates, buffer gates, and an AND gate, achieving a maximum bias margin of 22% at a bit error rate (BER) of 10−5. Operating at 1 MHz, the counter exhibits a static power consumption of 282 nW and a dynamic power consumption of approximately 2 nW at a maximum operation frequency of 17 MHz. The coincidence time window is tunable, with a minimum width below 1 ns, and its position can be adjusted via bias currents. This design offers compatibility with superconducting nanowire single-photon detectors in fabrication and operation, supporting monolithic integration for scalable quantum photonic systems.
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 development of superconducting electronics based on nanocryotrons has been limited so far to few device circuits, in part due to the lack of standard and robust logic cells. Here, we introduce and experimentally demonstrate designs for a set of nanocryotron-based building blocks that can be configured and combined to implement memory and logic functions. The devices were fabricated by patterning a single superconducting layer of niobium nitride and measured in liquid helium on a wide range of operating points. The tests show 10 − 4 bit error rates with above ± 20 % margins up to 50 MHz and the possibility of operating under the effect of an out-of-plane 36 mT magnetic field, with ± 30 % margins at 10 MHz. Additionally, we designed and measured an equivalent delay-flip-flop made of two memory cells to show the possibility of combining multiple building blocks to make larger circuits. These blocks may constitute a solid foundation for the development of nanocryotron logic circuits and finite-state machines with potential applications in the integrated processing and control of superconducting nanowire single-photon detectors.
Superconducting (SC) electronics (SCE) is one of the major requirements to develop a large-scale practical quantum computing system. (Figs. 1(a)) [1]. Currently, a room temperature conventional computer is used as the control processor which requires a huge number of lossy interconnects to connect with the qubits placed at a few millikelvin temperatures. This, in turn, limits the scalability of the quantum computing system up to hundreds of qubits which is not enough to achieve the full potential [2]. Therefore, along with a suitable cryogenic memory, a cryogenic (preferably superconducting) control processor is required to develop a quantum computing system with thousands of qubits. Moreover, SCE offers ultra-fast (hundreds of gigahertz) and highly energy efficient (sub attojoule/bit) performance [3]. Therefore, SCE is projected to achieve the department of energy (DOE) exascale goal which the conventional supercomputers failed to achieve (Fig. 1(b)) [4]. However, Josephson junction-based SC circuits suffer from limited integration density due to flux trapping, difficulty in driving large impedances, fanning out digital signals, and operating in noisy environments [5]. To address these existing issues, here, we present a novel reconfigurable SC logic circuit using a nano-cryotron (nTron) that can perform 1-input copy, 2-input AND and OR, and 3-input majority logic functionalities. This work can enable the implementation of highly scalable and energy-efficient SC processor since the use of only one device. To achieve the reconfigurable nature of the logic circuit, we utilize the multi-gate switching of nTron.
Lithium Niobate On Insulator (LNOI) has recently emerged as a promising platform for the realization of electro-optically reconfigurable waveguides combining small footprint and ultra-low propagation loss. Integration of these components with superconducting nanowire single photon detectors (SNSPDs) opens up the possibility to perform on-chip manipulation and detection of quantum states of light in a physically stable and scalable architecture. Here we report recent theoretical and experimental progress towards the realization of fast electro-optic modulators compatible with cryogenic temperatures and high-performance NbTiN superconducting nanowires single-photon detectors (SNSPDs) monolithically integrated in LNOI waveguides.
Quantum photonic processing via electro-optic components typically requires electronic links across different operation environments, especially when interfacing cryogenic components such as superconducting single photon detectors with room-temperature control and readout electronics. However, readout and driving electronics can introduce detrimental parasitic effects. Here we show an all-optical control and readout of a superconducting nanowire single photon detector (SNSPD), completely electrically decoupled from room temperature electronics. We provide the operation power for the superconducting detector via a cryogenic photodiode, and readout single photon detection signals via a cryogenic electro-optic modulator in the same cryostat. This method opens the possibility for control and readout of superconducting circuits, and feedforward for photonic quantum computing.