Afterpulsing reduction in InGaAs/InP SPADs with a compact, high-speed quenching circuit

InGaAs/InP Single-Photon Avalanche Diodes (SPADs) are widely employed in quantum optics applications, particularly in Quantum Key Distribution (QKD), due to their high sensitivity in the near-infrared range. In QKD, low noise and high timing resolution are essential to ensure secure and high-rate key exchange. One of the main limitations of InGaAs/InP SPADs is afterpulsing, which degrades the quantum bit error rate (QBER), limits the maximum count rate, and can compromise both the security and throughput of QKD systems. Reducing afterpulsing is thus crucial for enabling high-speed, reliable quantum communication. We present a newly developed gating and quenching circuit that significantly reduces afterpulsing. The circuit is located entirely outside the detector package, maintaining compatibility with any InGaAs/InP SPAD, and supports both gated and free-running operation. Unlike integrated ASIC-based solutions, which offer low parasitics but are often limited by process constraints (e.g., low maximum voltages), our approach uses high-speed discrete components, allowing high excess bias operation and design flexibility. While discrete designs are typically more prone to parasitics, our compact layout effectively mitigates these effects and preserves signal integrity. Experimental results show a strong afterpulsing reduction compared to our current commercial module. Additionally, the measured SPAD current highlights that the avalanche is completely quenched in less than a nanosecond after sensing. Eventually, the small footprint of the design paves the way for a new generation of compact, low-afterpulsing InGaAs/InP SPAD modules suitable for high-performance quantum optics applications.