Silicon Single-Photon Detectors

Updated 27 July 2025

Silicon single-photon detectors (Si SPDs) are semiconductor-based devices engineered to detect individual photons using SPADs, SiPMs, and upconversion techniques.
They achieve high photon detection efficiency, low dark count rates, and excellent timing resolution, supporting applications in quantum optics, LIDAR, and biomedical imaging.
Advanced designs integrate fast quenching, gating modes, and nanophotonic enhancements to optimize performance and enable seamless integration with silicon photonics.

Silicon single-photon detectors (Si SPDs) are semiconductor-based devices engineered for the reliable detection of individual photons, primarily in the visible to near-infrared spectral regions. Si SPDs encompass various architectures such as single-photon avalanche diodes (SPADs), silicon photomultipliers (SiPMs), and upconversion detectors, and they are widely used in quantum optics, quantum information, LIDAR, biomedical imaging, and satellite-based quantum communications. Silicon technology enables Si SPDs to achieve high photon detection efficiency (PDE), low dark count rates, excellent timing resolution, and robust practical implementation with compatibility for mass production and integration with silicon photonics.

1. Detector Architectures and Operating Principles

Silicon SPADs

Si SPADs operate in Geiger mode, wherein a reverse-bias voltage exceeding the avalanche breakdown threshold allows a single carrier—generated by photon absorption—to trigger a self-sustaining avalanche. The avalanche current is then rapidly quenched and the device reset, either passively with a ballast resistor or actively using a dedicated electronic circuit. The resulting output is a digital pulse corresponding to the detection of one or more photons. Up to 84.4% PDE at 785 nm with afterpulse probability of 2.9% and dark count rates as low as 260 cps at 268 K have been demonstrated for specially designed thick-junction SPADs with backside illumination and optimized doping profiles (An et al., 24 Jul 2025).

Silicon Photomultipliers (SiPMs)

SiPMs are arrays of many small SPADs (microcells) interconnected in parallel, each operated above breakdown. The summed current from multiple simultaneous avalanches yields a pulse with amplitude proportional to the number of detected photons. This structure inherently provides photon-number resolution and supports extremely high count rates, with demonstrated peak detection rates up to 430 MHz (0807.2320). Detection efficiency is given by the product η_det = η_Si × (fill factor), with typical fill factors around 31% and moderate intrinsic quantum efficiency.

Upconversion Detectors (SFG-Si APDs)

Sum-frequency generation (SFG) based detectors convert “difficult-to-detect” near-infrared photons (e.g., 1550 nm) into visible photons (600–800 nm) using nonlinear optical processes in materials such as periodically-poled lithium niobate. The upconverted photons are then detected by standard Si APDs. The overall system efficiency is η_total = η_upconversion × η_Si (0807.2320). Timing resolution is particularly notable, with jitter as low as 50 ps and quantum efficiencies above 10%, but operation is limited by nonlinear noise constraints.

2. Performance Metrics and Trade-Offs

Detector Type	Peak PDE	Timing Jitter	Count Rate Capability	Other Metrics
Thick-junction Si SPAD	up to 84.4%	360 ps	Free-running & gated, hybrid; flexible	DCR: 260 cps<br>AP: 2.9% (An et al., 24 Jul 2025)
SFG-Si APD	2–12%	~50 ps	Multi-MHz	Room-temperature operation (0807.2320)
SiPM	~16% (array fill factor limited)	~200 ps	Up to 430 MHz	Photon-number resolution (0807.2320)
Gated Si SPAD	45% ±5%	—	Gated advantages in background	DCR: 2×10⁻⁶/ns (-30°C) (1208.4205)

Photon Detection Efficiency (PDE): Recent Si SPADs achieve PDEs exceeding 84% at 785 nm using thick-junction, backside-illuminated structures and high-voltage, fast quenching/readout circuits. SiPMs are limited by fill factor and microcell quantum efficiency.
Timing Jitter: Nanostructured thin-film SPADs can reach jitter below 20 ps while maintaining high spectral absorption using nanocone gratings (Ma et al., 2015). Si PMs and thick Si SPADs offer sub-ns performance.
Dark Count Rate (DCR): Optimized cooled devices obtain DCRs < 5 cps (waveguide-integrated SPADs, 243 K) (Yanikgonul et al., 2019). Active and gated operation further reduce DCR and suppress afterpulsing.
Count Rate: SiPMs excel at MHz–100s MHz due to array parallelization; thick SPADs with optimized electronics achieve robust performance in free-running and gated modes.

3. Mode of Operation and Electronic Readout

Free-Running, Gated, and Hybrid Modes

Modern Si SPDs support several modes:

Free-running mode: The device is always ready to detect incident photons. High PDE (84.4%) and low DCR demonstrated in this mode (An et al., 24 Jul 2025).
Gating mode: The device is sensitized only during specific time windows to suppress noise and accommodate intense background or preceding pulses. Gated detectors outperform free-running ones in “hidden-photon” and “post-strong-pulse” scenarios (1208.4205).
Hybrid mode: Combines gated and free-running features; sensitive to signal photons during gate-on and deactivated outside these intervals.

Advanced active quenching and reset circuits enable fast (~30 ns quench, 10 ns reset) recycling with high overvoltages, thus allowing saturation of PDE with minimal afterpulsing (Fang et al., 2020).

Sine-Wave Gating

High-frequency sine-wave gating (e.g., 152 MHz, 50 Vpp drive) increases PDE (from 68.6% to 73.1% at 785 nm) with negligible increase in dark counts or afterpulsing. This technique synchronizes the detector’s Geiger mode with photon arrival and enhances multiphoton entanglement experiments by boosting coincidence rates by >30% (Zhou et al., 2017).

4. Device Engineering and Fabrication Innovations

Nanophotonics/Absorption Enhancements

SPADs with nanocone gratings on top/bottom surfaces surpass flat-film devices by maintaining >60–80% absorption from 400–1000 nm, enabling high PDE with thin depletion layers and thus low jitter (~20 ps at elevated bias) (Ma et al., 2015).

Red-NIR Enhanced SPADs and Array Integration

Red-enhanced SPADs use thick p– epitaxial absorption layers for higher PDE (70% at 650 nm; 45% at 800 nm) at the cost of increased junction capacitance/timing. Second-generation designs introduce deep trenches for electrical and optical isolation in arrays, maintain low timing jitter (95 ps FWHM), and suppress optical crosstalk, enabling dense SPAD arrays suitable for imaging and spectroscopy (Gulinatti et al., 2020).

Waveguide-Integrated Devices

On-chip, waveguide-coupled SPADs fabricated on silicon-on-insulator platforms demonstrate >6% PDE (488/532 nm), with DCR <100 kHz at room temperature. These devices use lateral PN junctions beneath silicon nitride waveguides for efficient evanescent coupling and are compatible with monolithic silicon photonics integration (Govdeli et al., 2023). Simulations show p-i-n+ junctions supporting up to 52.4% PDE, 10 ps jitter, and DCR <5 cps at 243 K (Yanikgonul et al., 2019).

5. Environmental and Reliability Considerations

Spaceborne and Radiation Hardness

For satellite-based quantum communications, Si SPDs require mitigation of radiation-induced displacement damage—manifested as increased dark counts—through multistage (aluminum/tantalum) shielding, deep cooling (down to –60°C), and temperature/voltage-tunable driver electronics. In-orbit DCR increments as low as 0.54 cps/day over 1029 days with PDE >45% have been demonstrated for COTS-grade cooled Si APDs (Yang et al., 2019). At higher altitudes (MEO/GEO), minimal radiation-induced DCR increase is achieved with ≥10 mm aluminum shielding (Wilson et al., 2021). After a high-altitude nuclear event, Si SPDs in low-Earth orbit may become unusable within a day due to artificial radiation belts, but higher-altitude platforms fare better.

Noise, Afterpulsing, and Charge Persistence

Thick absorption regions and high excess bias tend to increase DCR and afterpulse probability; advanced fabrication (doping-compensated regions, high-voltage integration) and deep trenches (for arrayed devices) counteract these effects (An et al., 24 Jul 2025, Gulinatti et al., 2020). Gating and fast active quenching suppress afterpulsing and “charge persistence” noise, the latter being more prominent at low temperatures and in devices with large absorption volumes (1208.4205).

6. Detector Calibration and Metrology

Absolute calibration of Si SPDs is critical for quantum information applications. Quantum-correlated bi-photon (SPDC) sources enable reference-free, on-site efficiency calibration via the Klyshko method, but care must be taken to avoid bias from multi-photon “heralding” and system losses. Detector POVMs and two-mode squeezed vacuum states accurately model counting statistics, and system transmittivities must be well-characterized to extract true device efficiency (Pani et al., 3 Dec 2024).

7. Applications and Impact

Si SPDs have enabled:

Quantum photonics: High-order entanglement generation, quantum key distribution, and quantum memory.
Advanced imaging: Fluorescence lifetime, LIDAR, low-light and single-molecule detection (Ma et al., 2015, Gulinatti et al., 2020).
Space quantum communications: Low-noise, radiation-hardened Si SPDs extend mission lifespans and support feats such as ground-to-satellite teleportation (Yang et al., 2019).
Integrated quantum photonics: Waveguide SPADs allow chip-scale quantum systems (Yanikgonul et al., 2019, Govdeli et al., 2023).

Broader developments such as hybrid Ge–Si SPADs for SWIR (e.g., 1310 nm at room temperature with 12% SPDP) (Na et al., 15 Jul 2024), and SiC SPADs for UV applications (Zhao et al., 7 Nov 2024), demonstrate the continued evolution and diversification of silicon-based single-photon detection technology.

Si SPDs remain at the forefront of research and application due to their scalability, robustness, high PDE, and integration potential. Ongoing advances in device engineering, quenching/readout, array integration, and environmental resilience ensure the continued expansion of their capabilities across the quantum technology landscape.