Short answer: Single-photon lidar measures distance by timing individual photons returning from a scene, which lets it work with very little light. Published experiments report depth images from about one detected photon per pixel, reconstruction through obscurants such as fog and smoke, and a 2025 design that approaches the standard quantum limit for intensity estimation. All of these are laboratory or field demonstrations.
How it works
A 2025 Frontiers in Physics paper describes lidar as using pulsed lasers to measure target reflectivity and distance, with depth estimated from the time of flight of photons. A 2021 Scientific Reports paper explains why single-photon detection is attractive: its high sensitivity and picosecond timing resolution allow high-resolution depth profiling of remote, complex scenes with low-power illumination, and it suits highly scattering environments such as fog and smoke.
Imaging with few photons
The Frontiers paper presents a noncoaxial scanning system that generates depth and reflectivity images with an average of only about one detected photon per pixel, using a single-photon avalanche diode. Its depth retrieval algorithm uses an l1-norm regularizer and does not need prior knowledge of the number of targets in the scene, which the authors say improves robustness. They report experimental results that validate the algorithm for photon-starved three-dimensional imaging.
Through obscurants
The 2021 paper reconstructs depth profiles of moving objects through obscurant equivalent to five attenuation lengths between transceiver and target, at stand-off distances up to 150 m. It uses a statistically based processing algorithm designed for real-time reconstruction from single-photon data, and provides uncertainty estimates in the depth reconstruction. The authors describe it as pointing a way forward for high-resolution imaging from mobile platforms in degraded visual environments.
Approaching the quantum limit
A 2025 Light: Science and Applications paper says quantum-inspired imaging has been effective for lidar with advances in single-photon detectors and algorithms, but that outdoor background noise and signal variation keep performance far from the limit set by quantum fluctuations of coherent probe light. The authors record echo photon numbers with a photon-number-resolving detector and use an active photon-number filter to overcome heavy background noise. They report approaching the standard quantum limit for intensity estimation over a wide photon-flux range, and a Fisher information only 0.04 dB below the quantum Fisher information when the mean signal photon number is 10. They demonstrate noise-free target reconstruction in daytime, and better reflectivity resolution than on/off detection when using only a thousandth of the measurements.
Limits
The three papers use different detectors, ranges and conditions. The term "quantum" here mostly refers to single-photon detection and to comparison with the standard quantum limit, not to entangled light, and none of the papers reports a deployed system.
FAQ
What makes single-photon lidar different?
It times individual returning photons, which gives picosecond timing and high sensitivity at low illumination power.
How few photons are needed?
The 2025 Frontiers paper reports images with about one detected photon per pixel.
Does it work in fog?
The 2021 paper reconstructs moving objects through obscurant equal to five attenuation lengths at up to 150 m.
What is the standard quantum limit here?
The limit on intensity estimation set by quantum fluctuations of coherent probe light, per the 2025 Light paper.
Is it entanglement-based?
These papers do not use entangled light.
All field notes