Feng et al. report on the use of a large area “fractal” superconducting nanowire single photon detector (SNSPD) to reconstruct 3D images of a scene behind a wall at wavelengths between 1500 and 1900 nm. In this spectral region, sun irradiance is significantly lower compared to more traditional wavelengths, thus promising high precision, non-line-of-sight imaging in real-world scenes.

The team uses a new generation SNSPD sensor that has a large area and is relatively insensitive to polarisation. They can then capture the remaining photons from a laser beam that is reflected from a surface, bounces behind a wall that is hiding a scene, from which photons are then retro-reflected back by the same path to the SNSPD sensor. The three-dimensional image of the hidden objects is reconstructed numerically from the time-of-flight data of the detected photons.

The SNSPD have a very high temporal resolution and this in turn allows to image objects beyond the line of sight with a transverse resolution of 1-2 cm. The detectors also have high sensitivity out to relatively long wavelengths in the 2 micrometer region. Estimations by the team indicate that the lower quantum efficiency at longer wavelengths is offset by the even lower solar radiation, thus resulting in an improved signal-to-noise ratio.

Despite the detector technology requiring cryo-cooling, advances towards miniaturisation and portability hold promise for future systems that could be deployed in the real-world for non-line-of-sight imaging.

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