Stefan Frick, University of Bristol, UK
Rangefinding has many applications in navigation, civil engineer, construction, military, surveillance and security. Most commonly rangefinders estimate the distance to an object by measuring the time of flight of light for the journey to and returning from the target. Conventional techniques use lasers for illumination in state of the art rangefinding systems. However, the particular state of light lasers produce makes them easy to detect.
Spontaneous parametric down-conversion is a process, happening in non-linear crystals, that allows for one photon of shorter wavelength to be destroyed while at the same time a photon pair of longer wavelength is created.One photon of such a pair will be in a quantum state of light much closer to background radiation than other light sources.Such a photon pair source can be utilised for rangefinding by keeping the first photon of the pair locally as a timing reference, while using the second photon to illuminate the target.
The principle of measuring distance via the time of flight of light is common to classical rangefinding and the rangefinding system presented here. However, the state of light produced in the down-conversion process, provides efficient camouflaging against background light.