Quantum Rangefinding

  • Stefan Frick

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)


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 as the light source of
choice in state of the art rangefinding systems. Lasers are nowadays cheap to manufacture and can produce amounts of optical powers excessive of the typically required microwatts, yet, the particular state of light they are producing makes them also easy to detect. Spontaneous parametric down-conversion is a quantum 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. If the
non-linear crystal is engineered carefully, one photon of such a pair will be in a quantum state of light much closer to background radiation than other light sources, and
especially much closer than the quantum state produced by lasers. 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 delay
between the first and the second photon, after its return from the target, can then be
used to estimate the time of flight to the target and back, effectively determining the
distance. 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 of one photon, produced in the down-conversion process, provides efficient camouflaging against background light and therefore makes the detection of the
rangefinding signal impossible.
Date of Award23 Jan 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorJohn G Rarity (Supervisor)

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