Measurement of the 'single-photon' velocity and classical group velocity in a standard optical fiber

02 Apr 2007

Jonathan Ingham and colleagues at the University of Cambridge, UK, have used kilometer lengths of standard optical fiber to demonstrate that changes in photon number states do not induce dispersion (Meas. Sci. Technol. 18 1538-1546).

Technologies based on single-photon phenomena in secure quantum communications are becoming increasingly important. This makes it vital to understand whether changes in the composition of photon states could alter the propagation of light in a dispersive fiber.

For example, it's possible that the velocity of groups of photons might vary in a dispersive waveguide or medium, not just with the modal frequency but also with the composition of the number states with different ranges of Schrödinger frequencies.

Ingham and colleagues investigated this possibility by comparing the velocity of approximate single-photon states and the classical optical group velocity. The experiments use the same source and the same novel technique with a single temporal standard to measure the time of flight of both 'single photons' and classical optical energy over standard optical fibre.

Elegant experiments to test such a question have previously been carried out, but only in centimeter lengths of bulk glass. The Cambridge team believes that this is the first such test on kilometer lengths of standard fiber.

The researchers conclude that, within the experimental accuracy (approximately ±0.2 ns in 30,000 ns, or 6.3 km of fiber), there is no evidence for changes of velocity as the composition of the number states varies. This suggests that photons travel in a dispersive fiber with a velocity that is independent of the Schrödinger frequencies.