Single Photons

All you ever wanted to know about photons…

Dr Peter Mosley, University of Bath (MA PhD Oxford)

BRLSI 27th April 2016 19:00

Notes by William Gaskell

This talk discusses how to make photons one at a time and the bespoke fibre technology, photonic crystal fibre pipeline developed in Bath.

A photon is a fundamental excitation of the electromagnetic field.

A beamsplitter is a half silvered mirror which has a 50% chance of either reflecting or transmitting a photon interacting with it. It divides the photon due to wave/particle duality but if we place detectors after the beamsplitter we would only detect a single photon arriving either after being transmitted or reflected. There is a random chance of the photon appearing at either detector in a 50% beamsplitter.

Quantum Information can use this effect for a very high quality random number generator (for example in Monte Carlo simulations).

Using a string of single photons as a key can more securely encrypt a signal and also can then detect if signal is intercepted by comparing the signal received with the generated signal – the interception would jumble the encryption.

Photonic quantum computation would require following setup:

Multiple single photon sources -> Quantum gates (half silvered mirror beamsplitters) -> multiple photon detectors.

Attenuation of a laser beam makes for a poor single photon source as distribution of different number of photons produced by attenuation from packets of 1 to n photons being produced in a Poisson or normal distribution depending on the laser source and attenuation material – can’t reliably get a single photon of a certain energy and characteristics.

Photon gun needs to reliably produce single photons which are identical in characteristics, spectrum, polarization, shape and size etc.

It is possible to verify the identical nature of photons using a second order coherence test, can only appear in pairs if identical after passing through test due to destructive interference making them cancel each other out in certain probabilistic outcomes; photons always bunch together due to interference in the filter which then compares photons to make sure they are identical. Techniques for this are either using quantum dots or using a non-linear response to a pulsed laser beam.

Dr Mosley uses a short pulse of light through a transparent field, electrons are perturbed a long way from their equilibrium position and their relaxation after the laser pulse produces a pair of identical photons as in a similar manner to the Einsteinian piezoelectric effect. This process turns billions of photons from laser beam into a photon pair, then the two identical photons are transformed into a non-identical pair using conservation of energy. Using dispersion [definition: the separation of white light into colours or of any radiation according to wavelength. Google] to control the relative velocity of different wavelengths of light to control the effect of the attenuation on the pulsed laser beam through the photonic crystal fibre in the apparatus and improve the statistics of only a single photon being produced by the gun, there would still be a probability that the photons could appear in packets of more than one in each bunch. The dispersion rate is set to allow one in a hundred pulses producing a single photon for the apparatus. Using the bespoke photonic crystal fibre technology.

The timing is slowed down to leave 1 second between each photon produced for the apparatus to handle the setup. The signal is then further processed using a four-way electronic multiplex to reduce multiple photon events.