A photon source and method of operating a photon source

摘要

A photon source for emitting polarisation entangled photons . s. +, . s. -, useful in quantum cryptography is described. A single quantum dot 1 of a quantum dot layer in a device is isolated. The quantum dot has a degenerate exciton level (13, fig 2). A naturally non-degenerate bright exciton level (25, fig 3) can be made degenerate by application of an electric field causing Stark shifts (fig 4), the magnitude of which can be optimised by choice of direction of application of the field though the dot, by magnetic field causing the Zeeman effect, or varying the strain across the crystal. A biexciton (11, fig 2) or higher order exciton state is excited within the quantum dot. A first photon is emitted, after which the remaining bright exciton decays emitting the second photon. A filter is used to transmit only the entangled photon pair, which may be separated using for example using a time gated switch or dichoric mirror.