New J. Phys. 17, 033033 (2015)

The orbital angular momentum (OAM) modes of photons are potentially useful for high-dimensional quantum key distribution (QKD) systems, offering advantages over polarization encoding, including an increase in information capacity and increased resilience against eavesdropping. However, the idea has largely remained impractical mostly due to the difficulty in efficiently sorting single photons using an OAM basis. Now, Mohammad Mirhosseini and co-workers from the USA, Austria, the UK and Canada have demonstrated a proof-of-principle experiment that uses a seven-dimensional alphabet encoded in the photons' OAM modes and angular position (ANG). A collimated beam from a He-Ne laser is processed by a binary hologram from a digital micro-mirror device to generate OAM and ANG modes at a rate of 4 kHz. The international team of scientists also used a mode sorter capable of sorting single photons based on their OAM and ANG content with a separation efficiency of 93%. The result was communication with an information capacity of 2.05 bits per shifted photon, which is more than twice the maximum allowable capacity of a two-dimensional QKD system. The symbol error rate was measured to be 10.5%, which was sufficient for proving unconditional security against coherent and individual eavesdropping attacks for an infinite key.