We demonstrate the transfer of information encoded as orbital angular momentum (OAM) states of a light beam. The transmitter and receiver units are based on spatial light modulators, which prepare or measure a laser beam in one of eight pure OAM states. We show that the information encoded in this way is resistant to eavesdropping in the sense that any attempt to sample the beam away from its axis will be subject to an angular restriction and a lateral offset, both of which result in inherent uncertainty in the measurement. This gives an experimental insight into the effects of aperturing and misalignment of the beam on the OAM measurement and demonstrates the uncertainty relationship for OAM.

We propose an interferometric method for measuring the orbital angular momentum of single photons. We demonstrate its viability by sorting four different orbital angular momentum states, and are thus able to encode two bits of information on a single photon. This new approach has implications for entanglement experiments, quantum cryptography and high density information transfer.

Some 16 years ago, Allen et al. [Phys. Rev. A 45, 8185 (1992)] recognised that laser beams which carried an angular momentum additional to photon spin, could be realized in the laboratory. Such beams have helical phase fronts and so have an azimuthal component to the Poynting vector, which results in angular momentum along the beam axis. This orbital angular momentum, very often combined with spin to make optical angular momentum, has given rise to many developments. These range from optical spanners for driving micro-machines to high dimensional quantum entanglement and new opportunities in quantum information processing. The concept of orbital angular momentum is now leading to new understanding of a wide range of phenomena, including fundamental processes in Bose-Einstein condensates, while the associated technologies have led to new applications in optical tweezing and microscopy.Orbital angular momentum phasefronts and interferogram

Entanglement of the properties of two separated particles constitutes a fundamental signature of quantum mechanics and is a key resource for quantum information science. We demonstrate strong Einstein, Podolsky, and Rosen correlations between the angular position and orbital angular momentum of two photons created by the nonlinear optical process of spontaneous parametric down-conversion. The discrete nature of orbital angular momentum and the continuous but periodic nature of angular position give rise to a special sort of entanglement between these two variables. The resulting correlations are found to be an order of magnitude stronger than those allowed by the uncertainty principle for independent (nonentangled) particles. Our results suggest that angular position and orbital angular momentum may find important applications in quantum information science.