Quantum key Distribution in a High-Dimensional State Space: Exploiting the Transverse Degree of Freedom of the Photon
R. W. Boyd, A. Jha, M. Malik, C. O'Sullivan, B. Rodenburg, and D. J. Gauthier, in Advances in Photonics of Quantum Computing, Memory, and Communication IV (International Society for Optics and Photonics, 2011), pp. 79480L.