Quantum key distribution with higher-order alphabets using   spatially-encoded qudits
  We propose and demonstrate a quantum key distribution scheme in higher-order $d$-dimensional alphabets using spatial degrees of freedom of photons. Our implementation allows for the transmission of 4.56 bits per sifted photon, while providing improved security: an intercept-resend attack on all photons would induce an error rate of 0.47. Using our system, it should be possible to send more than a byte of information per sifted photon. 
S. P. Walborn, D. S. Lemelle, M. P. Almeida and P. H. Souto Ribeiro