Quantum key distribution with higher-order alphabets using spatially encoded qudits

We present a proof of principle demonstration of 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 average error rate of 0.47. Using our system, it should be possible to send more than a byte of information per sifted photon.