This document summarizes a quantum key distribution scheme that uses higher-dimensional alphabets encoded in the spatial degrees of freedom of photons. It allows for the transmission of 4.56 bits per sifted photon while providing improved security, as an intercept-resend attack would induce an average error rate of 0.47. The scheme extends the BB84 protocol to higher dimensions using spatially encoded qudit states. It presents a proof-of-principle demonstration and illustrates the encoding method that uses optical systems and Fourier transforms to define the possible detection amplitudes across a 3D alphabet.

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The authors acknowledge financial
support from the Brazilian Millennium
Institute for Quantum Information,
CNPq, CAPES, FAPERJ, FUJB and
PRONEX.
“A verdade vos libertará”
S. P. Walborn, D. S. Lemelle, M. P. Almeida, and P. H. Souto Ribeiro
Instituto de Física, Universidade Federal do Rio de Janeiro,
Caixa Postal 68528, Rio de Janeiro, RJ 21941-972, Brazil
(Received 12 October 2005; published 8 March 2006)
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.
BB84 in d-dimensional AAllpphhaabbeettss
•Increasing the dimensionality using spatially encoded qudits.
•The major bit transmission rate per Photon and the major security of the QKD scheme.
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