  Quantum key distribution (QKD) promises information-theoretically secure
communication, and is already on the verge of commercialization. Thus far,
different QKD protocols have been proposed theoretically and implemented
experimentally [1, 2]. The next step will be to implement high-dimensional
protocols in order to improve noise resistance and increase the data rate
[3-7]. Hitherto, no experimental verification of high-dimensional QKD in the
single-photon regime has been conducted outside of the laboratory. Here, we
report the realization of such a single-photon QKD system in a turbulent
free-space link of 0.3 km over the city of Ottawa, taking advantage of both the
spin and orbital angular momentum photonic degrees of freedom. This combination
of optical angular momenta allows us to create a 4-dimensional state [8];
wherein, using a high-dimensional BB84 protocol [3, 4], a quantum bit error
rate of 11\% was attained with a corresponding secret key rate of 0.65 bits per
sifted photon. While an error rate of 5\% with a secret key rate of 0.43 bits
per sifted photon is achieved for the case of 2-dimensional structured photons.
Even through moderate turbulence without active wavefront correction, it is
possible to securely transmit information carried by structured photons,
opening the way for intra-city high-dimensional quantum communications under
realistic conditions.
