  High-dimensional quantum key distribution (HDQKD) is a promising avenue to
address the inherent limitations of basic QKD protocols. However, experimental
realizations of HDQKD to date have relied on indeterministic photon sources
that limit the achievable key rate. In this paper, we demonstrate a full
emulation of a HDQKD system using a single colloidal giant quantum dot (gQD) as
a deterministic, compact and room-temperature single-photon source (SPS). We
demonstrate a practical protocol by encoding information in a high-dimensional
space ($d = 3$) of the orbital angular momentum of the photons. Our
experimental configuration incorporates two spatial light modulators for
encoding and decoding the spatial information carried by individual photons.
Our experimental demonstration establishes the feasibility of utilizing high
radiative quantum yield gQDs as practical SPSs for HDQKD. We also demonstrate
experimentally secure qudit transmission exceeding one secure bit per photon,
thus already beating the traditional d=2 QKD capacity.
