  Quantum communication has been successfully implemented in optical fibres and
through free-space [1-3]. Fibre systems, though capable of fast key rates and
low quantum bit error rates (QBERs), are impractical in communicating with
destinations without an established fibre link [4]. Free-space quantum channels
can overcome such limitations and reach long distances with the advent of
satellite-to-ground links [5-8]. Shorter line-of-sight free-space links have
also been realized for intra-city conditions [2, 9]. However, turbulence,
resulting from local fluctuations in refractive index, becomes a major
challenge by adding errors and losses [10]. Recently, an interest in
investigating the possibility of underwater quantum channels has arisen, which
could provide global secure communication channels among submersibles and boats
[11-13]. Here, we investigate the effect of turbulence on an underwater quantum
channel using twisted photons in outdoor conditions. We study the effect of
turbulence on transmitted QBERs, and compare different QKD protocols in an
underwater quantum channel showing the feasibility of high-dimensional encoding
schemes. Our work may open the way for secure high-dimensional quantum
communication between submersibles, and provides important input for potential
submersibles-to-satellite quantum communication.
