  The ability to engineer the spatial wavefunction of photons has enabled a
variety of quantum protocols for communication, sensing, and information
processing. These protocols exploit the high dimensionality of structured light
enabling the encodinng of multiple bits of information in a single photon, the
measurement of small physical parameters, and the achievement of unprecedented
levels of security in schemes for cryptography. Unfortunately, the potential of
structured light has been restrained to free-space platforms in which the
spatial profile of photons is preserved. Here, we make an important step
forward to using structured light for fiber optical communication. We introduce
a smart high-dimensional encryption protocol in which the propagation of
spatial modes in multimode fibers is used as a natural mechanism for
encryption. This provides a secure communication channel for data transmission.
The information encoded in spatial modes is retrieved using artificial neural
networks, which are trained from the intensity distributions of experimentally
detected spatial modes. Our on-fiber communication platform allows us to use
spatial modes of light for high-dimensional bit-by-bit and byte-by-byte
encoding. This protocol enables one to recover messages and images with almost
perfect accuracy. Our smart protocol for high-dimensional optical encryption in
optical fibers has key implications for quantum technologies relying on
structured fields of light, particularly those that are challenged by
free-space propagation.
