Optical communications at the quantum limit

The ability to exchange messages between distant users is the cornerstone of the second industrial revolution and a crucial stepping stone of the current third industrial revolution, involving digital technologies and automation. As the bit rate approaches the nonlinear channel capacity, it is necessary to provide quantum-mechanical description of the information transmission and processing. We review the modeling approach of information carriers as quantum-optical states, assessing the advantages of considering the Holevo capacity compared to the Shannon capacity. We present a novel quantum-limited all-optical architecture based on Fourier codes generated by a single passive multiport encoder/decoder. We demonstrate that the proposed Fourier machine presents a coding efficiency larger than Shannon limit.