Optical channels, such as fibers or free-space links, are ubiquitous in todays telecommunication networks. They rely on the electromagnetic field associated with photons to carry information from one point to another in space. As a result, a complete
physical model of these channels must necessarily take quantum effects into account in order to determine their ultimate performances. Specifically, Gaussian photonic (or bosonic) quantum channels have been extensively studied over the past decades given their importance for practical purposes. In spite of this, a longstanding conjecture on the optimality of Gaussian encodings has yet prevented finding their communication capacity. Here, this conjecture is solved by proving that the vacuum state achieves the minimum output entropy of a generic Gaussian bosonic channel. This establishes the ultimate achievable bit rate under an energy constraint, as well as the long awaited proof that the single-letter classical capacity of these channels is additive. Beyond capacities, it also has broad consequences in quantum information sciences.
We propose a new architecture for implementing electronic interferometry in quantum Hall bars. It exploits scattering among parallel edge channels. In contrast to previous developments, this one employs a simply-connected mesa admitting serial concat
enation of building elements closer to optical analogues. Implementations of Mach-Zehnder and Hambury-Brown-Twiss interferometers are discussed together with new structures yet unexplored in quantum electronics.
