Quantum key distribution limits by using multichannel spectral- space scheme and entangled photons in optical fibers

We discuss achievable limits for high-speed quantum key distribution (QKD) over optical fibers serving as an optical quantum channel and the ways to achieve high rates in secure QKD over non-regenerative optical fiber links. The multicarrier transmission is a viable option to increase the secure rate and it can be combined by the employment of the orbital angular momentum of the photons in order to further increase the total secure rate. Our numerical results indicate that it is possible to achieve QKD transmission rate that exceeds 100 Mb/s over non-regenerative optical fiber links, which could scale above 50 km of deployed fibers. The higher speed QKD with enhanced security could be achieved in a similar manner by employing the total angular momentum and a modified BB84 protocol.