  Controlling and engineering continuous spectral modes of entangled photons
represents one of the promising approaches toward secure quantum
communications. By using the telecom bandwidth generated from a cascade-emitted
biphoton in atomic ensembles, a fiber-based long-distance quantum communication
can be feasible owing to its low transmission loss. With multiplexed photon
pairs, we propose to implement a nonorthogonal coding scheme in their spectral
modes and present an architecture of multiple channels enabling a high-capacity
transfer of codewords. Using the measures of the second-order correlations and
associated visibility and contrast, we further quantify the performance of the
proposed nonorthogonal coding scheme. Our results demonstrate the capability to
encode and decode quantum information beyond the orthogonal coding scheme. The
proposed scheme here can be applicable to a large-scale and multiuser quantum
communication and pave the way toward an efficient and functional quantum
information processing.
