A frequency-coded QKD scheme with an extension to qu-quarts

We propose a novel scheme to implement the BB84 quantum key distribution (QKD) protocol in optical fibers based on a quantum frequency-translation (QFT) process. Unlike conventional QKD systems, which rely on photon polarization/phase to encode qubits, our proposal utilizes photons of different frequencies. Qubits are thus expected to reach longer propagation distances due to the photon frequency state being more robust against mechanical and/or thermal fluctuations of the transmitting medium. Finally, we put forth an extension to a security-enhanced four-character-alphabet (qu-quarts) QKD scheme.