Quantum Key Distribution in Access Networks

This chapter presents a literature survey of data security in optical fiber networks. The evolution of data security in optical fibers starts from PHY-layer encryption based on ITU-T G.709 optical transport networks (OTN) standards, where the security of classical cryptography is based on computational complexities of intractable mathematical problems. The emerging quantum computers make threats to classical cryptography. Two strategies are developed to address these threats, i.e., post-quantum cryptography (PQC) and quantum key distribution (QKD). PQC algorithms are still based on computational complexities, whereas QKD provides information-theoretic security guaranteed by quantum mechanics. A comprehensive literature review of QKD is presented with a focus on quantum networks in optical fibers. Three issues hindering QKD technologies from wide commercial deployment include distance limit, noise interference, and QKD in passive optical networks (PONs). The key rate of a QKD link scales linearly with the channel transmittance. In optical fibers, the channel transmittance decays exponentially with fiber distance due to the absorption of photons. This limits the distances of fiber-based QKD links to hundreds of kilometers. To extend distances, trusted and untrusted relay technologies were developed, depending on key exposure at the relay node. Since QKD links are vulnerable to noise and interference from classical data traffic, most reported QKD systems need dedicated/dark fibers. But this is cost-prohibitive and impractical in real deployments. The only chance for the commercial success of QKD technologies is integration into existing fiber networks and sharing the same fibers with classical data traffic, which in turn brings the second issue, spontaneous Raman scattering (SpRS) noise from classical channels. Several noise mitigation strategies are discussed based on wavelength/time-division multiplexing (WDM/TDM) techniques. Finally, as the last mile of optical fiber networks, PONs are not only the bottleneck of data throughput but also the most vulnerable segment of data security. Most eavesdropping occurs in PONs leveraging the point-to-multipoint (P2MP) topology, where the downstream data are broadcast to all users. In PONs, the imbalanced attenuation to classical downstream data and the upstream QKD channel makes the downstream data in the feeder fiber become the dominant source of SpRS noise. A dual-feeder fiber method is introduced to mitigate SpRS noise by isolating QKD links in a dedicated feeder fiber.