There have been tremendous advances in quantum computing technologies over the past decade, from noisy intermediate-scale machines to the emergence of fault-tolerant ones. A growing number of vendors are now commercializing small-scale quantum computers built on a variety of qubit platforms. Many have plans to scale them to what is referred to as “utility scale”—devices capable of tackling problems beyond the reach of classical high-performance computing. These scaling strategies often rely on the notion of modularity, wherein quantum hardware units of manageable size are mass produced and interconnected via a quantum network. This trajectory is reminiscent of the evolution of classical communications and networking, which ultimately led to the distributed computing that is so ubiquitous today. The advances in quantum networks, however, have so far been motivated by the security concerns arising from developments in quantum computing. A sufficiently large quantum computer running Shor’s algorithm, a quantum computing method developed by Peter Shor in 1994 that enables the factorization of large integers exponentially faster than classical algorithms, would pose a direct threat to the current public-key cryptographic infrastructure if alternative solutions are not implemented. Quantum key distribution (QKD) is one such technology that can potentially update encryption keys at a faster rate than a quantum computer could compromise them via Shor’s algorithm, thus providing an additional layer of security at the physical layer of the network. Embedding QKD technology into modern optical communications and networking infrastructure is challenging, but significant progress has been made through the development of techniques for “quantum-classical coexistence.” Similar techniques are now being extended to support more advanced quantum protocols within the existing fiber optic communications infrastructure. The study of quantum-classical coexistence is nearly as old as experimental quantum communication itself, having been immediately identified as a central engineering challenge for real-world use and dictating a