We have developed a multi-user QKD network using wavelength multiplexing and high-dimensional arrival-time encoding. We have demonstrated an order of magnitude key rate improvement against recent implementations with >1 bits/photon efficiency.
Quantum Key Distribution (QKD) exploits the rules of quantum mechanics to generate and securely distribute a random sequence of bits to two spatially separated clients. Typically a QKD system can support only a single pair of clients at a time, and so a separate quantum link is required for every pair of users. We overcome this limitation with the design and characterization of a multi-client entangled-photon QKD system with the capacity for up to 100 clients simultaneously. The time-bin entangled QKD system includes a broadband down-conversion source with two unique features that enable the multi-user capability. First, the photons are emitted across a very large portion of the telecom spectrum. Second, and more importantly, the photons are strongly correlated in their energy degree of freedom. Using standard wavelength division multiplexing (WDM) hardware, the photons can be routed to different parties on a quantum communication network, while the strong spectral correlations ensure that each client is linked only to the client receiving the conjugate wavelength. In this way, a single down-conversion source can support dozens of channels simultaneously--and to the extent that the WDM hardware can send different spectral channels to different clients, the system can support multiple client pairings. We will describe the design and characterization of the down-conversion source, as well as the client stations, which must be tunable across the emission spectrum.
This chapter discusses the developments in the field of integrated nanophotonics for multi-user quantum key distribution (QKD) networks. It begins with the significance of QKD and presents some of the challenges faced by researchers and system developers, especially regarding the key rate, transmission distance, and implementation cost. The chapter considers various options to implement a multi- user QKD network compatible with the existing fiber-optic network infrastructure. It describes entanglement-based QKD and its wavelength-multiplexed implementation enabling efficient use of the available optical fiber bandwidth for QKD. The chapter shows how a centralized broadband entangled-photon source can be shared by many users in an access network and explains the benefits of wavelength- multiplexed entanglement distribution. It looks at recent advances in integrated broadband entangled-photon sources, waveguide-integrated photon detectors, and low-loss integrated passive devices. The chapter also provides an outlook for future research and development in this direction.
Quantum networks enable fundamentally secure communications and entanglement distribution in parallelized quantum computation. Here we describe our high-dimensional quantum network based on frequency-multiplexed time-bin encoding. Our multi-user 21-km experimental testbed is designed for robustness and scalability, with frequency- and temporal-multiplexing for high-rate interconnects while preserving minimal resource usage and multi-party access through simultaneous key or entanglement distribution. Our measurements are in the high-dimensional frequency and temporal conjugate subspaces, via our biphoton frequency comb qudit. Non-local Franson interferometry provide a measured lower-bound of our entanglement-of-formation, and the Schmidt mode decomposition provide an estimate of the accessible quantum information. We highlight adaptive optimization and singly-filtered biphoton combs as a practical architecture in our high-rate quantum network testbed. We describe the photon information efficiency, Shannon capacity, entanglement distribution, raw key rates, security measurements, dispersive bases, code-efficient non-binary error correction, and Holevo bounds embedded in our network.