Quantum networks scale the advantages of quantum communication protocols to more than just two distant users. Here we present a fully connected quantum network architecture in which a single entangled photon source distributes quantum states to a multitude of users. Our network architecture thus minimizes the resources required of each user without sacrificing security or functionality. As no adaptations of the source are required to add users, the network can readily be scaled to a large number of clients, whereby no trust in the provider of the quantum source is required. Unlike previous attempts at multi-user networks, which have been based on active components, and thus limited to some duty cycle, our implementation is fully passive and thus provides the potential for unprecedented quantum communication speeds. We experimentally demonstrate the feasibility of our approach using a single source of bi-partite polarization entanglement which is multiplexed into 12 wavelength channels to distribute 6 states between 4 users in a fully connected graph using only 1 fiber and polarization analysis module per user. I. QUANTUM KEY DISTRIBUTION NETWORKSQuantum Key Distribution (QKD) [1,2] has reached the level of maturity required for deployment in realworld scenarios [3][4][5][6][7], and has been shown to operate alongside classical communication in the same deployed telecommunication fiber [8-10] and even over long distances in both fiber [11,12] and free-space links [13][14][15][16][17].Despite these great advances, the practical applicability of QKD is severely curtailed by the fact that most implementations and protocols are limited to two communicating parties.The pressing need to adapt quantum communication to more than two users has motivated several attempts at quantum networks. The QKD networks demonstrated thus far can be roughly grouped into four types of configurations [18]:First, Quantum repeater networks [19] which use quantum memories and entanglement swapping to extend and route quantum states and form arbitrary network topologies. However, technological advancement in quantum memories are needed until quantum repeater networks can be considered practical. Note that quantum repeaters can also be used to improve the performance of the following types of quantum networks.Another approach to multi-user networks is to use high-dimensional/multi-partite entanglement to share entanglement resources between several users [20][21][22]. This way different users share different subspaces of the Hilbert space to generate their keys. However, adding * Correspondence and requests for materials should be addressed to Sören Wengerowsky and Rupert Ursin. † Soeren.Wengerowsky@oeaw.ac.at ‡ Rupert.Ursin@oeaw.ac.at or removing users requires changes in the dimensionality of the system which makes complex alterations of the source necessary.The third approach are trusted node networks: They amount to a mesh of point-to-point links, each requiring a complete two-party communication setup. While trusted nodes have been used t...

Quantum entanglement is one of the most extraordinary effects in quantum physics, with many applications in the emerging field of quantum information science. In particular, it provides the foundation for quantum key distribution (QKD), which promises a conceptual leap in information security. Entanglement-based QKD holds great promise for future applications owing to the possibility of device-independent security and the potential of establishing global-scale quantum repeater networks. While other approaches to QKD have already reached the level of maturity required for operation in absence of typical laboratory infrastructure, comparable field demonstrations of entanglement-based QKD have not been performed so far. Here, we report on the successful distribution of polarization-entangled photon pairs between Malta and Sicily over 96 km of submarine optical telecommunications fiber. We observe around 257 photon pairs per second, with a polarization visibility above 90%. Our results show that QKD based on polarization entanglement is now indeed viable in long-distance fiber links. This field demonstration marks the longest-distance distribution of entanglement in a deployed telecommunications network and demonstrates an international submarine quantum communication channel. This opens up myriad possibilities for future experiments and technological applications using existing infrastructure.

Quantum communication is rapidly gaining popularity due to its high security and technological maturity. However, most implementations are limited to just two communicating parties (users). Quantum communication networks aim to connect a multitude of users. Here, we present a fully connected quantum communication network on a city-wide scale without active switching or trusted nodes. We demonstrate simultaneous and secure connections between all 28 pairings of eight users. Our novel network topology is easily scalable to many users, allows traffic management features, and minimizes the infrastructure as well as the user hardware needed.