![]() Fortunately, fully connected networks (i.e., where every user is connected to every other user directly) can be achieved using multiplexing and bipartite entanglement ( 18). However, the extreme complexity of changing the dimensionality of the state produced by the source makes this approach unscalable. The last category is fully connected quantum networks, which can be based on high-dimensional/multipartite entanglement to share entanglement resources between several users ( 26, 27). Similarly, point-to-multipoint networks are useful in niche applications and have been shown using passive beam splitters (BSs) ( 20– 22), active switches ( 14– 17), and frequency multiplexing ( 17, 23– 25). The second category is actively switched or “access networks” where only certain pairs of users are allowed to exchange a key at a time ( 19). Furthermore, such networks tend to use multiple copies of both the sender and receiver hardware at each node, thereby increasing the cost prohibitively. In most practical networks, it is rare to be able to trust every connected node. The first category is trusted node networks ( 9– 12) where some or all nodes in a network are assumed to be safe from eavesdropping. So far, all demonstrated QKD networks fall in three broad categories. To achieve this, a quantum network must be scalable, must allow users with dissimilar hardware, must be compatible with traffic management techniques, must not limit permitted network topologies, and, as far as possible, must avoid potential security risks like trusted nodes. The ultimate goal of quantum communication research is to enable widespread connectivity, much like the current internet, with security based on the laws of physics rather than computational complexity. Thus far, quantum networks relied on one or more problematic features: trusted nodes ( 9– 13) that are a potential security risk active switching ( 14– 17), which restricts both functionality and connectivity and, most recently, wavelength multiplexing ( 18) with limited scalability. Despite real-world demonstrations of quantum key distribution (QKD) ( 3– 8), the difficulty of scaling the standard two-user QKD protocols to many users has prevented the large-scale adoption of quantum communication. Quantum communication networks present a revolutionary step in the field of quantum communication ( 1, 2).
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