The Qline architecture is a restricted Quantum Key Distribution (QKD) network topology in which all nodes are on a single line of fiber. The extremal points of the network have the ability to emit and measure single qubit quantum states, respectively, as in QKD. The intermediate nodes only have the ability to rotate the quantum states they receive before transmitting them.

Despite its simplicity, any pair of nodes on Qline can establish a shared secret key. The protocol is inspired by the celebrated “Single Qubit Quantum Secret Sharing” protocol implemented by Christian Schmid, Pavel Trojek, Harald Weinfurter, Mohamed Bourennane, Marek Zukowski, Christian Kurtsiefer in 2004.

The main advantage of this architecture is its scalability. Once the extremal nodes are setup, additional ones can be added using a single device. Moreover, while QKD is only point-to-point, Qline is inherently multipartite. This avoids key routing that reveals encryption keys to intermediate nodes, hence strengthening the overall security of the network.