Figure 1

The diagram of the multi-user round-trip QCS scheme in the quantum entanglement distribution network. (a) A demultiplexing equipment separates the entangled signal (idler) photons with the wavelength \(\lambda ^{j}_{s}\) and \(\lambda ^{j}_{i}\) for user \(U_{j}\), \(j \in [1,n]\). The signal photons are transmitted to the user, after passing through a round-trip detection (RTD) module. The idler photons are detected in the idler photon detection (IPD) module. (b) RTD module: the input photons pass through an optical circulator (OC) from port 1 to port 2 and the round-trip photons are detected by a detector. (c) IPD module: the entangled idler photons pass through a polarizing beam splitter (PBS), whose output photons \(|V_{i}\rangle \) and \(|H_{i}\rangle \) are detected, respectively. (d), (e) and (f) show the optical path of user \(U_{1}\), \(U_{2}\) and \(U_{n}\), respectively. The input single photons pass through an OC (from port 2 to port 3) and a PBS, which outputs photons \(| H_{s} \rangle \) and \(| V_{s} \rangle \). The \(| H_{s} \rangle \) photons are detected directly and the \(| V_{s} \rangle \) photons are sent back to the server through port 1 to port 2 of the OC. The server’s and user’s detectors are connected to their respective time-to-digital converters (TDC), which are synchronized by their local clocks