Ayan Kumar Nai

Please find the attached PDF. Overleaf version pasted below: Quantum communication networks have emerged as essential for connecting multiple users worldwide and for ensuring secure information exchange through entanglement-assisted quantum key distribution (QKD). The majority of untrusted node quantum networks have been demonstrated in fiber through wavelength-division-multiplexing (WDM) \cite{wengerowsky18} of entangled photon sources at 1550 nm, using commercially available WDM devices for classical communication, or space-division-multiplexing using multicore fiber. Here, we demonstrate a quantum network architecture (Fig. 1(a)) optimized for free-space communication using just one entanglement source, a vital necessity for long-distance quantum networks via satellite-based links. Harnessing the intrinsic spatial and temporal correlations and quantum randomness of the spontaneous parametric down-conversion process, we implement spatial division of the annular emission ring of the entangled photon source at 810 nm, and multiplex using beamsplitters to establish a proof-of-concept fully connected twelve-channel quantum network \cite{nai2025free} for seamless QKD among six users in the laboratory. The network achieves average coincidence rates exceeding $3 \times 10^{4}$ s$^{-1}$ between all user pairs (Fig. 1(b)) and a total sifted key rate of 407 kbps, even with Si-APD detectors. This fully passive, modular scheme provides a scalable, resource-efficient approach with unprecedented key rates and robust multi-user connectivity, suitable for deployment in terrestrial and satellite-based quantum communication infrastructures. \begin{figure}[ht] \centering \includegraphics[width=0.8\linewidth]{QCrypt.jpg} \caption{(a) Representation of the network topology with the free space channels and users. (b) Coincidence rate (c) Entangled state fidelity for each link connecting two users, and (d) Scalability in the twelve-channel network.} \label{Figure 1} \end{figure} \vspace{-1em} Experimentally, we generated the SPDC ring by pumping a 20-mm-long, type-0 periodically poled potassium titanyl phosphate (PPKTP) crystal kept in an oven inside a Sagnac interferometer. We then sectioned the SPDC ring into six diametrically opposite sections: S1-S6, shown in Fig. 1(a), to form three identical entangled photon sources. While each of these sources can support independent QKD between two parties, we have multiplexed the three sources by recombining two sections of the ring that are not diametrically opposite. Further, we shared through free-space channels to six users. Each user receives photons from two different sources and is connected to four other users via corresponding correlated photons from those sources, thereby forming a quantum network. In general, for N users, we can establish N(N-2)/2 links among users using N/2 sources. The entangled-state fidelity for all 12 links is in the range of 91-95\% shown in Fig. 1(c). Figure 1(d) demonstrates the network's scalability to accommodate more users. Additional results, both before and after multiplexing the sources to establish the quantum network, will be presented.