Quantum secure direct communication (QSDC) based on entanglement can directly transmit confidential information. However, the inability to simultaneously distinguish the four sets of encoded entangled states limits its practical application. Here, we explore a QSDC network based on time–energy entanglement and sum-frequency generation. In total,15 users are in a fully connected QSDC network, and the fidelity of the entangled state shared by any two users is >97%. The results show that when any two users are performing QSDC over 40 km of optical fiber, the fidelity of the entangled state shared by them is still >95%, and the rate of information transmission can be maintained above 1 Kbp/s. Our result demonstrates the feasibility of a proposed QSDC network and hence lays the foundation for the realization of satellite-based long-distance and global QSDC in the future.
Exploiting the fantastic features of quantum mechanics, a hyperentangled quantum network encoded in multiple degree of freedoms (DOF), e.g., polarization and orbital angular momentum DOFs, can encode more qubits per transmitted photon and offers a promising platform for many dramatic applications. Here, we demonstrate such a hyperentangled multiuser network with a fully connected network architecture by using dense wavelength division multiplexing and entanglement transfer technique. Three hyperentangled states in polarization and time-energy DOFs are multiplexed to three single mode fibers to form the fully connected network architecture. Then, three interferometric quantum gates are utilized for transferring quantum entanglement from time-energy to orbital angular momentum DOF. The experimental results reveal a high quality of the hyperentanglement of the constructed network with the entangled state fidelity of higher than 96%. Our approach can provide a novel way to construct a large-scale hyperentangled network that can support various kinds of quantum tasks like superdense coding and teleportation.
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