The future quantum communication network would integrate free space and optical-fiber links and be composed of heterogeneous quantum nodes. In order to accomplish different quantum tasks, the nodes of the network need to transfer the entangled states of information-carrying photons back and forth between different degrees of freedom (DOF), e.g., between orbital angular momentum (OAM) DOF in free space and time-energy DOF in optical fibers. This requires a reversible quantum entanglement transfer (QET) interface to perfectly perform such a function, which is a key technology for realizing quantum communication between nodes in the network.

Fig. 1 Experimental setup. a Schematic of the experimental setup of Spontaneous parametric down-conversion (SPDC) source. b Schematic of the experimental setup for characterizing the OAM entanglement. c Schematic of the main experimental setup of two-way entanglement transfer.

Here, we experimentally demonstrate this kind of transfer interface by using two interferometric cyclic gates. By using this quantum interface, we perform two-way entanglement transfer for the two DOFs. Two interferometric quantum gates consisting of a Franson-type interferemeter with spiral phase plates (SPP) inserted in different paths are utilized for transferring quantum entanglement information from time-energy to OAM DOF. we perform a quantum state tomography for the OAM entangled states and reconstruct their density matrices by using the maximum likelihood estimation. The average fidelity for OAM entangled states is calculated to be 94.1%, as shown in Fig. 2. Then, we use two OAM sorters followed by two Mach-Zehnder interferometers (MZIs) to implement QET from OAM to time-energy DOF. The visibility of the Franson-type interference fringe for time-energy entanglement is measured to be high than 92% (Fig. 3), which strongly suggests time-energy entanglement and implies that the quantum entanglement is coherently transferred into time-energy DOF. This interface can be used to prepare multichannel OAM entangled sources and paves a way for establishing entanglement between remote heterogeneous quantum nodes. Thus, our scheme has great potential applications in the future quantum communication networks, such as multi-DOF quantum entanglement swapping on multinode integrated space-to-fiber communication networks.

Fig. 2 The measured density matrices, fidelity and purity of the OAM entangled states.

Fig. 3 Two-photon interference fringes for time-energy entanglement after QET.

The research was published by “Yiwen Huang, Yuanhua Li, Zhantong Qi, Juan Feng, Yuanlin Zheng and Xianfeng Chen. A two-way photonic quantum entanglement transfer interface. npj Quantum Inf 8, 8 (2022).”

Link: https://doi.org/10.1038/s41534-022-00519-1