Quantum state modulation is a promising technique for fundamental physics research, which is closely related to quantum relay switches, and quantum storage and processing of quantum states. To date, optical orbital-angular-momentum (OAM) modes have become a popular choice for quantum information experiments. Since OAM single photons carry a unique phase intensity profile and multiple modes of operation, efforts to stimulate various quantum communication tasks are increasing, such as quantum key distribution, quantum teleportation, entanglement swapping, etc. Establishing connections between different quantum systems to transmit information requires some basic components, such as quantum memory and relay nodes. To connect these systems, it is necessary to establish a wavelength connection such as a frequency converter, which can effectively realize the manipulation and wavelength conversion of photonic states.

Here, we demonstrate the nonlinear quantum state transformation for a quantum repeater network with the cascaded sum-frequency generation (SFG) and EO polarization coupling of OAM single photons in a periodically polarized lithium niobate (PPLN) crystal. By using input photons with different OAM in orthogonal polarization components, we manage to adjust the weight of the spatial modes of the generated photons. The cascade process can be efficiently and conveniently balanced by applying an external electric field, realizing the superposition of quantum states with different OAM modes.

The fidelity of the OAM state is measured to be higher than 95% before and after the frequency conversion. The results show that the OAM state of single photons can be modulated on demand by applying a transverse electric field across the nonlinear crystal. Our work demonstrates the integration and high performance of our proposed transformation interface, and paves a route toward connecting different remote nodes based on quantum memories in a large-scale OAM-encoded quantum network.

Figure 1 Experimental setup.

Figure 2 (a) The measured density matrices of the OAM states. (b) Variation trends of the number of SFG photons are recorded by varying the applied voltage with dark count subtracted. (c) Output of the OAM superposition state represented on the Poincaré sphere as the applied voltage varies from 0 to 100 V.

The research was published in “Zhantong Qi, Yiwen Huang, Chuanyi Lu, Fengchao Ni, Yuanhua Li, Yuanlin Zheng, and Xianfeng Chen Orbital-angular-momentum quantum state interface via a nonlinear process, Physical Review Applied, 19, 014045 (2023).”

Link: https://doi.org/10.1103/PhysRevApplied.19.014045