As the paradigmatic quantum-mechanical resource, quantum entanglement has become the key ingredient in the rapidly expanding field of quantum-information science. Hyperentanglement in multiple degrees of freedom (DOFs) can encode more qubits per transmitted photon and significantly increases channel capacity, promising in many quantum-information applications such as entanglement distillation. Nowadays, constructing practical quantum networks has become a hot topic, among which polarization and time-energy entangled sources play a role. When scaling up quantum registers, a compact hyperentangled source in these two DOFs that can provide enough photon pairs in the communication band become essential for constructing a large-scale quantum network. On the other hand, high-fidelity quantum operations, like precise manipulation of polarization entangled state, represent a fundamental prerequisite for many quantum information technologies. Particularly in superdense coding, the precise manipulation of polarization entanglement is an indispensable part for encoding the quantum information and performing the complete Bell-state measurement.

Here, we demonstrate the generation and manipulation of hyperentanglement in polarization and time-energy degrees of freedom. The hyperentangled photon pairs are generated by cascaded second harmonic generation and spontaneous parametric down-conversion processes in a high-efficiency periodically poled lithium niobate (PPLN) waveguide in a fiber Sagnac loop. The states achieve the maximum value of Clauser-Horne-Shimony-Holt Bell inequality of S = 2.76 ± 0.03 with two-photon interference visibility of higher than 99% for polarization entanglement and the Franson-type two-photon interference visibility of 98.5% for time-energy entanglement. In addition, the polarization maximally entangled states can be fast switched to each other by using a PPLN on insulator chip through the electro-optic effect. After the entanglement manipulation, the two-photon interference visibility for polarization entanglement is still higher than 97% while keeping the quantum characteristics of time-energy degrees of freedom maintained. Our system paves a way toward reducing the complexity of quantum systems and has potential applications in many quantum-information tasks like fast superdense coding and teleportation.

FIG. 1. (a), (b) Real and imaginary parts of the reconstructed density matrix for the polarization entangled state. (c) Two-photon coincidence count as a function of signal polarization under two nonorthogonal projection bases. (d) Franson-type interference patterns for observing time-energy entanglement.

This research is published by “Yiwen Huang, Juan Feng, Yuanhua Li, Zhantong Qi, Chuangyi Lu, Yuanlin Zheng and Xianfeng Chen, High-performance hyperentanglement generation and manipulation based on lithium niobate waveguides, Physical Review Applied, 17, 054002 (2022)”。

Link: https://doi.org/10.1103/PhysRevApplied.17.054002