Quantum entanglement has been increasingly touted as a promising way to develop quantum cryptography and communication technology. One popular method entails entangling the polarization states of photon pairs (see the article by Barbara Terhal, Michael Wolf, and Andrew Doherty, Physics Today, April 2003, page 46). Recently, some physicists have developed a higher-dimensional approach that entangles a photon’s spatial modes of light, such as the transverse modes perpendicular to the light’s propagation direction. That approach allows for more information per photon to be transported and may be more resistant to hacking and background noise. But so far, it has required custom-made multimode fiber-optic cable that can accommodate the light’s various spatial modes. Now Jian Wang (Huazhong University of Science and Technology in Wuhan, China), Andrew Forbes (University of the Witwatersrand in Johannesburg, South Africa), and their colleagues have demonstrated that multidimensionally entangled photons can be transported through single-mode fiber.
The method is depicted in the figure. The researchers used a nonlinear optical process—spontaneous parametric down-conversion (blue box)—to convert one photon of higher energy into a pair of lower-energy photons (red), a signal and an idler. The idler photon (left), encoded with multidimensional orbital angular momentum states, traveled through free space. Meanwhile, the signal photon interacted with an optical plate to convert orbital angular momentum to spin. The resulting photon (right) had only one spatial mode, so it could travel through the single-mode fiber (yellow).
The researchers then accessed the orbital angular momentum of the signal photon under various polarizations by collecting those measurements from the idler photon. In previous experiments, photons with multidimensionally entangled states traveled no more than 1 m across multimode fiber. But Jun Liu, Isaac Nape, and the others who performed the experiments coaxed their photons to cruise through the single-mode fiber for 250 m. The approach may have practical applications if further testing indicates that the photons remain entangled even in the presence of various noise models. The single-mode fiber used in the experiments is the same cable that’s already used across various telecommunication networks. (J. Liu et al., Sci. Adv. 6, eaay0837, 2020.)
