Researchers have implemented entanglement swapping with independent sources over a 100 km optical fiber.
AsianScientist (Oct. 17, 2017) - Researchers from the University of Science and Technology of China (USTC) have successfully demonstrated long-distance entanglement swapping, bringing us one step closer to quantum communications. Their results have been published in Optica. Realizing long-distance entanglement swapping with independent sources under real-world conditions is important for both future quantum networks and the fundamental study of quantum theory. However, due to its high susceptibility to environmental effects, entanglement swapping has been achieved only for a few tens of kilometers of underground optical fiber, and there have been no reports of success in using optical fibers longer than 100 km.
To increase the distance, the USTC scientists exploited two independent 1-GHz-clock sequential time-bin entangled photon-pair sources, developed several automatic stability controls, and successfully implemented a field test of entanglement swapping over a 103 km optical fiber link composed of about 77 km of optical fiber inside the lab, 25 km of optical fiber outside the lab but kept underground, and 1 km of optical fiber suspended in the air outside the lab to account for various types of noise mechanisms in the real world.
The setup of this experiment provides a promising platform for many fundamental tests in the future. The configuration of the experiment allows the space-like separation between any two measurements of distance as performed in the experiments, and various time-space relations can be achieved by combining both coiled optical fiber and deployed optical fiber.
The results show that entanglement swapping between two cities is technically feasible, even if more suspended fiber is used. The experiment also verifies the feasibility of such technologies for long-distance quantum networks and opens up new possibilities for future applications in more complicated environments.
“Entanglement swapping over long distances is a crucial ingredient for quantum networks in optical fiber,” said Professor Lo Hoi-Kwong, associate editor of Optica. “The paper provides important details about the synchronization of the sources, the stabilization of the optical distance and polarization.”