Quantum-entangled drones share secure messages

Using quantum encryption to send confidential information through entangled photons offers much better protection than today’s encryption, but several challenges remain. So far researchers have transmitted quantum information as far as a few hundred kilometers using fiber-optic cables and 1200 km using satellite arrays (see Physics Today, August 2017, page 14 ). But constructing a communication network connected by fiber is costly and prone to loss problems; and a network of satellites suffers from low transmission rates and is only usable at night. Now Zhenda Xie, Yan-Xiao Gong, and Shi-Ning Zhu of Nanjing University in China have accelerated an alternative approach: Their experiment successfully demonstrates the transmission of an entangled state through air from one drone to another.

Aboard one drone, a pump laser shines on an inorganic crystal that is specially designed for nonlinear optical applications. The process, known as spontaneous parametric down-conversion, generates a pair of lower-energy entangled photons. Once the entangled pair is produced, it’s collimated and sent through a series of wave plates to prepare it for transmission through free space.

The entangled pair is then distributed to a second drone and a ground station known as Alice. The second drone works as an optical relay for the single photons and transmits the quantum information to Alice’s recipient station, Bob. The graph shows the signal loss due to diffraction in air for different beam apertures. To limit the diffraction loss, the drones were spaced within 200 m of each other, though the researchers could propel the photons farther by tweaking the optical relay.

The experiment succeeded in transmitting the entangled photon pair about 1 km between the stations Alice and Bob, and the fidelity of the entanglement was confirmed with a Bell-inequality test (see the article by Reinhold Bertlmann, Physics Today, July 2015, page 40 ). Because the drones are inexpensive and mobile, the optical relay could comprise more than two drones to transmit information between users who are more than a few kilometers apart. A network of drones, the authors say, could also be useful for other communication applications. Including quantum repeaters on the drones, for example, would boost the strength of information signals. (H.-Y. Liu et al., Phys. Rev. Lett. 126, 020503, 2021 .)