Quantum-Entangled Drones Share Secure Messages

(PhysicsToday) Zhenda Xie, Yan-Xiao Gong, and Shi-Ning Zhu of Nanjing University in China havesuccessfully demonstrated the transmission of an entangled state through air from one drone to another.
Using quantum encryption to send confidential information through entangled photons offers much better protection than today’s encryption, but several challenges remain. Until now, researchers have transmitted quantum information as far as a few hundred kilometers using fiber-optic cables and 1200 km using satellite arrays. 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.
Aboard one drone in this study, 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.
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.