Photo illustration of the beacon laser, used to track an entangled photon signal across Qinghai Lake. The statue is Padmasambhava at the Lotus Temple at Gangcha.
by Matthew Francis
It only works at night, but the technique sets a new distance record.
Sending signals through fiber optic cable is reliable and fast, but because of internal absorption and other effects, they will lose photons—which is a problem when the number of photons being sent is small. This is of particular concern in quantum networks, which typically involve a small number of entangled photons. Direct transmission through free space (vacuum or air) experiences less photon loss, but it’s very difficult to align a distant receiver perfectly with the transmitter so that photons arrive at their destination.