A quantum lighthouse on the roof: Brookhaven video shows quantum link through the air

Brookhaven National Laboratory in the US state of New York has published a video on its YouTube channel about an experiment meant to show the way to a quantum internet beyond fiber. In August, researchers from Brookhaven and Stony Brook University transmitted entangled photons over a 21-kilometre free-space link through the air – from the university's Quantum Watchtower to the Quantum Lighthouse on the roof of a seven-storey Brookhaven building. According to Stony Brook, it is the first demonstration of its kind in the US.

The photons are sent one at a time. They reach the stations via fiber, where telescopes take over: the beam is expanded from a fiber core about five microns wide to roughly 0.6 metres and coupled back into fiber on the other side. Deformable mirrors, reshaped thousands of times per second, correct for atmospheric distortion. Initially the link will operate mainly on clear nights.

The link extends an existing 161-mile (about 260-kilometre) quantum network with eight nodes. Next, a free-space link of about 48 kilometres across Long Island Sound to Yale University is planned. Justine Haupt, Brookhaven's lead scientist on the project, calls it one of the key technologies needed to make a quantum internet truly useful.