Can quantum information move beyond fibre? Researchers demonstrated a free-space optical link that transmitted entangled photons across 21 kilometres.
Researchers at Brookhaven National Laboratory and Stony Brook University demonstrated a free-space optical (FSO) link that transmitted quantum information through open air between the two institutions. The system connected Stony Brook’s Quantum Watchtower with Brookhaven’s Quantum Lighthouse, providing a wireless extension to a quantum network that had already spanned 161 miles across the region.
The demonstration used a laser to generate quantum states of light containing only a few photons. These photons travelled 21 kilometres through the atmosphere from a rooftop facility at Stony Brook to a detector at Brookhaven. The receiving system used an ultrafast camera to detect the photons after they were coupled into a small optical fibre.
The researchers then moved beyond single-photon transmission to test entangled photons. During nighttime experiments, entangled photon pairs were generated at Stony Brook, sent through fibre to the Quantum Watchtower, transmitted across the new FSO link, and detected at the Quantum Lighthouse.
The system relied on optical rather than radio-frequency communication because conventional radio signals introduce too much noise for fragile quantum information. The FSO setup instead used telescope-like optical systems to direct and receive the photons through the atmosphere.
Maintaining the quantum signal over the 21-kilometre path required precise control of the optical beam. Atmospheric turbulence can distort light as it travels, potentially affecting the quantum information carried by the photons. Brookhaven researchers therefore developed adaptive-optics technology to compensate for these distortions.
The rooftop facilities combined optics, mirrors, controls, communications, quantum sources, and detectors so that equipment separated by kilometres could operate as a single experiment. The researchers also explored infrared wavelengths, which could provide a more direct connection to quantum processors and other quantum technologies.
The next stage involves connecting Stony Brook with a facility at Yale University across a 48-kilometre path. The researchers ultimately aim to extend such free-space quantum links to satellites, potentially enabling quantum networks beyond the reach of terrestrial fibre. “In our new quantum wireless links, we are exploring infrared wavelengths that are native to quantum processors and related technologies,” says Eden Figueroa, Director, Stony Brook’s Quantum Institute.
