Quantum signals escape fiber, travel 13 miles in open air in New York

It may sound surprising, but one of the most stubborn problems in building quantum networks is that cables get in the way.

Fiber-optic cables—the same technology that carries much of today’s internet traffic—can transmit fragile quantum information over long distances. But they require physical infrastructure and are designed to work best with specific telecommunications wavelengths.

That creates a problem because quantum technologies can operate at other wavelengths, while quantum signals are so delicate that losses and disruptions in the fiber can limit how far they can travel.

Now, researchers at Brookhaven National Laboratory and Stony Brook University have demonstrated another route. 

They sent quantum states of light through open air across 13 miles (21 kilometers), marking the first US demonstration of its kind and adding a wireless component to the nation’s longest quantum network, which already spans 161 miles and connects eight nodes.

Sending quantum signals through the sky

During a daytime demonstration on August 21, researchers at Stony Brook’s Quantum Watchtower used a laser to create quantum states containing only a few photons at a time. The facility sits on the roof of the university’s Health Sciences Center in Stony Brook, New York.

The photons emerged from an optical fiber just 5 microns in diameter, less than one-tenth the width of a human hair. They then traveled 13 miles through the atmosphere to Brookhaven’s Quantum Lighthouse in Upton, New York, where an ultrafast camera detected them as they entered another tiny optical fiber.

Conventional wireless technology was not an option. Radio frequencies are too noisy to preserve the fragile quantum information carried by these photons, so the researchers turned to light instead. 

However, sending light through the atmosphere creates another problem, which is that turbulence can distort its path and potentially scramble the quantum information.

The team drew on expertise from astronomy, using telescope technology and adaptive optics to compensate for atmospheric turbulence and precisely collect and control the light.

“People think of telescopes as tools for looking up into space, but the same technologies that allow astronomers to precisely collect and control light are essential for these quantum experiments,” Justine Haupt, one of the researchers and a scientist at Brookhaven Lab (BNL), said.

The researchers also had to build specialized rooftop facilities and integrate their optics, control systems, communications equipment, quantum sources, and detectors so that equipment separated by 21 kilometers could operate as one experiment.

The bigger test involved entangled photons

The daytime experiment demonstrated that quantum states of light could cross the new free-space optical (FSO) link. The researchers then moved to a more demanding test involving entangled photons.

During nighttime tests, when background light was lower, they sent entangled photons from a Stony Brook physics laboratory to the Quantum Watchtower through fiber. The photons were then distributed across the open-air link and successfully received and measured at the Quantum Lighthouse.

Entangled photons have quantum properties that remain correlated even when the particles are separated by long distances. This makes them promising for future secure communications, quantum sensing, and networks linking quantum computers.

“In our long-distance fiber network, we routinely transmit entangled pairs of photons. However, such fiber networks are limited to the use of telecom wavelengths,” Eden Figueroa, one of the researchers and the director of Stony Brook’s Quantum Institute, said. 

“In our new quantum wireless links, we are exploring the use of infrared wavelengths that are native to quantum processors and related technologies. This will provide a direct route to create entangled atomic systems across long distances,” Figueroa added.

From Long Island to orbit

The new FSO connection is not intended to replace fiber. Instead, it could help quantum networks reach places and devices that are difficult to connect with conventional cables. 

It also gives researchers access to wavelengths beyond those favored by commercial telecommunications fiber, potentially making it easier to connect quantum processors and other technologies that operate at different wavelengths.

The next extension is already being developed. A third FSO facility at Yale University in New Haven, Connecticut, has been completed, while the Stony Brook-Yale connection is still under development. 

Once operational, it will allow researchers to send entangled photons 30 miles (48 kilometers) across Long Island Sound.

The researchers ultimately envision using the FSO network to connect quantum computers at Brookhaven and Stony Brook, allowing them to work together on problems that individual machines may struggle to solve. 

The long-term goal is sending quantum information through Earth’s atmosphere to satellites, potentially helping connect remote locations without extensive ground-based telecommunications infrastructure. 

Similar experiments have already shown that photons can be transmitted from aircraft to ground stations, pointing toward mobile and satellite-based quantum networks.

“Incorporating satellites into the network could help bring secure quantum communications to rural or remote locations with minimal ground-based telecommunications infrastructure, transforming the quantum network from a regional one to a global one,” Gabriella Carini, one of the researchers and a scientist at BNL, said.