Scientists in the United States have taken an important step toward a future “quantum internet” by sending quantum information through 13 miles of open air without relying on a fiber-optic cable.
Researchers from the U.S. Department of Energy’s Brookhaven National Laboratory and Stony Brook University successfully transmitted tiny particles of light, called photons, between the two institutions in New York.
The researchers say it is the first demonstration of its kind in the United States.
The achievement could help scientists expand quantum communication networks beyond the physical limits of fiber-optic cables and eventually connect them through satellites.
During a daytime demonstration, researchers generated special quantum states of light at Stony Brook University’s Quantum Watchtower. The facility sits on the roof of the university’s Health Sciences Center.
The photons began their journey inside an optical fiber with a core only about 5 microns wide, less than one-tenth the width of a human hair. They then left the fiber and traveled about 13 miles, or 21 kilometers, through open air before reaching Brookhaven’s Quantum Lighthouse.
There, highly precise equipment captured the incoming light and directed it into another extremely small optical fiber.
The researchers have also taken the experiment a step further by working with entangled photons. Entanglement is a strange feature of quantum physics in which two particles become closely linked, even when they are separated by large distances. Measuring one particle can reveal information about its partner.
This unusual property could eventually be useful for highly secure communications, powerful quantum sensors and networks of quantum computers.
During nighttime experiments, when interference from background light was lower, researchers sent entangled photons from a Stony Brook laboratory through fiber to the Quantum Watchtower. The photons were then transmitted across the open-air link and successfully detected at Brookhaven.
The wireless connection adds a new capability to an existing quantum network that stretches 161 miles, or 259 kilometers, across Long Island and the New York metropolitan area.
Sending quantum information through open air is much harder than ordinary wireless communication. Cellphones and satellites commonly transmit information using radio waves, but these signals are too noisy for fragile quantum information. Instead, the researchers use optical light.
To make the system work, the team borrowed technology and expertise from astronomy. Telescopes are designed to collect and precisely control light traveling through Earth’s atmosphere. Similar techniques can help keep delicate quantum signals on course.
The challenge is particularly difficult near the ground because atmospheric turbulence can distort light. Researchers therefore had to develop highly precise telescope systems and specialized rooftop facilities capable of working together across the 21-kilometer distance.
The next stage will be even more ambitious. A third facility has been completed at Yale University in Connecticut. Researchers plan to establish a 30-mile, or 48-kilometer, open-air quantum link between Stony Brook and Yale across Long Island Sound.
Eventually, the scientists hope to send quantum information through Earth’s atmosphere to satellites. Satellite connections could allow quantum networks to reach areas without extensive fiber infrastructure and potentially transform today’s regional experiments into a global network.
Researchers also hope to connect quantum computers at different institutions so they can work together. Much like today’s internet connects conventional computers around the world, these experiments could lay some of the groundwork for a future network linking powerful quantum machines.
