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OTTAWA, Ontario, Aug. 25, 2017 — A quantum-secured message containing more than one bit of information per photon has been sent through the air above the city of Ottawa, Ontario, Canada. According to scientists, this is the first time high-dimensional quantum encryption has been demonstrated with free-space optical communication in real-world conditions.

A research team from the University of Ottawa demonstrated 4D quantum encryption — so-called because each photon is encoded with two bits of information, providing the four possibilities of 00, 01, 10 or 11 — over a free-space optical network spanning two buildings 0.3 kilometers apart.

High-dimensio<em></em>nal quantum encryption demonstrated, University of Ottawa.

Researchers have demonstrated sending messages in a secure manner using high dimensional quantum cryptography in realistic city conditions. Courtesy of SQO team, University of Ottawa.

One of the primary problems faced during any free-space experiment is dealing with air turbulence, which can distort the optical signal. For the tests, the researchers brought their laboratory optical setups to two different rooftops and covered them with wooden boxes to provide some protection from the elements. After much trial and error, they successfully sent messages secured with 4D quantum encryption over their intracity link. The messages exhibited an error rate of 11 percent, below the 19 percent threshold needed to maintain a secure connection.

The researchers compared 4D encryption with 2D, finding that, after error correction, they could transmit 1.6 times more information per photon with 4D quantum encryption, even with turbulence.

In addition to sending more information per photon, high-dimensional quantum encryption can tolerate more signal-obscuring noise before the security of the transmission is threatened. Noise can arise from turbulent air, failed electronics, detectors that don't work properly or from attempts to intercept the data.

“This higher noise threshold means that when 2D quantum encryption fails, you can try to implement 4D because it, in principle, is more secure and more noise resistant,” said researcher Ebrahim Karimi.

As a next step, the researchers plan to implement their scheme into a network that includes three links that are about 5.6 kilometers apart, using adaptive optics to compensate for the turbulence. Eventually, the team hopes to link this network to one that already exists in the city.

“Our long-term goal is to implement a quantum communication network with multiple links but using more than four dimensions while trying to get around the turbulence,” said researcher Alicia Sit.

The demonstration showed that it could one day be practical to use high-capacity, free-space quantum communication to create a highly secure link between ground-based networks and satellites.

“Our work is the first to send messages in a secure manner using high-dimensional quantum encryption in realistic city conditions, including turbulence,” said Karimi. “The secure, free-space communication scheme we demonstrated could potentially link Earth with satellites, securely connect places where it is too expensive to install fiber, or be used for encrypted communication with a moving object, such as an airplane.”