Using quantum encryption to secure messages could make for much less hackable communication networks. The technique has been tested in the lab, but for it to really take off as a practical system it needs to work out in the real world, among other signals and natural air turbulence. Now, researchers from the University of Ottawa have successfully sent a message with high-dimensional quantum encryption between two building rooftops.

Quantum communication, at its most basic level, usually encodes information in a binary system: individual photons are sent between two points, with each representing one bit of information, either a one or a zero. But a technique called high-dimensional quantum encryption can theoretically squeeze twice the data into each photon, in turn allowing exponentially more information to be transmitted. Two bits of information per photon opens up four signal possibilities – 00, 01, 10 and 11 – giving it the title of 4D quantum encryption.

Not only can this technique fit more information into each particle, it's also more secure against deliberate attempts to intercept the message, as well as environmental factors like air turbulence and electronic interference. To keep out any prying eyes, this information can be encrypted with quantum key distribution, which uses the quantum states of light to encode a message and tell the receiving device how to decrypt it.

But outside of a lab, the real world is a noisy place, full of buildings, turbulent air and electronics. Before 4D quantum encryption can reach its potential, it needs to be tested in the kinds of environments it may eventually be used in. Since there's so much noise on the ground, sending a signal across a distance of 3 km (1.9 miles) horizontally is equivalent to the much greater distance of beaming a message through the relatively-clear air between the ground and a satellite in orbit.

The 3-km horizontal test is the next step, but for this proof of concept, the University of Ottawa researchers set about performing a 300 m (985 ft) test run between two rooftops in a city. They set up the lab equipment on the roof of each building, protected from the worst of the weather in wooden boxes.

The test was successful. Messages secured with 4D quantum encryption were beamed between the two stations, with an error rate of 11 percent – well below the threshold to make it a secure connection. Accounting for the error correction and turbulence, the system was able to transfer 1.6 times more data per photon than 2D encryption.

"Our work is the first to send messages in a secure manner using high-dimensional quantum encryption in realistic city conditions, including turbulence," says Ebrahim Karimi, lead researcher on the study. "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."

The researchers say the next step is to test the system across three points, placed 5.6 km (3.5 mi) apart, using adaptive optics to try to counteract the turbulence. Longer-term, the plan is to add more links and more encryption dimensions to the system.

The research was published in the journal Optica.