Graphic of quantum communication across city

Quantum encryption performed across a city for first time

Image credit: University of Ottawa

Researchers at the University of Ottawa have successfully sent a quantum-encrypted message containing more than 1 bit of information per photon across a city, in a major step towards making quantum encryption a practical possibility.

Quantum encryption uses photons – light particles – to carry information between sender and receiver, in the form of “quantum bits”. While the standard encryption we use today, which employs mathematical algorithms to encrypt and decrypt messages, can be broken with the aid of increasing computational power, quantum encryption is, in theory, impossible to eavesdrop on.

For “2D encryption”, each photon encodes 1 bit, a 1 or a 0, meaning that sending information is slow and requires an enormous number of photons. While it has been shown that a photon can contain more information than this (higher-dimensional quantum encryption), this has never been achieved in practice.

The University of Ottawa researchers built on previous, smaller-scale demonstrations by demonstrating 4D quantum encryption – where each photon encodes 2 bits – in a city setting, between the rooftops of two buildings 300m apart on the university campus.

“Our work is the first to send messages in a secure manner, using high-dimensional quantum encryption in realistic city conditions, including turbulence,” said Professor Ebrahim Karimi of the University of Ottawa, who led the research.

Sending these encrypted messages across 3,000m on Earth would be, the researchers say, equivalent to sending a signal from Earth to a satellite.

“The secure, free-space communication scheme we demonstrated could potentially link Earth with satellites, securely connect places where it is too expensive to install fibre, or be used for encrypted communication with a moving object, such as an airplane,” continued Professor Karimi.

According to the researchers, higher-dimensional quantum encryption can tolerate more noise – such as that caused by turbulence, technical failures and attempted eavesdropping – before the transmission is disturbed or becomes insecure.

The achievement demonstrated that it could be possible to use high-capacity, free-space quantum communication for unprecedented security, in links between ground-based networks and satellites in orbit around Earth. This is the foundation for creating a quantum encryption network.

Next, the researchers will attempt to implement the technology into a network of three links, 5,600m apart, and adopt adaptive optics – which corrects distortion by deforming a mirror – to compensate for turbulence.

In June, Chinese researchers successfully established quantum communication between Earth and space in a truly monumental achievement: the culmination of decades of theoretical and experimental work.

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