If you were walking across campus last November and looked up into the sky, you might have caught a glimpse of uOttawa researchers sending the first quantum-secured message containing more than one bit of information per photon through the air above a city.
The demonstration by lead researcher Ebrahim Karimi and his team showed that it might one day be practical to create a highly secure link between ground-based networks and space-based satellites, a requirement for creating a global quantum encryption network.
“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, who chose an image of the Canadian parliament as the message to encrypt and transmit. “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.”
Quantum encryption uses photons to encode information in the form of quantum bits. In its simplest form, known as 2D encryption, each photon encodes one bit: either a 1 or a 0. Scientists have shown that a single photon can encode even more information — a concept known as high-dimensional quantum encryption — but until now, this has never been demonstrated with free-space optical communication in real-world conditions.
As detailed in Optica, the Optical Society’s journal for high impact research, the researchers demonstrated 4D quantum encryption over a free-space optical network spanning two buildings 300 metres apart at the University of Ottawa. This high-dimensional encryption scheme is referred to as 4D because each photon encodes two bits of information, resulting in four possibilities: 01, 10, 00 or 11.
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.
“After bringing equipment that would normally be used in a clean, isolated lab environment to a rooftop that is exposed to the elements and has no vibration isolation, it was very rewarding to see results showing that we could transmit secure data,” said Alicia Sit, an undergraduate student in Karimi’s lab.
As a next step, the researchers are planning to implement their scheme into a network that includes three links that are about 5.6 kilometers apart and that uses a technology known as adaptive optics to compensate for turbulence. Eventually, they want to link this network to one that exists now in the city. “Our long-term goal is to implement a quantum communication network with multiple links but using a higher dimensional [encryption] than 4D while trying to get around the turbulence,” said Sit.
Karimi and his team collaborate closely with Robert Boyd, Gerd Leuchs and Christoph Marquardt at the Max Planck-uOttawa Centre for Extreme and Quantum Photonics.