Gilles Brassard, a University of Montreal computer scientist who made seminal contributions to the field of quantum cryptography, has been awarded the Breakthrough Prize in fundamental physics, the world’s largest science prize.

In an announcement from the Breakthrough Prize Foundation on Thursday, Prof. Brassard was named a co-winner of the US$3-million award, together with his long-time collaborator, Charles Bennett, a researcher with IBM in the United States. Oxford University physicist David Deutsch and Peter Shor, a professor of applied mathematics at the Massachusetts Institute of Technology, are also co-winners for separate contributions to the field of quantum information.

Their collective discoveries are the result of a once esoteric exploration that began in the early 1980s and morphed into an ambitious and potentially world-changing quest to develop quantum computers at commercial scale.

The award is just the latest for Dr. Brassard, who in 2018 won the prestigious Wolf Prize in physics.

He and Dr. Bennett are considered likely contenders for a future Nobel Prize for coming up with quantum key distribution – a practical way information can be sent securely and decoded using a digital key that is tied to the quantum properties of a physical system. Any attempt to eavesdrop on the transaction would disturb the quantum nature of the key so it could no longer be used.

Prof. Deutsch is known for working out how the principles of computer science operate when the digital bits on which a calculation is based are subject to the weird rules of quantum physics. In the 1990s, Dr. Shor, showed that if a quantum computer could be made to work, it would undermine RSA encryption — the conventional method by which much of the world’s digital information, including financial transactions, is kept private.

The Breakthrough Prize is awarded annually for achievements in physics, mathematics and life sciences. It was established in 2012 by Russian-Israeli entrepreneur Yuri Milner and is supported by Meta Mark Zuckerberg and Google co-founder Sergey Brin, among other Silicon Valley luminaries.

Robert Myers, director of the Perimeter Institute for Theoretical Physics in Waterloo, Ont., called this year’s winners “a wonderful choice” for the prize.

“I think it recognizes something that’s been growing and growing,” Dr. Myers said. “These are the people who set the foundations for quantum information.”

For Prof. Brassard, the journey began in the Caribbean Sea. It was 1979, and he had just taken up a position at the University of Montreal after completing his PhD. At a conference in Puerto Rico where he was scheduled to give a talk on cryptography, he went for a swim, and was approached by a stranger with an unlikely conversation starter.

“He swims up to me and starts telling me that he knows how to use quantum theory to make bank notes that are impossible to counterfeit,” Prof. Brassard said. “So I had to listen, and I realized that what he was saying was interesting, but impractical and useless.”

The stranger was Dr. Bennett and the conversation stimulated Prof. Brassard to see how the idea could be made useful – although still impractical – as he continued his swim. Soon afterward, the two were exchanging ideas and collaborating on what would become the basis for a science of quantum cryptography.

As a computer scientist, Prof. Brassard was not deeply plugged into fundamental physics, particularly the hard-to-interpret implications of quantum theory, which seemed to allow mind-bending possibilities like particles existing in more than one place at the same time until they are measured.

“I had never taken a course in quantum theory,” he said, “but when Bennett explained these things to me, I was totally seduced by how beautiful it is.”

For Prof. Deutsch, the attraction was in the reverse direction. As a physicist, he was exploring questions related to the nature of reality, including the idea that some of the contradictions in quantum theory can be explained if there are multiple universes, each representing a different outcome, such as a path that a particle can take.

Once again, it was Dr. Bennett that provided the impetus. After a conversation with the IBM researcher at a party, Dr. Deutch decided to look at how computers operate in a quantum world. As he and others would soon come to realize, it meant that bits of information – the ones and zeros that underpin conventional computer calculations – would be replaced by “qubits”, which carry some probability of being a one or a zero.

When harnessed together, qubits can effectively perform multiple calculations, using the same hardware, in the same time a conventional computer can perform a single operation. This can vastly speed up certain kinds of calculations, including those that today are central to data encryption.

Dr. Deutsch said he was happy to embrace the notion that a quantum computer achieves its power by working in multiple universes at the same time. However, it’s also possible that a deeper understanding of quantum physics will one day provide a better interpretation.

The catch, he said, is that such an interpretation “will be even weirder than what we have now.”

Since the recipients began their prize-winning work, technical advances have moved quantum computers into the realm of the possible and they are inching closer to being practical.

Dr. Brassard said the field is busier than ever, fuelled in part by the realization that the data world needs an urgent response to the impending obsolescence of today’s encryption systems.

When asked what he planned to do with his share of the prize money, Dr. Brassard, 67, said he had not yet thought about it, “but it’s not to retire.”