A Quantum Leap for the Turing Award

Today it’s widely acknowledged that the future of computing will involve the quantum realm. Companies like Google, Microsoft, IBM, and a few well-funded startups are frantically building quantum computers and routinely claiming advances that seem to bring this exotic, world-changing technology within reach. In 1979 all of this was unthinkable. But that summer, two scientists met in the Atlantic Ocean off the coast of Puerto Rico, and their aquatic conversation led to a body of work that created quantum information theory. In a larger sense, their contributions helped bring computer science into the quantum age.

Those water-logged scientists, Charles Bennett and Gilles Brassard, are now the latest recipients of the ACM A.M. Turing Award, the Nobel Prize of the field.

Until that 1979 meeting, there had been a disconnect between information science and physics. The latter field had experienced a disruption in the early 20th century when physicists discovered quantum mechanics, a deeper explanation of how the universe operated that superseded the classical physics of Issac Newton. Computer science, however, didn’t account for the quantum world, except for having to deal with its effects on tiny chips, where the behavior of electrons were relevant.

“In the 1950s through the 1980s people thought of quantum effects as occurring in very small things and as a source of noise—you had to understand quantum theory to build transistors,” explains Bennett. “People thought of quantum mechanics as a nuisance.” He and Brassard discovered methods—like quantum coin-tossing and quantum entanglement—that turned the perceived handicaps of quantum reality into a powerful tool.

At the time of their meeting, Bennett was at a career crossroads; he’d joined IBM in 1972, but had taken a years-long break from academic publishing. One source of continuing fascination was an idea shared by a college classmate, Steven Weisner—that using a quantum form of cryptography could enable digital money that could not be counterfeited. (Yep, Weisner envisioned cryptocurrency in the late 1960s!) At the 1979 conference, Bennett saw that a cryptographer named Brassard was in attendance—he had just completed a dissertation on public-key crypto—and located him offshore.

“So there I was swimming in the beach when a complete stranger came up to me and started telling me that a friend of his found that we can use quantum mechanics to make affordable banking notes out of nowhere,” Brassard tells me. “If I had been on firm ground, I would have run for my life, but I was trapped in the ocean, so I listened politely.” Though Brassard had no previous interest in physics, he was intrigued by the approach, and the pair eventually published a theory called BB84, essentially creating an alternative to classic public-key cryptography based on what would become quantum information theory. Suddenly, the world of the quantum became a source of solutions—if scientists could invent the mechanisms to make it happen. As Yannis Ioannidis—president of ACM, which bestows the Turing Award—put it in a statement, “Bennett and Brassard fundamentally changed our understanding of information itself.”

Both scientists take pains to say that their original work did not lead directly to the current scramble to build quantum computers. Bennett notes that in a 1981 conference at MIT, legendary physicist Richard Feynman “made the point that, since nature is quantum, probably some computational jobs would need to be done by a quantum computer.” He also credits physicist David Deutsch for key ideas about quantum computers. Bennett and Brassard became part of that effort.

“Quantum computing was invented independently from us, but then we jumped in,” says Brassard. “I was the first person to design a quantum circuit to do quantum teleportation.” Brassard and Bennett’s work on teleportation, while still in an experimental stage, is now part of the quantum lore. Brassard has said that “one day, it will fuel the quantum internet.”