One afternoon in October 1979, Gilles Brassard was swimming outside a beachfront hotel in San Juan, Puerto Rico, when a stranger swam up to him and changed the course of his career.
Without so much as an introduction, the man began describing a way to create currency that could not be forged, based on the laws of quantum physics. “I was trapped, so I listened politely,” Brassard later recalled.
The stranger was Charles H. Bennett, a physicist from IBM Research. The idea he pitched in that ocean would eventually become the only practical method for sharing encryption keys with security guaranteed not by mathematical complexity but by the laws of physics itself.
As governments and banks race to harden their cryptosystems as we enter the quantum age, the method Bennett proposed is looking less like a theoretical curiosity and more like a necessity. The two continued to build some of the underpinnings of quantum computing, and nearly half a century after that swim, the Association for Computing Machinery has named Bennett a co-recipient of the 2025 A.M. Turing Award, considered the Nobel Prize of computer science, together with Brassard, now a professor at the Université de Montréal. The USD 1 million prize marks the first time the Turing Award has recognized quantum research.
“Most of the applications,” Bennett told IBM in an interview, “are probably yet to be discovered.”
The field Bennett helped build rests on the idea that information follows physical rules —and that can include quantum physics. For most of the 20th century, as Bennett has described it, scientists treated information as abstract, a pattern independent of whatever physical medium carried it. Rolf Landauer, an IBM physicist who would recruit Bennett to the company, challenged that in a provocative 1961 paper, arguing that information is fundamentally physical and obeys the laws of thermodynamics.
Building on that work, Bennett showed in a 1973 paper that computation could in principle be carried out reversibly, run forward and then backward without any net energy cost, revealing a deep connection between physics and information that most scientists had not seen.
One strange property of quantum information is that it cannot be copied.
Classical information can be duplicated perfectly and infinitely. Copy a file, and you have two identical files. At the quantum level, this is false. A quantum state is disturbed the moment you try to measure or copy it. Bennett has a way of explaining this that tends to stop people cold.
“Quantum information,” he said to IBM, is “ like the information in a dream. As soon as you start trying to tell somebody about your dream, you begin to forget the dream, and you only remember what you said about it. The public version can be copied, but it’s not the same as the dream.”
What Bennett and his collaborators grasped was that this limitation was actually a tool. If quantum information cannot be copied, it cannot be secretly copied either. An eavesdropper who intercepts a quantum-encoded message necessarily disturbs it, leaving a trace. That is the premise behind quantum cryptography, which is theoretically unbreakable regardless of the computing power brought against it.
As Bennett later recalled it, that conversation was where the premise became a collaboration.
“Imagine my surprise when this complete stranger swims up to me and starts telling me, without apparent provocation on my part, about Wiesner’s quantum banknotes,” Brassard later wrote. “This was probably the most bizarre, and certainly the most magical, moment in my professional life.”
By 1984, the two had published the BB84 protocol. Alice and Bob, as cryptographers call the communicating parties, could establish a secret key by exchanging single photons, the smallest possible units of light. Any eavesdropper who intercepted them would inevitably disturb the photons, triggering an alert.
Digital security, as Bennett and Brassard wrote, held “even against an opponent with superior technology and unlimited computing power.” BB84 attracted little notice at first. The internet was emerging simultaneously, and the mathematical systems securing it seemed, for the moment, sufficient.
That changed in 1994, when mathematician Peter Shor, then at Bell Labs, showed that a quantum computer could crack the mathematical locks protecting most internet communications. Suddenly the method Bennett and Brassard had developed, by then used experimentally over distances of up to 1,200 kilometers between a satellite and Earth, according to Britannica, looked urgent.
The first working demonstration had come years earlier. In 1989, according to IBM, Bennett built the first quantum cryptography machine in his office at IBM, a two-meter-long device assembled from mirrors, polarizers and photon detectors, with software written by Brassard and his students. Four years after that came a paper introducing quantum teleportation: not the science-fiction kind, but the transfer of a quantum state from one location to another using entanglement, a phenomenon in which measuring one particle instantly affects another regardless of the distance between them.
Still keeping an office at IBM, where Landauer recruited him more than 50 years ago, Bennett is the seventh IBM-affiliated researcher to receive the Turing honor.
Jay Gambetta, Director of IBM Research and an IBM Fellow, said the legacy of that early work runs directly into what the company’s quantum teams are building now.
“When many researchers saw quantum mechanics as a problem to solve for shrinking electronic components rather than a tool to be developed, he recognized the same physics could become a powerful new way to process and transmit information,” Gambetta said. “That insight, and the decades of work that followed, helped lay the intellectual foundation for one of the most important scientific and technological frontiers of our time.”
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