It began 25 years ago in the warm coastal waters of Puerto Rico when a stranger swam over to Gilles Brassard and struck up a conversation about using quantum physics to make bank notes impossible to counterfeit. "I had no idea who he was," recalled Brassard, then a 24-year-old prodigy and computer-science professor at the Universite de Montreal. "He just started talking nonsense about quantum physics."

The stranger turned out to be U.S. physicist Charles H. Bennett. Their chance meeting while attending a theoretical computer science conference would end up revolutionizing the art of code making, also known as cryptography.

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