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 ...