One bright October afternoon on a beach in San Juan, Puerto Rico, two scientists found the solution to a problem that didn't yet exist. It was 1979. Gilles Brassard, then a newly graduated Ph.D. from Cornell University, was immersed in the warm Caribbean water when someone swam toward him. The dark-haired stranger launched into a pitch about how to make a currency that could not be counterfeited. The scheme, invented several years earlier by a Columbia University graduate student named Stephen Wiesner, involved embedding photons—particles of light—in banknotes. By the laws of quantum mechanics, any attempt to measure or copy the photons would instantaneously change their properties. Each bill would have its own string of photons, a quantum serial number that could never be duplicated.