In 2000, the world braced for Y2K. It came with a date and a remedy. There was panic about doomsday but as I and other programmers stretched the year field from two to four characters, apart from scattered hiccups, the lights stayed on. Everything about Y2K was known—the problem, the solution, and the deadline. Q-Day is something else entirely. Q-Day is shorthand for the moment when quantum computing crosses a line we assumed would hold—when the mathematics that secures modern life can be broken, and broken quickly. On Q-Day the locks will be quietly and rapidly picked. And the unsettling part