This was not just any math problem. On Wednesday, Google said its researchers had performed a calculation that the largest supercomputers could not complete in under 10,000 years. And they had done it in 3 minutes 20 seconds. With that calculation, the company said, its research lab in Santa Barbara, Calif., achieved a milestone that scientists had been working toward since the 1980s -- a breakthrough called "quantum supremacy" that could allow new kinds of computers to do calculations at speeds that are inconceivable with today's technology. The long-sought breakthrough could have implications for national security and cryptography and even pave the way for the creation of new medicines.