Today, silicon microchips underlie every aspect of digital computing. But their dominance was never a foregone conclusion. Throughout the 1950s, electrical engineers and other researchers explored many alternatives to making digital computers. One of them seized the imagination of the U.S. National Security Agency (NSA): a superconducting supercomputer. Such a machine would take advantage of superconducting materials that, when chilled to nearly the temperature of deep space-just a few degrees above absolute zero-exhibit no electrical resistance whatsoever. This extraordinary property held the promise of computers that could crunch numbers and crack codes faster than transistorbased systems while consuming far less power. For six decades, from the mid-1950s to the present, the NSA has repeatedly pursued this dream, in partnership with industrial and academic researchers. Time and again, the agency sponsored significant projects to build a superconducting computer. Each time, the effort was abandoned in the face of the unrelenting pace of Moore’s Law and the astonishing increase in performance and decrease in cost of silicon microchips.