Free Computing with Quantum Cats by John Gribbin

a Hungarian citizen) did not recognize his American divorce. So he had to renounce his Hungarian citizenship and marry as an American citizen, which in turn enabled Klári to obtain the necessary visa as his wife. The formalities took months, and itwas November 18, 1938, before the couple were finally married in Budapest. The following month they sailed for New York from Southampton on the Queen Mary , arriving a week before Christmas. The marriage lasted for the rest of Johnny's life, although it did not always run smoothly, and many personal insights into his character come from a memoir Klári wrote, now in the possession of Johnny's daughter Marina. One intriguing tidbit is that Johnny, like so many gifted mathematicians, suffered from a mild compulsive disorder, one manifestation of which was that whenever he turned a light on or off he had to do so by flipping the switch exactly seven times.
    The couple were childless. Klári suffered a miscarriage in the summer of 1942, and with Johnny often away on secret war work, in 1943 she took a job with the Office of Population Research at Princeton University. The training she gained there in statistical work would prove invaluable when she later worked with Johnny as one of the first computer programmers—but that is getting ahead of our story.
    In spite of the distractions of his personal life, and the parties (which continued with Klári as hostess), Johnny was one of the early stars of the Institute, proof that Feynman's reservations did not apply to everyone there. Between 1933 and the end of 1941, when America was forced into war, he published thirty-six scientific papers and, even more significantly, met Alan Turing and learned about his work. But long before the Japanese attack on Pearl Harbor, Johnny was becoming involved with the military. He became a US citizen on January 8, 1937, and was so concerned about the situation in Europe that he soon applied for a commission inthe reserve of the US Army, being turned down solely on grounds of age (he had just passed the cut-off age of 35 by the time he had completed the required examinations). Instead, he became a consultant to the US Army's Ordnance Department, and then (in 1940) a member of the scientific advisory board for its Ballistic Research Laboratory. Other appointments followed, including membership of the National Defense Research Council (NDRC) and work for the US Navy (which, among other things, took him on an extended visit to Britain). By the time the Japanese bombs fell on Pearl Harbor on December 7, 1941, he was the go-to man for advice about large explosions, had a reputation for being able to break down complex problems into their simple components, and was a skillful chairman of committees who got things done. Inevitably—although he kept fingers in many pies—he was drawn towards the Manhattan Project.
    JOHNNY AND THE BOMB
    In July 1946, von Neumann received both the US Navy's Distinguished Civilian Service Award and President Harry S. Truman's Medal for Merit. The citation for the latter said that he was “primarily responsible for fundamental research by the United States Navy on the effective use of high explosives, which has resulted in the discovery of a new ordnance principle for offensive action.” The reference was, of course, to the nuclear bomb.
    Von Neumann had been called back from England to join the Manhattan Project in 1943, and by the autumn he was in Los Alamos, where he made two key contributions to the project. 5 The first was to point out (and prove mathematically) that such a bomb would be more effective ifexploded at altitude rather than at ground level, because both the heat and blast would affect a wider area. The second contribution was much more profound.
    A fission bomb works by bringing together forcefully a sufficient amount (“critical mass”) of a radioactive material such as uranium or plutonium. Under such conditions, particles