Emil Julius Klaus Fuchs (29 December 1911 – 28 January 1988) was a German theoretical physicist and atomic spy who, in 1950, was convicted of supplying information from the American, British, and Canadian Manhattan Project to the Soviet Union during and shortly after the Second World War. While at the Los Alamos National Laboratory, Fuchs was responsible for many significant theoretical calculations relating to the first nuclear weapons, and later, early models of the hydrogen bomb.
https://en.m.wikipedia.org/wiki/Klaus_Fuchs