Soviet knowledge of the Manhattan Project was extensive. German-born Klaus Fuchs, a theoretical physicist, fled to England. He was a member of "the British Mission", where he made major contributions in the theory of gaseous diffusion cascades, and in implosion theory. He, along with David Greenglass, passed secrets to the Soviets through the spy Harry Gold, which helped the Soviet Union get a head start on its research and stay aware of what was going on at Los Alamos. Fuchs passed detailed designs about the implosion bomb, as well as some early information on the hydrogen bomb.
Fuchs was finally arrested in England in 1950. His arrest led to the arrests of Gold, and Greenglass, his sister Ethel Rosenberg and her husband Julius. The Rosenbergs were convicted of passing atomic secrets and were sentenced to death, which drew worldwide protest. They were executed in 1953. Fuchs served nine years of a 14-year sentence. He then immigrated to East Germany, where he became deputy director of their nuclear research institute. He died on January 28, 1988.