The German theoretical physicist Klaus Fuchs helped develop the first effective atomic bomb at Los Alamos during the WWII Manhattan Project.
German born, Fuchs fled Nazi Germany and became a British citizen in August 1942. As a long-time Communist sympathizer, Fuchs soon began providing Soviet KGB operatives with classified information on the progress of Britain’s atomic energy research.
In 1943, Fuchs went to Columbia University in New York with a British delegation of scientists to work on the Manhattan Project. In 1944 he began releasing classified information to a KGB agent in America.
In August 1944 Fuchs was transferred to Los Alamos where he calculated the approximate energy yield of an atomic explosion, and specialized in researching implosion methods.
In addition to secrets regarding the American atomic bomb project, Fuchs also passed detailed information about the hydrogen bomb to the Soviets.
At the end of the war, Fuchs returned to England and continued his work on the British atomic bomb project.
In 1949 decrypted cables from the U. S. Army Signal Intelligence Service revealed Fuchs was a Soviet spy.
Fuchs was arrested in January 1950 and charged with violating Britain’s Official Secrets Act. After his testimony led to the arrests of KGB agents (including Julius and Ethel Rosenberg) he was released after nine of his 14 year prison sentence.
In 1959 Fuchs returned to the (East) German Democratic Republic as Deputy Director of the Central Institute for Nuclear Research at the Helmholtz-Zentrum Dresden-Rossendorf laboratory. In 1979 he received the Karl Marx Medal of Honor.