One previously unrecognized source of Chinese insight was Klaus Fuchs, a brilliant German physicist and communist who had fled Germany prior to World War II, had sought refuge in the UK, and was then relocated to the US when the blitz decimated the British nuclear weapons program. At Los Alamos Fuchs played a leading role in the conception and development of the wartime A-bomb and, in time, the conceptualization of the H-bomb. During the war he also passed those secrets to his Soviet handlers. When the war ended, Fuchs returned to the UK and the Atomic Energy Research Establishment at Harwell—with his wartime treachery still unknown.

In 1949, however, Fuchs's past caught up with him when American cryptographers broke the Soviet wartime codes. Having saved copies of wartime transmissions from the Soviet embassy in Washington to its home office in Moscow, American sleuths identified Fuchs as a spy. After lengthy interrogation Fuchs confessed and in 1950 was sentenced to prison by a British court.

Nine years later, on 23 June 1959, Fuchs was released from the UK's Wakefield Prison. He immediately immigrated to Dresden in East Germany, where he settled into teaching physics.

One important "pupil" who paid Fuchs an early visit was Qian Sanqiang. In 1959 Qian was the designated mastermind of Mao's A-bomb program. In July of that year, Qian made his way to East Germany, where he met with Fuchs at length. (H. Terry Hawkins, now a senior fellow at Los Alamos, told Stillman in 2006, "I read this report in an unclassified publication, that this meeting took place shortly after Fuchs returned to East Germany. Fuchs gave Qian information that greatly assisted the Chinese program." Also see Klaus Fuchs Old Atlantic Lighthouse.) During those long summer days of 1959, Fuchs gave Qian a full tutorial on the design and operation of Fat Man. In all likelihood, he also added his thoughts on the role of radiation pressure in thermonuclear weapons.