Klaus Emil Julius Fuchs (29 December 1911 – 28 January 1988) was a German theoretical physicist and atomic spy who supplied information from the American, British, and Canadian Manhattan Project to the Soviet Union during and shortly after World War II.

While at the Los Alamos Laboratory, Fuchs was responsible for many significant theoretical calculations relating to the first nuclear weapons and, later, early models of the hydrogen bomb.

[5][3] In the March 1932 German presidential election, the SPD supported Paul von Hindenburg for President, fearing that a split vote would hand the job to the Nazi Party (NSDAP) candidate, Adolf Hitler.

However, when the Communist Party of Germany (KPD) ran its own candidate, Ernst Thälmann, Fuchs offered to speak for him, and was expelled from the SPD.

Fuchs correctly assumed that opposition parties would be blamed for the fire, and surreptitiously removed his hammer and sickle lapel pin.

In August 1933, he attended an anti-fascist conference in Paris chaired by Henri Barbusse, where he met an English couple, Ronald and Jessie Gunn, who invited him to stay with them in Clapton, Somerset.

Fuchs published papers with Born on "The Statistical Mechanics of Condensing Systems" and "On Fluctuations in Electromagnetic radiation" in the Proceedings of the Royal Society.

She visited her brother, Klaus Fuchs, in England en route to America, where she eventually married an American communist, Robert Heineman, and settled in Cambridge, Massachusetts.

[21] Kristel arranged for mathematics professor Israel Halperin, the brother-in-law of a friend of hers, Wendell H. Furry, to send Fuchs some magazines, likely scientific journals.

[22] In May 1941, he was approached by Rudolf Peierls of the University of Birmingham to work on the "Tube Alloys" programme – the British atomic bomb research project.

She was a German communist, a major in Soviet Military Intelligence and an experienced agent who had worked with Richard Sorge's spy ring in the Far East.

[28] In late 1943, Fuchs (codename: "Rest"; he became "Charles" in May 1944)[29] transferred along with Peierls to Columbia University, in New York City, to work on gaseous diffusion as a means of uranium enrichment for the Manhattan Project.

[30] Although Fuchs was "an asset" of GRU in Britain, his "control" was transferred to the NKGB (Russian: Народный Kомиссариат Государственной Безопасности), the Soviet Union's civilian intelligence organisation, when he moved to New York.

[33] He was the author of techniques (such as the still-used Fuchs-Nordheim method) for calculating the energy of a fissile assembly that goes highly prompt critical,[34] and his report on blast waves is still considered a classic.

[45] Fuchs was highly regarded as a scientist by the British, who wanted him to return to the United Kingdom to work on Britain's postwar nuclear weapons programme.

[47] From late 1947 to May 1949 he gave Alexander Feklisov, his Soviet case officer, the principal theoretical outline for creating a hydrogen bomb and the initial drafts for its development as the work progressed in England and America.

Meeting with Feklisov six times, he provided the results of the test at Eniwetok Atoll of uranium and plutonium bombs and the key data on production of uranium-235.

After a great deal of research for his 2019 biography, Trinity, Frank Close confirmed that while MI5 suspected Fuchs for over two years, "it was decrypters at GCHQ who supplied clear proof of his guilt ... not the crack American team that is normally given all the credit", according to a review of the book.

[54] In October 1949, Fuchs had approached Henry Arnold, the head of security at Harwell, with the news that his father had been given a chair at the University of Leipzig in East Germany, and this information became a factor as well.

[59] Fuchs told interrogators that the NKGB had acquired an agent in Berkeley, California, who had informed the Soviet Union about electromagnetic separation research of uranium-235 in 1942 or earlier.

[60] Fuchs's statements to British and American intelligence agencies were used to implicate Harry Gold,[61] a key witness in the trials of David Greenglass and Julius and Ethel Rosenberg in the United States.

Nancy Thorndike Greenspan quotes from police notes on a visit between fellow scientist Peierls and Fuchs in detention shortly after the news of the arrest broke.

[48] Considering that the pace of the Soviet programme was set primarily by the amount of uranium that it could procure, it is difficult for scholars to counterfactually judge how much time or effort was saved.

And in many respects, the Soviet programme was not simply a recapitulation of the Manhattan Project: it explored several approaches that were not done during the war, such as the successful development of the gas centrifuge method of enrichment.

[68] He told Perrin that he was himself "extremely surprised" at the speed at which the Soviets developed their own atomic bomb, "as he had been convinced that the information he had given could not have been applied so quickly and that the Russians would not have had the engineering design and construction facilities that would be needed to build large production plants in such a short time.

"[67] The information that Fuchs was able to give the Soviet Union about the Manhattan Project was much more extensive, and much more technically precise, than that available from other, later-discovered atomic spies like David Greenglass or Theodore Hall.

Most scholars agree with Hans Bethe's 1952 assessment, which concluded that by the time Fuchs left the thermonuclear programme in mid-1946, too little was known about the mechanism of the hydrogen bomb for his information to be useful to the Soviet Union.

"Whatever hopes had existed for a tightly-integrated programme with the British and Canadians died with the Fuchs revelation", as historians Richard G. Hewlett and Francis Duncan put it.

[71] Fuchs was prosecuted by Sir Hartley Shawcross[72] and was convicted on 1 March 1950 of four counts of breaking the Official Secrets Act by "communicating information to a potential enemy.

[80] On arrival at Berlin Schönefeld Airport in the GDR, Fuchs was met by Margarete "Grete" Keilson, a friend from his years as a student communist.