Klaus Fuchs
Klaus Fuchs was a German physicist, atomic spy, and communist who leaked US/UK/Canadian nuclear secrets to the Soviet Union during the Cold War.
~11 min readMay 6, 2026 · 07:28 AM
<p>Klaus Emil Julius Fuchs (29 December 1911 – 28 January 1988) was a German theoretical physicist, atomic spy, and communist 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. After his conviction in 1950, he served nine years in prison in the United Kingdom, then migrated to East Germany where he resumed his career as a physicist and scientific leader.<br/>The son of a Lutheran pastor, Fuchs attended the University of Leipzig, where his father was a professor of theology, and became involved in student politics, joining the student branch of the Social Democratic Party of Germany (SPD), and the Reichsbanner Schwarz-Rot-Gold, an SPD-allied paramilitary organisation. He was expelled from the SPD in 1932, and joined the Communist Party of Germany (KPD). He went into hiding after the 1933 Reichstag fire and the subsequent persecution of communists in Nazi Germany, and fled to the United Kingdom, where he received his PhD from the University of Bristol under the supervision of Nevill Francis Mott, and his DSc from the University of Edinburgh, where he worked as an assistant to Max Born.<br/>After the Second World War broke out in Europe, he was interned in the Isle of Man, and later in Canada. After he returned to Britain in 1941, he became an assistant to Rudolf Peierls, working on "Tube Alloys"—the British atomic bomb project. He began passing information on the project to the Soviet Union through Ursula Kuczynski, codenamed "Sonya", a German communist and a major in Soviet military intelligence who had worked with Richard Sorge's spy ring in the Far East. In 1943, Fuchs and Peierls went to Columbia University, in New York City, to work on the Manhattan Project. In August 1944, Fuchs joined the Theoretical Physics Division at the Los Alamos Laboratory, working under Hans Bethe. His chief area of expertise was the problem of implosion, necessary for the development of the plutonium bomb. After the war, he returned to the UK and worked at the Atomic Energy Research Establishment at Harwell as head of the Theoretical Physics Division.<br/>In January 1950, Fuchs confessed that he had passed information to the Soviets over a seven-year period beginning in 1942. A British court sentenced him to fourteen years' imprisonment and he was subsequently stripped of his British citizenship. He was released in 1959, after serving nine years, and migrated to the German Democratic Republic (East Germany), where he was elected to the Academy of Sciences and became a member of the Socialist Unity Party of Germany (SED) central committee. He was later appointed deputy director of the Central Institute for Nuclear Physics in Dresden, where he served until his retirement in 1979.<br/>Post Cold War declassified information states that the Russians freely acknowledged that Fuchs gave them the fission bomb.</p>
<p>Early life<br/>Klaus Emil Julius Fuchs was born in Rüsselsheim, Grand Duchy of Hesse, on 29 December 1911, the third of four children of a Lutheran pastor, Emil Fuchs, and his wife Else Wagner. His father served in the army during World War I but later became a pacifist and a socialist, joining the Social Democratic Party of Germany (SPD) in 1912. He eventually became a Quaker. Fuchs had an older brother Gerhard, an older sister Elisabeth, and a younger sister, Kristel. The family moved to Eisenach, where Fuchs attended the Martin-Luther Gymnasium, and took his Abitur. At school, Fuchs and his siblings were taunted over his father's unpopular socialist political views, which they came to share. They became known as the "red foxes", Fuchs being the German word for fox.<br/>Fuchs was left-handed, but was forced to write with his right hand.<br/>Fuchs entered the University of Leipzig in 1930, where his father was a professor of theology. He became involved in student politics, joining the student branch of the SPD, a party that his father had joined in 1921, and the Reichsbanner Schwarz-Rot-Gold, the party's paramilitary organisation. His father took up a new position as professor of religion at the Pedagogical Academy in Kiel, and in the autumn Fuchs transferred to the University of Kiel, which his brother Gerhard and sister Elisabeth also attended. Fuchs continued his studies in mathematics and physics at the university. In October 1931, his mother committed suicide by drinking hydrochloric acid. The family later discovered that his maternal grandmother had also taken her own life.<br/>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. That year Fuchs and all three of his siblings joined the KPD. Fuchs and his brother Gerhard were active speakers at public meetings, and occasionally attempted to disrupt NSDAP gatherings. At one such gathering, Fuchs was beaten up and thrown into a river.<br/>When Hitler became Chancellor of Germany in January 1933, Fuchs decided to leave Kiel, where the NSDAP was particularly strong and he was a well-known KPD member. He enrolled at the Kaiser Wilhelm Institute for Physics in Berlin. On 28 February, while on a train to Berlin for a secret KPD meeting, he read a newspaper article on the Reichstag fire. Fuchs correctly assumed that opposition parties would be blamed for the fire, and surreptitiously removed his hammer and sickle lapel pin. Fellow party members urged him to continue his studies in another country. He went into hiding for five months in the apartment of a fellow party member. 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. He was expelled from the Kaiser Wilhelm Institute in October 1933.</p>
<p>Refugee in Britain<br/>Fuchs arrived in England on 24 September 1933. Jessie Gunn was a member of the Wills family, the heirs to Imperial Tobacco and benefactors of the University of Bristol. She arranged for Fuchs to meet Nevill Francis Mott, Bristol's professor of physics, and he agreed to take Fuchs on as a research assistant. Fuchs earned his PhD in physics there in 1937. A paper on "A Quantum Mechanical Calculation of the Elastic Constants of Monovalent Metals" was published in the Proceedings of the Royal Society in 1936. <br/>By this time, Mott had a number of German refugees working for him, and lacked positions for them all. He did not think that Fuchs would make much of a teacher, so he arranged a research post for Fuchs, at the University of Edinburgh working under Max Born, who was himself a German refugee. 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. He also received a Doctorate in Science degree from Edinburgh. Fuchs proudly posted copies back to his father, Emil, in Germany.<br/>In Germany, Emil had been dismissed from his academic post, and, disillusioned with the Lutheran Church's support of the NSDAP, had become a Quaker in 1933. He was arrested for speaking out against the government and was held for a month. His daughter, Elisabeth, married a fellow communist, Gustav Kittowski, with whom she had a child they named Klaus. Elisabeth and Kittowski were arrested in 1933, and sentenced to 18 months imprisonment but were freed at Christmas. Emil's son, Gerhard, and his wife Karin were arrested in 1934 and spent the next two years in prison. Gerhard, Karin, Elisabeth and Kittowski established a car rental agency in Berlin, which they used to smuggle Jews and opponents of the government out of Germany.<br/>After Emil was arrested in 1933, his other daughter, Kristel, fled to Zurich, where she studied education and psychology at the University of Zurich. She returned to Berlin in 1934, where she too worked at the car rental agency. In 1936, Emil arranged with Quaker friends in the United States for Kristel to attend Swarthmore College there. 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. She became a permanent resident in the United States in May 1938. <br/>In 1936, Kittowski and Elisabeth were arrested again, and the rental cars were impounded. Gerhard and Karin fled to Czechoslovakia. Elisabeth was released and went to live with her father, Emil, while Kittowski, sentenced to six years, later escaped from prison and also made his way to Czechoslovakia. In August 1939, Elisabeth died by suicide by throwing herself off a bridge in the path of an oncoming train, leaving Emil to raise her young son, Klaus.</p>
<p>Second World War<br/>In August 1939, Fuchs applied to become a British subject, but his application had not been processed before the Second World War broke out in Europe in September 1939. There was a classification system for enemy aliens, but Born provided Fuchs with a reference that said that he had been a member of the SPD from 1930 to 1932, and an anti-Nazi. There matters stood until June 1940, when the police arrived and took Fuchs into custody. He was first interned on the Isle of Man. In July, he was sent to internment camps in Canada, first on the Plains of Abraham in Quebec City and later at a site near Sherbrooke, Quebec. During his internment in 1940, he continued to work and published four more papers with Born: The Mass Centre in Relativity, Reciprocity, Part II: Scalar Wave Functions, Reciprocity, Part III: Reciprocal Wave Functions and Reciprocity, Part IV: Spinor Wave Functions, and one by himself, On the Statistical Method in Nuclear Theory.</p>
<p>While interned in Quebec, he joined a communist discussion group led by Hans Kahle. Kahle was a KPD member who had fought in the Spanish Civil War. After fleeing to Britain with his family, Kahle had helped Jürgen Kuczynski organise the KPD in Britain. 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. Max Born lobbied for his release. On Christmas Day 1940, Fuchs and Kahle were among the first group of internees to board a ship to return to Britain.<br/>Fuchs returned to Edinburgh in January, and resumed working for Born. 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. Despite wartime restrictions, he became a British subject on 31 July 1942 and signed an Official Secrets Act declaration form. As accommodation was scarce in wartime Birmingham, he stayed with Rudolf and Genia Peierls. Fuchs and Peierls did some important work together, which included a fundamental paper about isotope separation.<br/>Soon after, Fuchs contacted Jürgen Kuczynski, who was now teaching at the London School of Economics. Kuczynski put him in contact with Simon Davidovitch Kremer (codename: "Alexander"; 1900-1991), the secretary to the military attaché at the Soviet Union's embassy, who worked for the GRU (Russian: Главное Разведывательное Управление), the Red Army's foreign military intelligence directorate. After three meetings, Fuchs was teamed up with a courier so he would not have to find excuses to travel to London. She was Ursula Kuczynski (codename: "Sonya"), the sister of Jürgen Kuczynski. 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.<br/>In late 1943, Fuchs (codename: "Rest"; he became "Charles" in May 1944), 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. 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. He spent Christmas 1943 with Kristel and her family in Cambridge. He was contacted by Harry Gold (codename: "Raymond"), an NKGB agent in early 1944.</p>
<p>From August 1944, Fuchs worked in the Theoretical Physics Division at the Los Alamos Laboratory, under Hans Bethe. His chief area of expertise was the problem of imploding the fissionable core of the plutonium bomb. At one point, Fuchs did calculation work that Edward Teller had refused to do because of lack of interest. 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, and his report on blast waves is still considered a classic. Fuchs was one of the many Los Alamos scientists present at the Trinity test in July 1945.<br/>Socially, Fuchs was later judged as someone who kept to himself, and never talked about politics. He was fairly well-liked. He dated grade school teachers Evelyn Kline and Jean Parker, and occasionally served as a babysitter for other scientists. He befriended Richard Feynman. Fuchs and Peierls were the only members of the British Mission to Los Alamos who owned cars, and Fuchs lent his Buick to Feynman so Feynman could visit his dying wife in a hospital in Albuquerque. When Fuchs was discovered to be a spy, his former colleagues were shocked. The postwar director of Los Alamos, Norris Bradbury, later said that:</p>
<p>Fuchs was a strange man. I knew him, though not well. A very popular, very reticent bachelor, who was welcome at parties because of his nice manners. He worked very hard; worked very hard for us, for this country. His trouble was that he worked very hard for Russia, too. Basically, he hated the Germans bitterly. He had an undying hatred and he simply thought this country was not working hard enough to assist the Russians to defeat the Germans. Well, he was in his own odd way loyal to the United States. He suffered from a double loyalty.<br/>Fuchs's main courier in the United States was Harry Gold, a chemist who lived in Philadelphia, but was willing to travel to wherever Fuchs was. Allen Weinstein, the author of The Haunted Wood: Soviet Espionage in America (1999), has pointed out: "The NKVD had chosen Gold, an experienced group handler, as Fuchs's contact on the grounds that it was safer than having him meet directly with a Russian operative, but Semyon Semyonov was ultimately responsible for the Fuchs relationship."<br/>Gold reported after his first meeting with Klaus Fuchs:</p>
<p>He (Fuchs) obviously worked with our people before and he is fully aware of what he is doing. … He is a mathematical physicist … most likely a very brilliant man to have such a position at his age (he looks about 30). We took a long walk after dinner. … He is a member of a British mission to the U.S. working under the direct control of the U.S. Army. … The work involves mainly separating the isotopes... and is being done thusly: The electronic method has been developed at Berkeley, California, and is being carried out at a place known only as Camp Y. … Simultaneously, the diffusion method is being tried here in the East. … Should the diffusion method prove successful, it will be used as a preliminary step in the separation, with the final work being done by the electronic method. They hope to have the electronic method ready early in 1945 and the diffusion method in July 1945, but (Fuchs) says the latter estimate is optimistic. (Fuchs) says there is much being withheld from the British. Even Niels Bohr, who is now in the country incognito as Nicholas Baker, has not been told everything. <br/>After the end of the war, in April 1946, he attended a conference at Los Alamos that discussed the possibility of a thermonuclear weapon; one month later, he filed a patent with John von Neumann, describing a thermonuclear weapon design the two had collaborated on. Though it was not a viable design, it was the first instance of the idea of radiation implosion being part of a weapon design. Radiation implosion would later become a core part of the successful Teller–Ulam design for thermonuclear weapons, but its importance was not appreciated at the time. Bethe considered Fuchs "one of the most valuable men in my division" and "one of the best theoretical physicists we had."</p>
<p>Post-war activities<br/>At the request of Norris Bradbury, who had replaced Robert Oppenheimer as director of the Los Alamos Laboratory in October 1945, Fuchs stayed on at the laboratory into 1946 to help with preparations for the Operation Crossroads weapons tests. The US Atomic Energy Act of 1946 (McMahon Act) prohibited the transfer of information on nuclear research to any foreign country, including Britain, without explicit official authority, and Fuchs supplied highly classified U.S. information to nuclear scientists in Britain and to his Soviet contacts.<br/>As of 2014, British official files on Fuchs were still being withheld. As of 2020, the National Archives listed one dossier on Fuchs, KV 2/1263, including the "Prosecution file. With summary of early interrogations ... and details of the scientific/technical information passed to the Russians," which was released in 2003. According to an October 2020 book review, author Nancy Thorndike Greenspan "appears to have had access to some of the Fuchs files that have been withheld at Kew, such as the AB/1 [Tube Alloys] series, which has been closed for access for most human beings," although others are available to view onsite, or are available to download, such as KV 2/1263 and some AB 1 fi</p>
<hr/><p><em>Based on Wikipedia article: <a href="https://en.wikipedia.org/wiki/Klaus_Fuchs" target="_blank" rel="noopener">Klaus Fuchs</a> – licensed under CC BY-SA.</em></p>
