Klaus Fuchs

• 1. Introduction

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  Klaus Fuchs, a German-born theoretical physicist, gave a plain reason for passing atomic secrets to the Soviet Union. Rudolf Peierls, a fellow scientist, visited him in detention shortly after the news of his arrest broke and asked him why. Fuchs answered: "Knowledge of atomic research should not be the private property of any one country but should be shared with the rest of the world for the benefit of mankind."

  The secrets came from the joint American, British and Canadian Manhattan Project. He passed them on during the Second World War and shortly after it. At the Los Alamos Laboratory he carried out many of the theoretical calculations behind the first nuclear weapons. Later his work touched early models of the hydrogen bomb.

  How does a man driven out of Germany by the Nazis end up inside the most guarded laboratory of the war? What exactly did he give away, and did it truly change history? And what did Britain, and later East Germany, make of him afterwards?

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• 2. Red Foxes

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  Klaus Emil Julius Fuchs was born in Rüsselsheim, in the Grand Duchy of Hesse, on the 29th of December 1911. He was the third of four children. His father, Emil Fuchs, was a Lutheran pastor, and his mother was Else Wagner. Klaus had an older brother, Gerhard, an older sister, Elisabeth, and a younger sister, Kristel.

  Emil Fuchs served in the army during the First World War but later became a pacifist and a socialist. He joined the Social Democratic Party of Germany, known as the SPD, and eventually became a Quaker.

  The family settled in Eisenach, where Klaus attended the Martin-Luther Gymnasium and sat his Abitur. Classmates mocked the children for their father's unpopular socialism, and the children came to share his views. The mockery produced a nickname, the "red foxes", a pun on the fact that Fuchs is the German word for fox. Klaus was also left-handed, but he was forced to write with his right hand.

  In 1930 Fuchs entered the University of Leipzig, where his father was a professor of theology. He joined the SPD's student branch and the Reichsbanner Schwarz-Rot-Gold, a paramilitary organisation allied with the party. Emil then took a post as professor of religion at the Pedagogical Academy in Kiel. In the autumn Klaus followed him to the University of Kiel, where Gerhard and Elisabeth were also students, and he continued with mathematics and physics.

  In October 1931 his mother died by suicide, drinking hydrochloric acid. The family later discovered that his maternal grandmother had also taken her own life.

  The March 1932 presidential election ended his time in the SPD. The party backed Paul von Hindenburg, fearing that a divided vote would hand the presidency to Adolf Hitler. When the Communist Party of Germany, the KPD, ran Ernst Thälmann as its own candidate, Fuchs offered to speak for Thälmann. The SPD expelled him. That same year Fuchs and all three of his siblings joined the KPD. He and Gerhard became active speakers at public meetings and sometimes tried to disrupt Nazi gatherings. At one of them Fuchs was beaten up and thrown into a river.

  Hitler became Chancellor in January 1933, and Fuchs decided to leave Kiel, where the Nazis were especially strong and he was a known communist. He enrolled at the Kaiser Wilhelm Institute for Physics in Berlin. On the 28th of February he was riding a train to a secret KPD meeting when he read a newspaper report on the Reichstag fire. He guessed that opposition parties would be blamed, and he quietly removed the hammer and sickle pin from his lapel.

  Fellow party members urged him to continue his studies abroad. He spent five months hiding in the apartment of a party comrade. In August 1933 he attended an anti-fascist conference in Paris chaired by Henri Barbusse. There he met an English couple, Ronald and Jessie Gunn, who invited him to stay with them in Clapton, Somerset. The Kaiser Wilhelm Institute expelled him in October 1933.

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• 3. Refugee Years

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  Fuchs arrived in England on the 24th of September 1933. Jessie Gunn belonged to the Wills family, heirs to Imperial Tobacco and benefactors of the University of Bristol. She introduced him to Nevill Francis Mott, the university's professor of physics, who took him on as a research assistant. Fuchs earned his PhD there in 1937. Among his early work was a 1936 paper in the Proceedings of the Royal Society, "A Quantum Mechanical Calculation of the Elastic Constants of Monovalent Metals".

  Mott already employed several German refugees and lacked posts for all of them. He doubted that Fuchs would make much of a teacher, so he arranged a research position at the University of Edinburgh under Max Born, himself a German refugee. With Born, Fuchs published "The Statistical Mechanics of Condensing Systems" and "On Fluctuations in Electromagnetic radiation", both in the Proceedings of the Royal Society. He also earned a Doctorate in Science from Edinburgh and proudly mailed copies of his papers to his father.

  In Germany, Emil Fuchs had lost his academic post. Disillusioned by the Lutheran Church's support for the Nazi Party, he became a Quaker in 1933. He was arrested for speaking out against the government and held for a month. Elisabeth had married a fellow communist, Gustav Kittowski, and they had a son named Klaus. Both were arrested in 1933 and sentenced to eighteen months, but they were freed at Christmas.

  Gerhard and his wife Karin were arrested in 1934 and spent two years in prison. After their release, Gerhard, Karin, Elisabeth and Kittowski opened a car rental agency in Berlin. They used it to smuggle Jews and opponents of the government out of Germany. Kristel fled to Zurich after her father's arrest and studied education and psychology there. She returned to Berlin in 1934 and worked at the agency too.

  In 1936 Emil arranged with Quaker friends in the United States for Kristel to attend Swarthmore College. She visited her brother Klaus in England on the way. She later married an American communist, Robert Heineman, and settled in Cambridge, Massachusetts. She became a permanent resident of the United States in May 1938.

  Back in Berlin, Kittowski and Elisabeth were arrested again in 1936, and the rental cars were impounded. Gerhard and Karin fled to Czechoslovakia. Kittowski was sentenced to six years, escaped from prison and also reached Czechoslovakia. Elisabeth was released and went to live with her father. In August 1939 she died by suicide, throwing herself off a bridge into the path of an oncoming train. Emil was left to raise her young son.

  That same month Fuchs applied to become a British subject, but war broke out in September before the application was processed. Enemy aliens were sorted into categories, and Born supplied a reference describing Fuchs as an anti-Nazi who had belonged to the SPD from 1930 to 1932. In June 1940 the police arrived and took him into custody. He was interned first on the Isle of Man. In July he was sent to camps in Canada, first on the Plains of Abraham in Quebec City and later near Sherbrooke, Quebec.

  He kept working behind the wire. During 1940 he published four more papers with Born and one of his own, "On the Statistical Method in Nuclear Theory". In Quebec he joined a communist discussion group led by Hans Kahle, a KPD member who had fought in the Spanish Civil War. After fleeing to Britain, Kahle had helped Jürgen Kuczynski organise the KPD there.

  Kristel arranged for Israel Halperin, a mathematics professor, to send Fuchs some magazines. Halperin was the brother-in-law of her friend Wendell H. Furry. Meanwhile Born lobbied for the release. On Christmas Day 1940, Fuchs and Kahle were among the first internees to board a ship back to Britain. Fuchs returned to Edinburgh in January 1941 and went back to work for Born.

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• 4. Recruited

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  In May 1941 Rudolf Peierls of the University of Birmingham asked Fuchs to join "Tube Alloys", the British atomic bomb research project. Despite wartime restrictions, Fuchs became a British subject on the 31st of July 1942 and signed an Official Secrets Act declaration. Lodgings were scarce in wartime Birmingham, so he stayed with Peierls and his wife Genia. The two men wrote a fundamental paper on isotope separation.

  Soon afterwards Fuchs contacted Jürgen Kuczynski, who was now teaching at the London School of Economics. Kuczynski put him in touch with Simon Davidovitch Kremer, codenamed "Alexander". Kremer was secretary to the military attaché at the Soviet embassy and worked for the GRU, the Red Army's foreign military intelligence directorate. After three meetings, Fuchs was paired with a courier so that he would not need excuses to travel to London.

  The courier was Ursula Kuczynski, codenamed "Sonya", the sister of Jürgen. She was a German communist and a major in Soviet military intelligence. She was also an experienced agent who had worked with Richard Sorge's spy ring in the Far East. Fuchs himself carried the codename "Rest" and became "Charles" in May 1944.

  In late 1943 Fuchs moved with Peierls to Columbia University in New York City to work on gaseous diffusion, a method of enriching uranium for the Manhattan Project. The move changed his handlers. Though the GRU had run him in Britain, control passed to the NKGB, the Soviet Union's civilian intelligence organisation. He spent Christmas 1943 with Kristel and her family in Cambridge. In early 1944 an NKGB agent named Harry Gold, codenamed "Raymond", made contact.

  Gold was a chemist from Philadelphia who would travel to wherever Fuchs happened to be. The historian Allen Weinstein notes that Gold was an experienced handler of agent groups. The NKVD judged a go-between safer than a face-to-face meeting with a Russian operative, though Semyon Semyonov remained ultimately responsible for the relationship.

  Gold wrote up their first meeting. He judged that Fuchs had plainly worked with Soviet agents before and knew exactly what he was doing. He guessed Fuchs was a brilliant mathematical physicist, noting that he looked about thirty. Fuchs described the diffusion and electromagnetic methods of separating uranium isotopes and said that much was being withheld from the British. Even Niels Bohr, then in the country incognito as Nicholas Baker, had not been told everything.

  From August 1944 Fuchs worked in the Theoretical Physics Division at Los Alamos under Hans Bethe. His chief specialty was implosion, the problem of squeezing the fissionable core of the plutonium bomb. At one point he took on calculation work that Edward Teller had refused to do because he lacked interest in it.

  Fuchs authored techniques that are still in use, including the Fuchs-Nordheim method for calculating the energy of a fissile assembly that goes highly prompt critical. His report on blast waves is still regarded as a classic. In July 1945 he was among the many Los Alamos scientists present at the Trinity test.

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• 5. The Quiet Colleague

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  At Los Alamos, colleagues later remembered Fuchs as someone who kept to himself and never talked politics. They also found him fairly well liked. He dated two grade school teachers, Evelyn Kline and Jean Parker. He sometimes babysat for other scientists' children.

  He befriended Richard Feynman. Fuchs and Peierls were the only members of the British Mission who owned cars. Fuchs lent his Buick to Feynman so that Feynman could visit his dying wife in a hospital in Albuquerque.

  When the spying came out, his former colleagues were stunned. Norris Bradbury, who later directed Los Alamos, called him a very popular and very reticent bachelor, welcome at parties because of his nice manners. Bradbury said Fuchs worked very hard for the United States and, unfortunately, just as hard for Russia. In Bradbury's account, Fuchs bitterly hated the Germans and believed America was not doing enough to help the Russians defeat them. Bradbury concluded: "He suffered from a double loyalty."

  Bethe's judgment of the scientist was warm. He called Fuchs "one of the most valuable men in my division" and "one of the best theoretical physicists we had."

  Bradbury replaced Robert Oppenheimer as director in October 1945 and asked Fuchs to stay on into 1946. Fuchs helped prepare the Operation Crossroads weapons tests. The US Atomic Energy Act of 1946, known as the McMahon Act, barred sharing nuclear research with any foreign country without explicit official authority, Britain included. Fuchs nonetheless supplied highly classified American information both to nuclear scientists in Britain and to his Soviet contacts.

  British officials rated him highly and wanted him home for Britain's postwar weapons programme. In August 1946 he returned and took charge of the Theoretical Physics Division at the Atomic Energy Research Establishment at Harwell.

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• 6. Confession and Trial

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  By September 1949, the American Venona project had given GCHQ evidence that Fuchs was a spy. British intelligence was wary of revealing where the information came from. The Soviets had already broken off contact with him in February. Fuchs may have been tipped off by Kim Philby.

  Frank Close, in his 2019 biography Trinity, concluded that MI5 had suspected Fuchs for over two years. According to a review of the book, it was GCHQ decrypters who supplied clear proof of his guilt, not the American team that usually gets the credit.

  In October 1949 Fuchs told Henry Arnold, the head of security at Harwell, that his father had been given a chair at the University of Leipzig in East Germany. That news became a factor in what followed. In early January 1950 Fuchs was told he must resign from Harwell because of his father's appointment, and he was offered help finding a university post.

  MI5 officer William Skardon questioned him at an informal meeting in December 1949. Fuchs denied being a spy and was not detained. The author Nancy Thorndike Greenspan reports that Skardon told him an admission could let him keep working at Harwell.

  At their fourth meeting, on the 24th of January 1950, Fuchs confessed of his own accord. Greenspan notes that Skardon's report left out his promise about staying at Harwell. A debriefing agreed to keep Fuchs "in his present state of mind". So that he could be prosecuted, Fuchs wrote a signed statement at the War Office in London. It included the line "I was given the chance of admitting it and staying at Harwell or clearing out."

  Three days later he gave a more technical statement to Michael Perrin, deputy controller of atomic energy at the Ministry of Supply. He said the NKGB had an agent in Berkeley, California, who had told the Soviet Union about electromagnetic separation of uranium-235 in 1942 or earlier. He said he had last handed over information in February or March 1949. His statements to British and American agencies were used to implicate Harry Gold, a key witness in the trials of David Greenglass and Julius and Ethel Rosenberg.

  Fuchs was arrested on the 2nd of February 1950 and charged under the Official Secrets Act. Sir Hartley Shawcross prosecuted him. On the 1st of March 1950 he was convicted of four counts of "communicating information to a potential enemy." Fuchs pleaded guilty. His barrister, Derek Curtis-Bennett, confined the defence to a plea in mitigation based on his state of mind and his wish to help the Soviets defeat the Nazis.

  Fuchs accepted advice not to raise the question of inducement. The trial lasted less than 90 minutes. Lord Goddard sentenced him to fourteen years, the maximum for espionage. The judge reasoned that the crime could not count as treason, a capital offence, because the Soviet Union was then classed as an ally. On the 21st of February 1951 Fuchs was formally stripped of his British citizenship.

  In 1952 Sir William Penney, head of the British H-bomb project, visited him in prison. Fuchs also played chess with Seamus Murphy, an Irish Republican Army prisoner, and helped Murphy escape. Fuchs served at Wakefield Prison.

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• 7. What Moscow Received

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  After his arrest Fuchs sat down with Michael Perrin and listed what he had given the Soviets. Implosion came first. He passed on all he knew about the theory, dimensions and components of the implosion bomb, and why it was being pursued. The reason was that plutonium bred in a reactor could not be used in a gun-type design, a fact that had caught the United States by surprise in mid-1944.

  His disclosures included a sketch of the bomb with its important dimensions marked. They also covered the "urchin" neutron source, or initiator, of the plutonium bomb, on which he was credited as a key inventor.

  He described the gaseous diffusion process for enriching uranium. He gave the critical masses of plutonium-239 and uranium-235, which had taken the United States considerable time to settle. He added metallurgical details of the plutonium allotrope used in the final weapon and how gallium stabilises it.

  In June 1945 he told his contacts that the Americans planned to test an atomic bomb that July, said where, and warned that it was meant for use against Japan. He also supplied approximate American production rates for plutonium-239 and uranium-235, and the amounts of material in each bomb type. From those figures the Soviets could estimate the size and growth of the US stockpile, a key early Cold War secret. He even described ideas for future fission weapons, including composite cores that stretch fissile material further.

  On the hydrogen bomb, Fuchs called the American picture "confused". In April 1946 he attended a Los Alamos conference on a thermonuclear weapon. A month later he filed a patent with John von Neumann for a design the two had developed together. It was not viable, but it was the first time radiation implosion appeared in a weapon design.

  He passed along Enrico Fermi's autumn 1945 lectures on the physics of the "Super", the then-secret inverse Compton effect, and the basic idea of Teller's "Runaway Super". All of this reached the Soviets in 1947, after Fuchs had left the United States and only when his contact asked for it directly. From late 1947 to May 1949 he met his case officer, Alexander Feklisov, six times. He also gave results from the Eniwetok Atoll tests and key data on uranium-235 production.

  How much did it matter? Bethe once said Fuchs was the only physicist he knew to have truly changed history. Scholars struggle to measure the effect, because the Soviet programme's pace was set mainly by how much uranium it could obtain. Before the Soviet archives opened in the early 1990s, analysts assumed Moscow had applied technical intelligence as directly as possible.

  Later work showed that Lavrenti Beria, head of the Soviet atomic project, trusted neither his spies nor his own scientists. He used intelligence as a third-party check and guide instead. Most scientists on the bomb project did not know they were using any intelligence at all, and the Soviets often redid the research themselves. Their programme also explored paths the Manhattan Project never took, including the successful gas centrifuge method of enrichment.

  Fuchs told Perrin he thought he had saved the Soviets "several years", and "one year at least". His reasoning was that they could begin developing the explosive before the fissionable material was ready. He said he gave them nothing that would speed up plutonium production. He was "extremely surprised" by how fast the Soviets built their own bomb, since he doubted they had the engineering and construction facilities for large production plants so quickly.

  His information was much more extensive and technically precise than anything from later-discovered atomic spies such as David Greenglass or Theodore Hall. Post-Cold War declassified information says the Russians freely acknowledged that Fuchs gave them the fission bomb.

  The hydrogen bomb question is still debated. Most scholars accept Bethe's 1952 assessment that, by the time Fuchs left the thermonuclear programme in mid-1946, too little was known about the mechanism for his information to be useful. His work belonged to the original, mistaken "Runaway Super" idea, which was not abandoned until early 1950. The United States found its successful Teller-Ulam design only in early 1951, so neither Fuchs nor anyone else knew the secret before his arrest.

  The Soviets also chased the "Runaway Super" and found it a dead end. They reinvented the Teller-Ulam principle independently, by a different route. The Soviet physicist German Goncharov has noted that Fuchs's early work contained the seed of radiation implosion. In retrospect it was closer to the right approach than anyone recognised. Fuchs's work was a "precursor", recognisable as one only after the fact.

  The revelation also cost the Western allies. Before it, the United States and the United Kingdom had planned fuller nuclear cooperation, which had been on hold since the 1946 Atomic Energy Act. The historians Richard G. Hewlett and Francis Duncan wrote: "Whatever hopes had existed for a tightly-integrated programme with the British and Canadians died with the Fuchs revelation." The Anglo-American "Special Relationship" was repaired only by the mid-1950s.

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• 8. Second Career

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  On the 23rd of June 1959, Fuchs was released from Wakefield Prison after nine years and four months. In England, long-term prisoners were then entitled by law to one third off for good behaviour. He promptly emigrated to the German Democratic Republic.

  At Berlin Schönefeld Airport he was met by Margarete "Grete" Keilson, a friend from his student communist days. They married on the 9th of September 1959.

  In East Germany he resumed his scientific career and became a prominent research leader. He joined the central committee of the Socialist Unity Party in 1967. In 1972 he was elected to the Academy of Sciences, where from 1974 to 1978 he headed the research area of physics, nuclear and materials science. He then became deputy director of the Central Institute for Nuclear Physics in Rossendorf, Dresden, and retired in 1979.

  His working life continued after that. From 1984 he headed the scientific councils for energetic basic research and for fundamentals of microelectronics. His honours included the Patriotic Order of Merit, the Order of Karl Marx and the National Prize of East Germany.

  One claim about his later influence remains disputed. In The Nuclear Express (2009), Thomas Reed and Daniel Stillman argue that a tutorial Fuchs gave to Qian Sanqiang and other Chinese physicists helped them build the first Chinese atomic bomb, the 596. It was tested five years later. Three historians of nuclear weapons, Robert S. Norris, Jeremy Bernstein and Peter D. Zimmerman, called the assertion "unsubstantiated conjecture". They described the book as "an ambitious but deeply flawed book".

  Fuchs died in East Berlin on the 28th of January 1988. He was cremated and honoured with burial in the Pergolenweg Ehrengrab section of Berlin's Friedrichsfelde Cemetery.

  He has remained a screen subject. Denis Forest played him in the 1987 television miniseries Race for the Bomb. A documentary, Väter der tausend Sonnen, meaning Fathers of a Thousand Suns, followed in 1990. Richard Rhodes's 1995 book Dark Sun includes a clerihew about him. Christopher Denham played him in the 2023 film Oppenheimer. In 2022 Fuchs was the primary focus of the second season of the BBC World Service podcast The Bomb.

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