The Teller–Ulam design is a nuclear weapon design which is used in megaton-range thermonuclear weapons, and is more colloquially referred to as "the secret of the hydrogen bomb". It is named after two of its chief contributors, Hungarian-born physicist Edward Teller and Polish-born mathematician Stanisław Ulam, who developed the design in 1951. The idea is thought to pertain specifically to the use of a fission bomb "trigger" placed near an amount of fusion fuel, known as "staging", and the use of "radiation implosion" to compress the fusion fuel before igniting it. There are a number of other additions and variations to this idea posited by different sources.
The first device to be based on this principle was detonated by the United States in the "Ivy Mike" nuclear test in 1952. In the Soviet Union, this design was known as Andrei Sakharov's "Third Idea". Similar devices were developed by the United Kingdom, France and China though no specific code names are known for their designs. The most powerful thermonuclear device ever tested was the circa 50 megaton Soviet Tsar Bomba test.