Garwin and the Mike shot

Richard Garwin designed the first H-Bomb, based on the Teller-Ulam mechanism, while still in his early twenties. See also One hundred thousand brains.

From Kenneth Ford's Building the H-Bomb: A Personal History:

... In 1951 Dick Garwin came for his second summer to Los Alamos. He was then twenty-three and two years past his Ph.D.* Edward Teller, having interacted with Garwin at the University of Chicago, knew him to be an extraordinarily gifted experimental physicist as well as a very talented theorist. He knew, too, that Fermi had called Garwin the best graduate student he ever had. [5] So when Garwin came to Teller shortly after arriving in Los Alamos that summer (probably in June 1951) asking him “what was new,” [6] Teller was ready to pounce. He referred Garwin to the Teller-Ulam report of that March and then asked him to “devise an experiment that would be absolutely persuasive that this would really work.” Garwin set about doing exactly that and in a report dated July 25, 1951, titled “Some Preliminary Indications of the Shape and Construction of a Sausage, Based on Ideas Prevailing in July 1951,”[7] he laid out a design with full specifics of size, shape, and composition, for what would be the Mike shot fired the next year. ...

Wikipedia: Ivy Mike was the codename given to the first test of a full-scale thermonuclear device, in which part of the explosive yield comes from nuclear fusion. It was detonated on November 1, 1952 by the United States on Enewetak, an atoll in the Pacific Ocean, as part of Operation Ivy. The device was the first full test of the Teller-Ulam design, a staged fusion bomb, and was the first successful test of a hydrogen bomb. ...