Before the first successful test of creating a chain reaction with uranium Edward Teller and a few other physicists had already conceived of a way to take a still theoretical atomic bomb and use it to trigger a new weapon they called a ‘Super’ which would fuse light weight atoms into a heavier stable atom which would release even more energy than an atomic bomb. In 1942 the idea for the hydrogen bomb was already being worked on.

(Book – The Making of the Atomic Bomb – Richard Rhodes)