Born in Zurich, Switzerland and educated at the Eidgenössische Technische Hochschule (ETH) there, Bretscher gained a PhD degree in organic chemistry at Edinburgh in 1926.

[6] He returned to Zurich as privat docent to Peter Debye, later moving in 1936 to work in Rutherford’s laboratory at the Cavendish in Cambridge as a Rockefeller Scholar.

Bretscher used to joke that his main contribution to physics occurred in the summer of 1930, when he was climbing in the Bergell region near Engadin with another student, Felix Bloch, in the Swiss Alps.

In 1944 he became a part of the British Mission to the Manhattan Project in Los Alamos, New Mexico led by James Chadwick, where he made the first measurements on the energy released in fusion processes.

His pictures, which are now held by the Churchill Archives Centre, include photographs of Enrico Fermi, Edward Teller and the Trinity site in New Mexico after the first atomic bomb was detonated, showing the surface light brown sand turned to a green-blue glass.