Photo from the Nobel Foundation archive.
Born: 3 May 1902, Guebwiller, Germany (now France)
Died: 7 January 1984, Bandol, France
Affiliation at the time of the award: École Normale Supérieure, Paris, France
Prize motivation: "for the discovery and development of optical methods for studying Hertzian resonances in atoms."
Prize share: 1/1
Electrons in atoms and molecules have fixed energy levels, according to the principles of quantum physics. When there are transitions among different energy levels, light with certain frequencies is emitted or absorbed. In 1950 Alfred Kastler presented the idea that electrons, with the help of light or other electromagnetic radiation, can be pumped up to fixed higher energy levels and then fall back to fixed lower levels. This made precise determination of energy levels possible and also was of fundamental importance for development of the laser.