= Alfred Kastler
{c}
{tag=Physicist}
{title2=1902–1984}
{wiki}
= A. Kastler
{c}
{synonym}
<Alfred Kastler> developed <optical pumping>, using selective <absorption> and <emission> of <light> to redistribute populations among atomic <Zeeman sublevels>. <Angular momentum> <selection rules> make some transitions accessible and others forbidden, so repeated illumination can produce a strongly unequal population. A subsequent <magnetic resonance> changes that distribution and therefore the detected <light>. Optical preparation and optical readout connect a weak <radio frequency> excitation with a sensitive optical signal.<Optical methods for studying Hertzian resonances by Alfred Kastler>[Nobel lecture: population redistribution, optical pumping and optical detection of resonance]{ref}
A <physicist>.https://www.lkb.fr/en/laboratory/presentation/history/alfred-kastler-2/[Independent professional profile]{ref}
\Image[https://commons.wikimedia.org/wiki/Special:Redirect/file/Alfred%20Kastler%20French%20physicist.jpg?width=250]
{title=Portrait of <Alfred Kastler>}
{description=Unknown photographer; scan from Problemy, April 1967, p. 221. Public domain per Commons PD-Poland: qualifying Polish publication without clear copyright notice before 23 May 1994; File also states U.S. non-restoration conditions. https://commons.wikimedia.org/wiki/File:Alfred_Kastler_French_physicist.jpg[Source usage terms], via Wikimedia Commons.}
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{width=250}
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