In 1988, The NIST group in Washington led by William Phillips first measured temperatures below the Doppler limit in sodium atoms in an optical molasses, prompting the search for the theoretical underpinnings of sub-Doppler cooling. The next year, Jean Dalibard and Claude Cohen-Tannoudji identified the cause as the multi-photon process of Sisyphus cooling, and Steven Chu 's group likewise modeled sub-Doppler cooling as fundamentally an optical pumping scheme. As a result of their efforts, Phillips, Cohen-Tannoudji, and Chu jointly won the 1997 Nobel Prize in Physics. T.W. Hänsch, et al., first outlined the theoretical formulation of gray molasses in 1994, and a four-beam experimental realization in cesium was achieved by G. Grynberg the next year. It has since been regularly used to cool all the other alkali (hydrogenic) metals.