Laser Cooling
Ever since the time of Kepler, and perhaps even earlier, there were ideas of radiation pressure. Optical forces (light pressure) were derived by Maxwell who found that the force on an object absorbing P watts of light is F = P/c. This result survived intact through the development of both relativity and quantum optics because E=pc relativistically (p is momentum). Light pressure was studied in the 1908 Ph.D. thesis of Peter Debye on comet tails.
By now the fundamental principles of laser cooling are widely understood. A good review is found in the Nobel Lecture of Bill Phillips in Reviews of Modern Physics in 1998, and in the Springer book by Harold Metcalf and Peter van der Straten (1999). Optical forces on atoms irradiated with a single frequency of light have been extensively studied for many years, both theoretically and experimentally. The two-level atom model has been used to describe a wide range of optical force phenomena and to exploit successfully a large range of applications.
New areas of study were opened up as a result of unexpected experimental discoveries that revealed cooling below the "Doppler limit". This limit arises from the two-level atom model, but it is an oversimplified view of the interactions between atoms and light and could not explain these new discoveries. Thus the multiple levels of real atoms had to be considered. The interval between the observations and the theoretical description was indeed an exciting time, but a consistent theory eventually emerged. That led to our discovery of an additional type of sub-Doppler cooling that we called MILC.