William D. Phillips


William Phillips, an atomic physicist, shared the 1997 Nobel Prize in physics with Steven Chu and Claude Cohen-Tannoudji “for development of methods to cool and trap atoms with laser light.” William Phillips first pursued laser cooling for fun after reading a paper in 1978 describing an idea for slowing down an atomic beam with a laser, and then trapping the atoms. Using neutral sodium atoms with laser beams he quickly realized a limitation to his initial cooling method. Atoms have many nearby energy levels (hyperfine structure). The motion of the atoms causes them to see a frequency shifted laser (Doppler shift) resulting in electrons going to a non-desired energy level, which would not allow further cooling. He solved this problem using a magnetic field to separate the hyperfine structure (Zeeman splitting) and cool the atoms further. This method became one of the standards of laser cooling and is called Zeeman cooling. By 1983 his group was one of two groups in the world to have published a paper on experimental laser cooling of neutral atoms. By 1985 they had slowed atoms down enough for trapping. Phillips’ group continued to work in laser cooling and found sub-Doppler temperatures. He collaborated with Chu and Cohen-Tannoudji, as well as others who helped move the field forward. Laser cooling eventually led to many applications including Bose Einstein condensates.