Laser cooling and trapping of neutral atoms*

In 1978, while I was a postdoctoral fellow at MIT, I read a paper by Art Ashkin (1978) in which he described how one might slow down an atomic beam of sodium using the radiation pressure of a laser beam tuned to an atomic resonance. After being slowed, the atoms would be captured in a trap consisting of focused laser beams, with the atomic motion being damped until the temperature of the atoms reached the microkelvin range. That paper was my first introduction to laser cooling, although the idea of laser cooling (the reduction of random thermal velocities using radiative forces) had been proposed three years earlier in independent papers by Hänsch and Schawlow (1975) and Wineland and Dehmelt (1975). Although the treatment in Ashkin’s paper was necessarily over-simplified, it provided one of the important inspirations for what I tried to accomplish for about the next decade. Another inspiration appeared later that same year: Wineland, Drullinger and Walls (1978) published the first laser cooling experiment, in which they cooled a cloud of Mg ions held in a Penning trap. At essentially the same time, Neuhauser, Hohenstatt, Toschek and Dehmelt (1978) also reported laser cooling of trapped Ba ions. Those laser cooling experiments of 1978 were a dramatic demonstration of the mechanical effects of light, but such effects have a much longer history. The understanding that electromagnetic radiation exerts a force became quantitative only with Maxwell’s theory of electromagnetism, even though such a force had been conjectured much earlier, partly in response to the observation that comet tails point away from the sun. It was not until the turn of the century, however, that experiments by Lebedev (1901) and Nichols and Hull (1901, 1903) gave a laboratory demonstration and quantitative measurement of radiation pressure on macroscopic objects. In 1933 Frisch made the first demonstration of light pressure on atoms, deflecting an atomic sodium beam with resonance radiation from a lamp. With the advent of the laser, Ashkin (1970) recognized the potential of intense, narrow-band light for manipulating atoms and in 1972 the first ‘‘modern’’ experiments demonstrated the deflection of atomic beams with lasers (Picqué and Vialle, 1972; Schieder et al., 1972). All of this set the