The manipulation of neutral particles* Steven Chu Stanford University, Departments of Physics and Applied Physics, Stanford, California 94305-4060 [S0034-6861(98)01003-4] The written version of my lecture is a personal ac- sey, to head the Quantum Electronics Research Depart- count of the development of laser cooling and trapping. ment at the Holmdel branch in the fall of 1983. During Rather than give a balanced history of the field, I chose conversations with Art Ashkin, an office neighbor at to present a personal glimpse of how my colleagues and Holmdel, I began to learn about his dream to trap atoms I created our path of research. with light. He found an increasingly attentive listener I joined Bell Laboratories in the fall of 1978 after and began to feed me copies of his reprints. That fall I working with Eugene Commins as a graduate student was also joined by my new post-doc, Leo Hollberg. and post-doc at Berkeley on a parity nonconservation When I hired him, I was planning to construct an elec- experiment in atomic physics (Conti et al., 1979; see also tron energy-loss spectrometer based on threshold ion- Chu, Commins, and Conti, 1977). Bell Labs was a re- ization of a beam of atoms with a picosecond laser. We searcher’s paradise. Our management supplied us with hoped to improve the energy resolution compared to funding, shielded us from bureaucracy, and urged us to existing spectrometers by at least an order of magnitude do the best science possible. The cramped labs and of- and then use our spectrometer to study molecular adsor- fice cubicles forced us to rub shoulders with each other. bates on surfaces with optical resolution and electron Animated discussions frequently interrupted seminars, sensitivity. However, Leo was trained as an atomic and casual conversations in the cafeteria would some- physicist and was also developing an interest in the pos- times mark the beginning of a new collaboration. sibility of manipulating atoms with light. In my first years at Bell Labs, I wrote an internal Leo and I spontaneously decided to drive to Massa- memo on the prospects for x-ray microscopy and chusetts to attend a workshop on the trapping of ions worked on an experiment investigating energy transfer and atoms organized by David Pritchard at MIT. I was in ruby with Hyatt Gibbs and Sam McCall as a means of ignorant of the subject and lacked the primitive intuition studying Anderson Localization (Chu, Gibbs, McCall, that is essential to add something new to a field. As an and Passner, 1980; Chu, Gibbs, and McCall, 1981). This example of my profound lack of understanding, I found work led us to consider the possibility of Mott or Ander- myself wondering about the dispersive nature of the ‘‘di- son transitions in other exciton systems such as GaP:N pole force.’’ The force is attractive when the frequency with picosecond laser techniques (Hu, Chu, and Gibbs, of light is tuned below the resonance, repulsive when 1980). During this work, I accidentally discovered that tuned above the resonance, and vanishes when tuned picosecond pulses propagate with the group velocity, directly on the atomic resonance. Looking back on these even when the velocity exceeds the speed of light or early fumblings, I am embarrassed by how long it took becomes negative (Chu and Wong, 1982). me to recognize that the effect can be explained by While I was learning about excitons and how to build freshman physics. On the other hand, I was not alone in picosecond lasers, I began to work with Allan Mills, the my lack of intuition. When I asked a Bell Labs colleague world’s expert on positrons and positronium. We began about this effect, he answered, ‘‘Only Jim Gordon really to discuss the possibility of working together while I was understands the dipole force!’’ still at Berkeley, but did not actually begin the experi- By 1980, the forces that light could exert on matter 1 ment until 1979. After three long and sometimes frus- were well understood. Maxwell’s calculation of the mo- trating years, a long time by Bell Labs standards, we mentum flux density of light (Maxwell, 1897) and the finally succeeded in exciting and measuring the 1S-2S laboratory observation of light pressure on macroscopic energy interval in positronium. (Chu and Mills, 1982; objects by Lebedev (1901) and by Nichols and Hull Chu, Mills, and Hall, 1984). (1903) provided the first quantitative understanding of how light could exert forces on material objects. Ein- MOVING TO HOLMDEL AND WARMING UP TO LASER stein (1917) pointed out the quantum nature of this COOLING force: an atom that absorbs a photon of energy hn will receive a momentum impulse hn/c along the direction My entry into the field of laser cooling and trapping of the incoming photon pin . If the atom emits a photon was stimulated by my move from Murray Hill, New Jer- with momentum pout , the atom will recoil in the oppo- site direction. Thus the atom experiences a net momen- tum change Dpatom5pin– pout due to this incoherent scat- *The 1997 Nobel Prize in Physics was shared by Steven Chu, Claude N. Cohen-Tannoudji, and William D. Phillips. This lec- ture is the text of Professor Chu’s address on the occasion of 1A more complete account of this early history can be found the award. in Minogin and Letokhov, 1987. Reviews of Modern Physics, Vol. 70, No. 3, July 1998 0034-6861/98/70(3)/685(22)/$19.40 © 1998 The Nobel Foundation 685 686 Steven Chu: Manipulation of neutral particles FIG. 2. A schematic diagram of the first particle trap used by Ashkin. Confinement in the axial direction is due to an imbal- ance of the scattering forces between the left and right propa- gating beams. Confinement in the radial direction results from the induced dipole force, which must overcome the outwardly directed scattering force. This reaction force is also called the ‘‘dipole force.’’ The oscillating electric field E of the light induces a di- FIG. 1. A photograph of a 10-mm glass sphere trapped in wa- pole moment p on the particle. If the induced dipole ter with green light from an argon laser coming from above. moment is in phase with E, the interaction energy The picture is a fluorescence image taken using a green- 2p•E is lower in high-field regions. If the induced di- blocking, red-transmitting filter. The exiting (refracted) rays pole moment is out of phase with the driving field, the show a notable decrease in beam angles relative to the incident particle’s energy is increased in the electric field and the rays. The increased forward momentum of the light results in particle will feel a force ejecting it out of the field. If we an upward force on the glass bead needed to balance the model the atom or particle as a damped harmonic oscil- downward scattering force. The stria in the forward-scattered lator, the sign change of the dipole force is easy to un- light is a common Mie-scattering ring pattern (courtesy A. derstand. An oscillator driven below its natural resonant Ashkin). frequency responds in phase with the driving field, while an oscillator driven above its natural frequency oscillates tering process. In 1930, Frisch (1933) observed the out of phase with the driving force. Exactly on reso- deflection of an atomic beam with light from a sodium nance, the oscillator is 90 degrees out of phase and 5 resonance lamp where the average change in momen- p•E 0. The dipole force was first discussed by Askar’yan tum was due to the scattering of one photon. (1962) in connection with plasmas as well as neutral at- Since the scattered photon has no preferred direction, oms. The possibility of trapping atoms with this force the net effect is due to the absorbed photons, resulting was considered by Letokhov (1968) who suggested that in scattering force, Fscatt5Npin , where N is the number atoms might be confined along one dimension in the of photons scattered per second. Typical scattering rates nodes or antinodes of a standing wave of light tuned far for atoms excited by a laser tuned to a strong resonance from an atomic transition. In 1970, Arthur Ashkin had line are on the order of 107 to 108/sec. As an example, succeeded in trapping micron-sized particles with a pair the velocity of a sodium atom changes by 3 cm/sec per of opposing, focused beams of laser light, as shown in absorbed photon. The scattering force can be 105 times Fig. 2. Confinement along the axial direction was due to the gravitation acceleration on Earth, feeble compared the scattering force: a displacement towards either of the to electromagnetic forces on charged particles, but focal points of the light would result in an imbalance of stronger than any other long-range force that affects scattered light that would push the particle back to the neutral particles. center of the trap. Along the radial direction, the out- There is another type of force based on the lensing wardly directed scattering force could be overcome by (i.e., coherent scattering) of photons. A lens alters the the attractive dipole force. In the following years, other distribution of momentum of a light field, and by New- stable particle-trapping geometries were demonstrated ton’s third law, the lens must experience a reaction force by Ashkin (1980), and in 1978 he proposed the first equal and opposite to the rate of momentum change on three-dimensional traps for atoms (Ashkin, 1978).