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ChemPhysChem

When Atoms Behave as Waves: Bose–Einstein Condensation and the Atom Laser (Nobel Lecture)

Wolfgang Ketterle

Published Sep 16, 2002• 30 citations
Publisher

Abstract

Atom plus atom equals…︁ vacuum! The destructive interference of atoms is just one of the surprising effects of coherent atoms. The long and winding road from trapping atoms to the atomic laser, passing the peak of the Bose – Einstein condensate, won Ketterle a share in the 2001 Nobel Prize for Physics. The graphic shows pulses of coherent sodium atoms (each containing 105 – 106 atoms produced every 5 ms) falling under gravity and expanding due to mutual interatomic repulsion.

DOI
10.1002/1439-7641(20020916)3:9<736::aid-cphc736>3.0.co;2-u

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Atom plus atom equals…︁ vacuum! The destructive interference of atoms is just one of the surprising effects of coherent atoms. The long and winding road from trapping atoms to the atomic laser, passing the peak of the Bose – Einstein condensate, won Ketterle a share in the 2001 Nobel Prize for Physics. The graphic shows pulses of coherent sodium atoms (each containing 105 – 106 atoms produced every 5 ms) falling under gravity and expanding due to mutual interatomic repulsion.
When Wolfgang Ketterle and his colleagues at MIT demonstrated an “atom laser” recently, the announcement excited the physics community and naturally grabbed the attention of the popular press. One essential property of a laser is, of course, its coherence, and the evidence provided by the MIT group is striking: Sharp interference fringes formed when two Bose–Einstein condensates from the same trap expanded and overlapped after being released from confinement. The fringes are reminiscent of the interference between coherent light waves emanating from a double slit. (See the figure below.)
The experimental observation of Bose–Einstein condensation (BEC) in rubidium in 1995 demonstrated that a macroscopic number of bosons could be produced in a single quantum state of trapped atoms. The occupation of a single quantum state by a large number of identical bosons is the matter-wave analog of the storage of photons in a single mode of a laser cavity. In a conventional laser, one extracts a coherent beam of photons from a cavity by using a partially transmitting mirror as an output coupler. In 1997, Wolfgang Ketterle and his collaborators at MIT built a pulsed output coupler that extracted matter waves from a condensate, and they observed interference between atoms from two separate condensates, thereby demonstrating an atom laser for the first time (see PHYSICS TODAY, March 1997, page 17).
Bose–Einstein condensates of particles behave in many ways like coherent radiation fields, and the realization of Bose–Einstein condensation in dilute gases has opened up the experimental study of many aspects of interactions between coherent matter waves. In addition, the existence of these dilute trapped quantum gases has prompted a re-examination of a number of theoretical issues. This field is a vast one, and in this chapter we shall touch briefly on selected topics. In Sec. 13.1 we describe the classic interference experiment, in which two clouds of atoms are allowed to expand and overlap. Rather surprisingly, an interference pattern is produced even though initially the two clouds are completely isolated. We shall analyse the reasons for this effect. The marked decrease in density fluctuations in a Bose gas when it undergoes Bose–Einstein condensation is demonstrated in Sec. 13.2. Gaseous Bose–Einstein condensates can be manipulated by lasers, and this has made possible the study of coherent matter wave optics. We describe applications of these techniques to observe solitons, Bragg scattering, and non-linear mixing of matter waves in Sec. 13.3. The atom laser and amplification of matter waves is taken up in Sec. 13.4. How to characterize Bose–Einstein condensation microscopically is the subject of Sec. 13.5, where we also consider fragmented condensates. Interference of two condensates One of the striking manifestations of the wave nature of Bose–Einstein condensates is the observation of an interference pattern when two condensed and initially separated clouds are allowed to overlap. An example is shown in Fig. 13.1.
WHEN the 2001 Nobel Prize for Physics was awarded to Eric Cornell and Carl Wieman from JILA in Boulder, Colorado, and to Wolfgang Ketterle from the Massachusetts Institute of Technology, they were recognized for creating and studying Bose–Einstein condensates – dilute gases of atoms that are all in the same quantum state. Their remarkable achievement enabled "quantum gases" to be studied experimentally for the first time, and jump-started new sub-fields of research, including gaseous superfluidity, atom lasers and matter–wave optics.