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¹ 2002 WILEY-VCH Verlag Gmbh & Co. Kgaa, Weinheim 1439-4235/02 736 ¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1439-4235/02/03/09 $ 20.00+.50/0 CHEMPHYSCHEM 2002, 3, 736 ± 753 When Atoms Behave as Waves: Bose ± Einstein Condensation and the Atom Laser (Nobel Lecture) Wolfgang Ketterle*[a] KEYWORDS: alkali elements ¥ Bose ± Einstein condensates ¥ cooling ¥ low temperature physics ¥ quantum optics The lure of lower temperatures has attracted physicists for the past century, and with each advance towards absolute zero, new and rich physics has emerged. Laypeople may wonder why ™freezing cold∫ is not cold enough. But imagine how many aspects of nature we would miss if we lived on the surface of the sun. Without inventing refrigerators, we would only know gaseous matter and never observe liquids or solids, and miss the beauty of snowflakes. Cooling to normal earthly temper- atures reveals these dramatically different states of matter, but this is only the beginning: Many more states appear with further cooling. The approach into the kelvin range was rewarded with the discovery of superconductivity in 1911 and of superfluidity in Figure 1. Annual number of published papers, which have the words ™Bose∫ and helium-4 in 1938. Cooling into the millikelvin regime revealed ™Einstein∫ in their title, abstracts, or keywords. The data were obtained by the superfluidity of helium-3 in 1972. The advent of laser cooling searching the ISI (Institute for Scientific Information) database. in the 1980s opened up a new approach to ultralow temperature physics. Microkelvin samples of dilute atom clouds were and with mass m can be regarded as quantum-mechanical generated and used for precision measurements and studies wavepackets that have a spatial extent on the order of a thermal of ultracold collisions. Nanokelvin temperatures were necessary de Broglie wavelength l (2ph2/mk T)1/2. The value of l is to explore quantum-degenerate gases, such as Bose ± Einstein dB B dB the positional uncertainty associated with the thermal momen- condensates first realized in 1995. Each of these achievements in tum distribution and increases with decreasing temperature. cooling has been a major advance, and recognized with a Nobel When atoms are cooled to the point where l is comparable to prize. dB the interatomic separation, the atomic wavepackets ™overlap∫ This paper describes the discovery and study of Bose ± Einstein and the gas starts to become a ™quantum soup∫ of indistin- condensation (BEC) in atomic gases from my personal perspec- guishable particles. Bosonic atoms undergo a quantum-me- tive. Since 1995, this field has grown explosively, drawing chanical phase transition and form a Bose ± Einstein condensate researchers from the communities of atomic physics, quantum (Figure 2), a cloud of atoms all occupying the same quantum optics, and condensed matter physics. The trapped ultracold mechanical state at a precise temperature (which, for an ideal vapor has emerged as a new quantum system that is unique in gas, is related to the peak atomic density n by nl3 2.612). If the precision and flexibility with which it can be controlled and dB the atoms are fermions, cooling gradually brings the gas closer manipulated. At least thirty groups have now created conden- to being a ™Fermi sea∫ in which exactly one atom occupies each sates, and the publication rate on Bose ± Einstein condensation low-energy state. has soared following the discovery of the gaseous condensates Creating BEC is thus simple in principle: Make a gas extremely in 1995 (Figure 1).[*] cold until the atomic wave packets start to overlap! However, in The phenomenon of Bose ± Einstein condensation was pre- most cases quantum degeneracy would simply be pre-empted dicted long ago, in a 1925 paper by Albert Einstein[1] using a by the more familiar transitions to a liquid or solid. This more method introduced by Satyendra Nath Bose to derive the conventional condensation into a liquid and solid can only be blackbody spectrum.[2] When a gas of bosonic atoms is cooled below a critical temperature Tc , a large fraction of the atoms condenses in the lowest quantum state. Atoms at temperature T [a] Prof. W. Ketterle Department of Physics MIT± Harvard Center for Ultracold Atoms and Research Laboratory of Electronics [*] This paper is my Nobel Lecture and takes a personal look at the development Massachusetts Institute of Technology of Bose ± Einstein condensation. The 2001 Nobel Prize for Physics was shared Cambridge, MA 02139 (USA) with Carl E. Wieman and Eric A. Cornell, and their Lecture was published http://cua.mit.edu/ketterle_group/ earlier (E. A. Cornell, C. E. Wieman, ChemPhysChem 2002, 3, 476). E-mail: [email protected] CHEMPHYSCHEM 2002, 3, 736 ± 753 ¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1439-4235/02/03/09 $ 20.00+.50/0 737 W. Ketterle (which is proportional to the inverse density squared) is stretched to seconds or minutes. Since the rate of binary elastic collisions drops only proportionally to the density, these collisions are much more frequent. Therefore, thermal equilibrium of the translational degree of freedom of the atomic gas is reached much faster than chemical equilibrium, and quantum degeneracy can be achieved in an effectively metastable gas phase. However, such ultralow density lowers the temperature requirement for quantum degeneracy into the nano- to microkelvin range. The achievement of Bose ± Einstein condensation required first the identification of an atomic system which would stay gaseous all the way to the BEC transition and, second, the development of cooling and trapping techniques to reach the required regime of temperature and density. Even around 1990, it was not certain that nature would provide us with such a system. Indeed, many people doubted that BEC could ever Figure 2. Criterion for Bose ± Einstein condensation. At high temperatures, a be achieved, and it was regarded as an elusive goal. Many weakly interacting gas can be treated as a system of ™billiard balls∫. In a simplified believed that pursuing BEC would result in new and interesting quantum description, the atoms can be regarded as wavepackets with an physics, but whenever one would come close, some new l extension of their de Broglie wavelength dB . At the BEC transition temperature, l phenomenon or technical limitation would show up. A news dB becomes comparable to the distance between atoms and a Bose condensate forms. As the temperature approaches zero, the thermal cloud disappears, article in 1994 quoted Steve Chu: ™I am betting on nature to hide leaving a pure Bose condensate. Bose condensation from us. The last 15 years she's been doing a great job.∫[3] In brief, the conditions for BEC in alkali gases are reached by avoided at extremely low densities, about a hundred thousandth combining two cooling methods. Laser cooling is used to of the density of normal air. Under those conditions, the precool the gas. The principle of laser cooling is that scattered formation time of molecules or clusters by three-body collisions photons are on average blue-shifted with respect to the incident laser beam. As a result, the scattered light carries away more Wolfgang Ketterle, born October 21, energy than has been absorbed by the atoms, resulting in net 1957 in Heidelberg, Germany, was one cooling. Blue shifts are caused by Doppler shifts or ac Stark shifts. of three scientists jointly awarded the The different laser cooling schemes are described in the 1997 [4±6] 2001 Nobel Prize in Physics. He un- Nobel Lectures in Physics. After the precooling, the atoms are dertook undergraduate studies at the cold enough to be confined in a magnetic trap. Wall-free University of Heidelberg and post- confinement is necessary, otherwise the atoms would stick to the graduate work at the Max Planck surface of the container. It is noteworthy that similar magnetic Institute for Quantum Optics in confinement is also used for plasmas, which are too hot for any Garching. He had postdoc positions material container. After magnetically trapping the atoms, forced [7±9] at both of the above institutions then, evaporative cooling is applied as the second cooling stage. In in 1990, accepted a third at the this scheme, the trap depth is reduced, allowing the most Massachusetts Institute of Technology, where he is currently the energetic atoms to escape while the remainder rethermalize at John D. MacArthur Professor of Physics. His group continues to steadily lower temperatures. Most BEC experiments reach work on novel methods to cool, trap, and manipulate atoms with quantum degeneracy between 500 nK and 2 mK, at densities 14 15 À3 the goal of exploring novel aspects of ultracold atomic matter. He between 10 and 10 cm . The largest condensates are of co-invented the Dark SPOT trap and combined laser cooling and 100 million atoms for sodium, and a billion for hydrogen; the evaporative cooling, which became key techniques to obtain smallest are just a few hundred atoms. Depending on the Bose ± Einstein condensation in dilute atomic gases. He was magnetic trap, the shape of the condensate is either approx- among the very first scientists to observe this phenomenon in 1995, imately round, with a diameter of 10 to 50 mm, or cigar-shaped, and realized the first atom laser in 1997. His other awards include a about 15 mm in diameter and 300mm in length. The full cooling David and Lucile Packard Fellowship (1996), the Rabi Prize of the cycle that produces a condensate may take from a few seconds American Physical Society (1997), the Gustav Hertz Prize of the to as long as several minutes. German Physical Society (1997), the Discover Magazine Award for After this short overview, I want to provide the historical Technological Innovation (1998), the Fritz London Prize in Low context for the search for BEC and then describe the develop- Temperature Physics (1999), the Dannie Heineman Prize of the ments which led to the observation of BEC in sodium at MIT.
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