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The Base-Einstein Condensate

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The Base-Einstein Condensate The Base-Einstein Condensate Three years ago in a Colorado laboratory, scientists realized a long-standing dream, bringing the quantum world closer to the one of everyday experience by Eric A. Cornell a nd Carl E. Wieman n June 1995 our research group at long quest to prove Einstein's rheory. Bur the Joint Institute for Laboratory most of the fascination now stems from I Astrophysics (now called JILA) in the fact that the condensate offers a Boulder, Colo., succeeded in creating a macroscopic window into the Strange minuscule but marvelous droplet. By world of quantum mechanics, the theory cooling 2,000 rubidium atoms to a of matter based on the observation that temperature less than 100 billionths of elemenrary particles, such as electrons, a degree above absolute zero (100 bil­ have wave properties. Quantum me­ lionths of a degree kelvin), we caused chanics, which encompasses the famous the atoms ro lose for a full 10 seconds Heisenberg uncertainty principle, uses their individual idenrities and behave as these wavelike properties to describe though they were a single "superatom." the structure and interacrions of matter. The atoms' physical propercies, such as We can rarely observe the effects of their motions, became identical ro one quantum mechanics in the behavior of another. This Bose-Einstein condensate a macroscopic amount of material. In (BEC), ehe first observed in a gas, can ordinary, so-called bulk matter, the in­ be thought of as rhe matter counterpart coherent contributions of the uncount­ of ehe laser-except rhat in the conden­ ably !arge number of constituent parti­ sare it is aroms, rathcr rhan photons, cles obscure the wave nature of quan­ that dance in perfect unison. tum mechanics, and we can only infer its Our short-livcd, gelid sample was the effects. But in Bose condensarion, the experimental realization of a theoretical wave nature of cach atom is precisely in construct that has intrigued scientists phase with that of every other. Quan­ ever since it was predicted some 73 years tum-mechanical waves extend across ago by the work of physicists Albert Ein­ the sample of condensate and can be stein and Satyendra Nath Bose. At ordi­ observed with the naked eye. The sub­ nary temperatures, rhe atoms of a gas microscopic rhus becomes macroscopic. are scattered throughout the container holding them. Some have high energies New Light on Old Paradoxes (high speeds); orhers have low ones. Ex­ panding on Bose's work, Einstein he creation of Bose-Einstein con­ showed that if a sample of atoms were Tdensates has cast new light on long­ cooled sufficiently, a !arge fraction of standing paradoxes of quantum me­ them would settle into the single lowest chanics. For example, if two or more possible energy state in the container. In atoms are in a single quantum-mechan­ mathematical rerms, their individual ical state, as thcy are in a condensare, it wave equations-which describe such is fundamentally impossible to distin­ physical characreristics of an atom as guish them by any measurcment. The its position and velocity-would in ef­ cwo atoms occupy rhe same volume of fect merge, and each atom would be­ space, move at the identical speed, scat­ come indistinguishable from any ocher. ter light of the same color and so on. Progress in creating Bose-Einstein con­ Nothing in our experience, based as densates has sparked great interest in the it is on familiarity with matter at nor­ physics community and has even gener­ mal temperatures, hclps us comprehend ated coverage in the mainstream press. this paradox. That is because at normal At first, some of the atrention derived temperatures and at the size scales wc from the drama inherenr in ehe decades- are a ll familiar with, it is possible to de- 26 Sc tENTlf-lC ..\~mu CA!': March 1998 Thc ßose-Ei11stein Co11de11satc scribe rhe posirion and morion of each we can expecr ro find rhc arom. As a er disringuish one arom from anorher. ancl every objecr in a collecrion of ob­ collecrion of aroms bernmcs colcler, rhe The currenr excitemenr over rhese jecrs. Thc numberecl Ping-Pong balls size of each wa ve packe r grows. As long conclensares conrrasrs sharply wirb rhe bouncing in a roraring drum usecl ro se­ as each wave packet is spariall y sepa­ reacrion ro Einstein 's discovery in 1925 lecr lorrcry numbers exemplify the mo­ rarecl from rhe orhers, ir is possible, ar rhat rhey coulcl exist. Perbaps because rions describable by classical mechanics. least in principle, ro teil aroms apart. of rhe impossibiliry rhen of reacbing rhe Ar exrremely low remperarures or ar When rhe remperarure becomes suffi ­ required remperatures-less rhan a mil­ small size scales, on rhe orher hand, rhe cienrly low, however, each arom's wave lionrh of a degree kelvin-tbe hyporhe­ usefulness of classical mechanics begins packet begins ro overlap wirb rhose of sized gaseous condensate was consid­ to wane. The crisp analogy of atoms as neighboring aroms. When rhis bappens, ered a curiosiry of quesrionable validiry Ping-Pong balls begins to blur. We can­ rbe aroms "Bose-condense" inro rbe ancl lirrle physical significance. For per­ nor know the exact posirion of each lowesr possible energy srare, and rbe specrive, even rhe coldesr deprhs of in­ arom, which is berrer thought of as a wave packers coalesce inro a single, mac­ rergalacric space are mi llions of times blurry spot. This spor-known as a wa,·e roscopic packet. Tbe aroms undergo a too hor for Bose condensarion. packet-is rhe region of space in which quanrum idenriry crisis: we can no long- In rhe intervening decades, however, The Bose-Ei11stei11 Co11densate <;('JFNTJFJ(" .A. \1FRJ('•\N lvbrrh 1 C)C)R 27 Bose condensation came back imo fash­ putting the '·super" in superfluidity. than the rare for hydrogen atoms. These ion. Physicists realized that the concept Not until the late 1970s d id refrigera­ much !arger atoms bounce off one an­ could explain superfluidity in liquid he­ tion rechnology advance to the point other at much higher rares, sharing en­ lium, which occurs at much higher tem­ that physicists could emertain the no­ ergy among rhemselves more quickly, peratures than gaseous ßose condensa­ tion of creating something like Ei nstein's which allows the condensare to form tion. ßelow 2.2 kelvins, the viscosity of original concept of a BEC in a gas. Lab­ before clumping can occur. liquid helium completely disappears- oratory workers at M.I.T., rhe University Also, it looked as if it mighr be rela­ of Amsterdam, the University of British tively easy and inexpensive ro ger these Columbia and Cornell Universiry bad atoms very cold by combining ingenious to confront a fundamental difficulty. To rechniques developed for laser cooling achieve such a BEC, they bad to cool and trapping of alkali atoms with the the gas to far below the temperature ar techniques for magnetic trapping and which the aroms would normally freeze evaporative cooling developed by the re­ into a solid. In other words, they had to searchers working wich hydrogen. These creare a supersaturared gas. Their ex­ ideas were developed in a series of dis­ pecrarion was rhat hydrogen would su­ cussions with our friend and former persaturare, because the gas was known teacher, Daniel Kleppner, the co-leader to resist the arom-by-atom clumping of a group at M.I.T. that is attempting that precedes bulk freezing. to create a condensate with hydrogen. Although rhese invesrigarors have not Our hypothesis about alkali atoms yet succeeded in crearing a Bose-Ein­ was ultimately fruitful. Just a few stein condensate wirb hydrogen, rhey months after we succeeded with rubidi­ did develop a much better undersrand­ um, Wolfgang Ketterle's group at M .J.T. ing of the difficulties and found clever produced a Bose condensare with sodi­ approaches for attacking them, which um atoms; since that time. Ketterle's benefited us. In 1989, inspired by the team has succeeded in crearing a con­ hydrogen work and encouraged by our densate wirb 10 million aroms. Ar the own research on the use of lasers ro trap time of this writing, there are at least and cool alkali aroms, we began ro sus­ seven teams producing condensates. pect rhar rhese atoms, which include ce­ Besides our own group, others working sium, rubidium and sodium, would with rubidium are Daniel J. Heinzen of make much better candidates than hy­ the University of Texas at Ausrin, Ger­ drogen for producing a Bose conden­ hard Rempe of the University of Kon­ sate. Although the clumping properties stanz in Germany and Mark Kasevich of cesium, rubidium and sodium are not of Yale University. In sodium, besides Superior ro those of hydrogen, the rate Kctterle's at M .I.T., there is a group led at which those atoms transform rhem­ by Lene Vestergaard Hau of rhe Row­ selves inro condensate is much faster land Institute for Science in Cambridge, EVAPORATIVE COOLING occurs in a magnetic trap, which can be thoughr of as a deep bowl (blue). The most energetic atoms, depicted with the longest green trajectory arrows, escape from the bowl (above, left). T hose that remain collide with one another frequently, apportioning out the remaining energy (left). Eventually, the atoms move so slowly and are so closely packed at the bottom of the bowl that their quantum nature becomes more pronounced. So-called wave packets, representing the region where each atom is likely tobe found, become less distinct and begin to overlap (below, left). Ulti­ mately, two atoms collide, and one is left as close to stationary as is allowed by Heisen­ berg's uncertainty principle.
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