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sing one of the light beams, the light, from the atom's perspective, increases in frequency. The light that has been shifted up in frequency is then likely to be absorbed by the atom. The light that the atom absorbs exerts a scattering force that slows the atom down. How does the atom interact with the light traveling in the same direction? The atom is less likely to absorb the light because the light, again from the atom's perspective, has been shifted down in frequency. The net effect of both of the beams is that a scattering force is gen- erated, opposing the motion of the tom. The beauty of this idea is that an tom moving in the opposite direction experience a scattering force it toward zero velocity. By sur- the atom with three sets of ounteGropagating beams along three tually perpendicular axes, the atom an be cooled in all three dimensions. shkin, Leo Hollberg, John E. , Alex Cable and I at AT&T were able to cool sodium 240 millionths of a kelvin. Be- field acts as a viscous orce, we dubbed the combination of beams used to create the drag "optical molasses." Although not a the atoms were confined in the us medium for periods as long as second, until eventually thev would out of the cooling beams. Optical molasses enabled us to solve the three major problems that stood in the way of constructing a laser trap. First, by cooling the atoms to extremely low temperatures, we could reduce the random thermal motions of the atoms, making them easy to trap. Second, we could easily load the atoms into the trap. Simply by focusing the trapping beam in the center of the optical molasses, atoms would be snagged as they randomly wandered into the trapping beam. Third, by alternating between trapping and cooling light, we could reduce the heating effects of the trapping light. A year after we had perfected optical molasses, atoms could finally be trapped with light. Even with the loading technique used in our first trap, an optical trap with a larger capture volume was desirable. A trap that could use the scattering force would need much less light intensity, which meant the constraints imposed by the Optical Earnshaw Theorem had to be circumvented. The important clue of how to design such a trap came from David E. Pritchard of the Massachusetts Institute of Technology and Carl E. Wie- man of the University of Colorado and their colleagues. They pointed out that if magnetic or electric fields that varied over space were applied to atoms, the scattering force caused by the laser light would not necessarily be proportional to the light intensity. This suggestion led Jean Dalibard of the fcole Nonnale Superieure in Paris to propose a "magneto-optic" trap, which used a weak magnetic field and circu- larly polarized light. In 1987 Pritchard's group and my own at AT&T collabo- rated to construct such a trap. Three years later Wiernan's team went on to show that this technique could be used to trap atoms in a glass cell, using in- expensive diode lasers. Their method eliminated the precoolmg procedures needed in our first tra ments. The fact species of atoms, isotopes, could be the most widely used optical trap tod eanwhile researchers were ing rapid progress in cooling. Phillips and his leagues discovered that under used to cool atoms to temperatures far below the lower limit predicted by the existing theory. This discovery prompt- ed Dalibard and Claude Cohen-Tan- noudji of the College de France and the hole Normale and my group at Stan- ford to construct a new theory of laser cooling based on a complex but beauti- ful interplay between the atoms and their interaction with the light fields. Currently atoms can be cooled to a temperature with an average velocity equal to three and a half photon recoils. For cesium atoms, it means a temperature lower than three microkelvHis.
Going beyond optical molasses, Co- hen-Taimouai, Alain Aspect, Ennip Ari- mondo, Robin Kaiser sod., Nathalie Van- steoddste, then all at the Ecole Normale, invented an ingenious scheme capable of cooling helium atoms below the recoil velocity of a single scattered photon. Helium atoms have been cooled to two microkelvins along one dimension, and work is under way to extend this technique to two and three dimensions. This cooling method captures an atom in a well-defined velocity state in much the same way atoms were trapped in space in our fast optical trap. As the atom scatters photons, its velocity randomly changes. The French experiment establishes conditions that allow an atom to recoil and land in a particular quantum state, which is a combination of two states with two distinct velod- ties close to zero. Once in this state, the chance of scattering more photons is greatly reduced, meaning that addi- tional photons cannot scatter and increase the velocity. If the atom does not happen to land in this quantum state, it continues to scatter photons and has more opportunities to seek out the desired low-velocity state. Thus, the atoms are cooled by lett&lg tiasai ran- ddy walk into a "velocity trapped" quantum state. Besides the cooling and trapping of atoms, investigators have demonstrat- ed various atomic lenses, mirrors and diffraction gratings for manipulating atoms. They have also fashioned de- vices that have no counterpart In light optics. Researchers at Stanford and the University of Bonn have made "atomic funnels" that transform a collection of hot atoms into a well-controlled stream of cold atoms. The Stanford group has also made an "atomic trampoline" in which atoms bounce off a sheet of light extending out from a glass surface. With a curved glass surface, an atom trap based on gravity and light can be made. Clearly, we have learned to push atoms around with amazing facility, but what do all these tricks enable us to do? With very cold atoms in vapor form, physicists are in a position to study how the atoms interact with one another at extremely low temperatures. According to quantum theory, an atom behaves like a wave whose length is equal to Planck's constant divided by the parti- cle's momentum. As the atom is cooled, COILS GENERATE MAGNETIC RELD its momentum decreases, creasing its wavelength. At sufficiently low temperatures, the average wavelength becomes comparable to the average distance between the atoms. At, these low temperatures and high densi- ties, quantum theory says that a sig- nificant fraction of all the atoms will condense into a single quantum ground state. This unusual form of matter, called a Bose-Einstein condensation, has been predicted but never observed in a vapor of atoms. Thomas J. Greytak and Daniel Kleppner of M.LT. and Jook T. M. Walraven of the University of Amster- dam are trying to achieve such a condensation with a collection of hydrogen atoms in a magnetic trap. Meanwhile other groups are attempting the same feat in a laser-cooled sample of alkali atoms such as cesium or lithium. Atom-manipulation techniques are also offering new opportunities in high- resolution spectroscopy. By combining several such techniques, the Stanford group has created a device that will allow the spectral features of atoms to be measured with exquisite accuracy. We have devised an atomic fountain that launches ultra-cold atoms upward ATOMIC FOUNTAIN allows preelse measurements of the en- several light beams. After about 10 million atoms have accu- ergy states of atoms. Atoms are injected into the apparatus undated in die trap, the atoms are launched upward. At the and slowed by a laser beam. The atoms are then captured top of the tratectory, microwave pulses excite the atom from and cooled by the combined effects of a mgiwtic field and one enemy state to another. 74 SCIENTIFIC AMERICAN February 1992
Page 1 and 2: Laser Trapping of Neutral Particles
Page 6 and 7: . - 'gently enough to have gravity
Page 8: SCIENTIFIC - AMERICAN New drugs may

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