Stopping an atom in its tracks.Stopping an Atom in its Tracks
For a long time physicists have sought methods of studying a single atom in isolation. To do this they must trap the atoms -- that is, somehow stop or at least sharply curtail the various motions in which an atom naturally engages and hold the atom more or less at rest in the laboratory where they study it. One motivation for doing this is to study the basic foundations of quantum mechanics quantum mechanics: see quantum theory.
Branch of mathematical physics that deals with atomic and subatomic systems. It is concerned with phenomena that are so small-scale that they cannot be described in classical terms, and it is , the science of how things happen inside atoms and smaller structures. For nearly 80 years, physicists have built up this science on the basis of inferences drawn from the behavior of large aggregates of atoms. Physicists -- or at least some of them -- are eager to see whether the basic processes proposed by quantum mechanics really happen on the level of the single atom.
experimenters have pursued two approaches to atomic trapping more or less in parallel. One is to use ions, atoms that have either lost a few electrons or gained a few extra and so are electrically charged, and confine them through the influence of electric and magnetic forces. The other method is to take atoms as they usually come -- that is, electrically neutral -- and confine them under the influence of beams of laser light. This technique has just succeeded in the trapping of single atoms of sodium.
This method of trapping neutral bodies is applicable to a wide range of neutral objects, from atoms, which are characteristically 1 angstrom angstrom (ăng`strəm), abbr. Å, unit of length equal to 10−10 meter (0.0000000001 meter); it is used to measure the wavelengths of visible light and of other forms of electromagnetic radiation, such as ultraviolet across, to objects up to 100,000 times as large, including viruses, bacteria, small solid spheres and tiny oil drops. The procedure holds promise in studies of very basic chemistry -- how one atom relates to and binds itself to another atom -- and in microbiology microbiology: see biology.
Scientific study of microorganisms, a diverse group of simple life-forms including protozoans, algae, molds, bacteria, and viruses. . Physicists John E. Bjorkholm, Steve Chu, Arthur Ashkin Arthur Ashkin is a retired scientist who worked at Bell Laboratories and Lucent Technologies. He invented the optical tweezers in 1986. He has pioneered the optical trapping process that eventually was used to manipulate atoms, molecules, and biological cells. and Alex Cable, from AT&T Bell Laboratories in holmdel, N.J., developed the method. Ashkin and Bjorkholm described it last week at the International Quantum Electronics Quantum electronics
A loosely defined field concerned with the interaction of radiation and matter, particularly those interactions involving quantum energy levels and resonance phenomena, and especially those involving lasers and masers. Conference '86 in San Francisco San Francisco (săn frănsĭs`kō), city (1990 pop. 723,959), coextensive with San Francisco co., W Calif., on the tip of a peninsula between the Pacific Ocean and San Francisco Bay, which are connected by the strait known as the Golden . It is the outcome of an effort that began in 1970 with some theoretical papers by Ashkin.
Light pressure from a laser beam will stop the forward motion of a small object, but in the absence of some restoring force, the light pressure will then kick the object backward. For objects a good deal larger than the wavelength of the light, gravity will work as the restoring force: The object's weight balances the thrust of a laser beam coming from below. For objects very much smaller than a light wave -- and an atom's size lies between one-thousandth and one-ten-thousandth of a light wave -- gravity is not a sufficient restoring force.
It seemed for a long time that there was no way to get one, but in 1978 Ashkin showed that if the laser beam were sharply focused, it would supply the restoring force itself. The shape of the light wave fron near the focus produces the restoring force, and if everything is properly arrnaged, an object will be trapped at a point just short of the focus.
With this arrangement the experimenters in the late 1970s were able to trap small solid particles and oil drops. They tried tobacco mosaic viruses tobacco mosaic virus
A retrovirus that causes a disease in tobacco and some other plants and is widely used in the study of viruses and viral diseases. , which are cylinders 180 angstroms in diameter and 3,000 angstroms long. The trap could hold them for days without damage, and it also inhibited the tumbling that they naturally do. At this point, according to according to
1. As stated or indicated by; on the authority of: according to historians.
2. In keeping with: according to instructions.
3. Ashkin, some bugs swam into range (the experiments were performed in water). They turned out to be real biological bugs -- bacteria, that is; the trap could hold a living bacterium bacterium /bac·te·ri·um/ (bak-ter´e-um) pl. bacte´ria [L.] in general, any of the unicellular prokaryotic microorganisms that commonly multiply by cell division, lack a nucleus or membrane-bound organelles, and possess a cell for several hours. Then the experimenters decided, as Ashkin puts it, to "optocute" it. They increased the power of the laser beam until it killed the bacterium. The carcass carcass, carcase
1. the body of an animal killed for meat. The head, the legs below the knees and hocks, the tail, the skin and most of the viscera are removed. The kidneys are left in and in most instances the body is split down the middle through the sternum and the vertebral stayed in the trap also.
A limitation of these optical traps is that the objects must be cooled -- in physical terms, their thermal vibrations must be damped. Otherwise they skitter skit·ter
v. skit·tered, skit·ter·ing, skit·ters
1. To move rapidly along a surface, usually with frequent light contacts or changes of direction; skip or glide quickly: out of range before they can be trapped. A viscous viscous /vis·cous/ (vis´kus) sticky or gummy; having a high degree of viscosity.
1. Having relatively high resistance to flow.
2. Viscid. liquid (water) will cool the larger objects, but single atoms are too small for that.
The solution, worked out last year and performed only a few weeks before the meeting, is what the experimenters call "optical molasses molasses, sugar byproduct, the brownish liquid residue left after heat crystallization of sucrose (commercial sugar) in the process of refining. Molasses contains chiefly the uncrystallizable sugars as well as some remnant sucrose. ." It consists of shooting a number of laser beams at the atom from all sides. This procedure looks like trapping, Bjorkholm says, but it really isn't, as there are no restoring forces. The "molasses" laser beams bounce the atoms in such a way as to reduce their thermal vibrations. Then the sharply focused trapping beam comes on, and the atoms are caught.
Ashkin and Bjorkholm point out that the optical molasses produces what amounts to a new state of matter, an ultra-cooled gas. These atoms are cooled to a temperature of 1 millikelvin, a thousandth of a degree above absolute zero, which is well below the solidification temperature for any substance under ordinary circumstances, yet they remain physically a gas. The experimenters expect fascinating new physics from the also.