Riding the atomic waves: with the magic of quantum mechanics, an atom goes two ways at once.Physicists have added a new trick to their experimental repertoire. In their latest feat, called atom interforometry, they paradoxically divide and recombine re·com·bine
To undergo or cause genetic recombination; form new combinations. single atoms, aided by a beautiful but enigmatic assistant known as 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 .
Four teams have recently performed atom interferometry, each using a different technique to accomplish this same bit of seeming magic. Unlike real magicians, however, these physicists eagerly explain the mysteries of their craft. In the past few months, all four groups have shared the secrets of their atom interferometers.
Interferometers are highly sensitive Adj. 1. highly sensitive - readily affected by various agents; "a highly sensitive explosive is easily exploded by a shock"; "a sensitive colloid is readily coagulated" instrument that provide exact measurements of extremely small distances and physical properties such as wavelength. Scientists use them mainly for experimentation, but the devices have several commercial applications as well. Laser interferometers, for instance, play a vital part in advanced gyroscopes.
In the past, interferometers accomplished their precise measurements by manipulating electrons, neutrons or light. Researchers have now made even more sensitive instruments that extend those manipulations to atoms.
In the paradoxical world of quantum mechanics, an atom--like a photon--can be thought of as both a particle and a wave. But atoms have a great advantage over photons when it comes to interferometry. The wavelenth of an atom, known as its de Broglie de Broglie. For persons thus named use Broglie. wavelenth, is based on its momentum and can be 10,000 times shorter than that of visible light. The smaller the wavelength used, the greater an interferometer's precision.
With the new atom interferometers, physicists plan to conduct difficulties tests of atomic properties, general relatively and quantum mechanics. One such device shows promise for measuring gravitational acceleration In physics, gravitational acceleration is the acceleration of an object caused by the force of gravity from another object. An interesting fact is that any object will accelerate towards a large object at the same rate, regardless of the mass of the object. with record-breaking precision.
The key to all types of interferometry lies in quantum mechanics' wave-particle duality wave-particle duality
Principle that subatomic particles possess some wavelike characteristics, and that electromagnetic waves, such as light, possess some particlelike characteristics. . The instruments take a particle and break the single wave that represents it into multiple (usually two) distinct components. In an atom interferometer, for example, "each atom has been split and is going both ways at once," explains David E. Pritchard of the Massachusetts Institute of Technology Massachusetts Institute of Technology, at Cambridge; coeducational; chartered 1861, opened 1865 in Boston, moved 1916. It has long been recognized as an outstanding technological institute and its Sloan School of Management has notable programs in business, in Cambridge. Yet an observer attempting to witness this counterintuitive coun·ter·in·tu·i·tive
Contrary to what intuition or common sense would indicate: "Scientists made clear what may at first seem counterintuitive, that the capacity to be pleasant toward a fellow creature is ... split will see only one wave--a phenomenon arising from the quirks of quantum mechanics. With quantum mechanics, notes Pritchard "you beat your intuition into submission."
After traveling their divergent paths, the wave components recombine at an awaiting detector. If their path difference is exactly the particle's wavelength or an integer multiple of it, the waves are "in phase" and harmoniously merge with each other--an effect known as constructive interference. In other words Adv. 1. in other words - otherwise stated; "in other words, we are broke"
put differently , the crests and troughs of each wave coincide and reinforce one another.
But at other spots on the detector, the path difference amounts to only a fraction of the wavelength, and the components are out of phase. The waves still recombine, but with destructive interference. At these places, where the merging waves are out of alignment, probability dictates that fewer particles will appear.
Ligth interferometers, for example, often produce an interference pattern interference pattern
An overall pattern that results when two or more waves interfere with each other, generally showing regions of constructive and of destructive interference. consisting of a series of dark strips, where few photons emerge, and light strips, where many photons are detected. Atom interferometers show similar patterns based on the number of atoms at each spot on the detector. In effect, "you get ligth and dark spots of atoms," Pritchard says.
Careful examination of these interference patterns can reveal the minute fraction of a wavelength by which the atom's paths differed (the phase shift) and can even reveal the particle's wavelength.
Pritchard's team and a German group unveiled their atom interferometers in the May 27 PHYSICAL REVIEW LETTERS Physical Review Letters is one of the most prestigious journals in physics. Since 1958, it has been published by the American Physical Society as an outgrowth of The Physical Review. . The German researchers, led by Jurgen Mlynek of the University of Konstanz The University of Konstanz (German: Universität Konstanz) is a university in the city of Konstanz in Baden-Württemberg, Germany. It was founded in 1966, and the main campus on the Gießberg was opened in 1972. , created their device by adapting the classic double-slit experiment “Slit experiment” redirects here. For other uses, see diffraction.
In the double-slit experiment, light is shone at a solid thin plate that has two slits cut into it. A photographic plate is set up to record what comes through those slits. of English physicist Thomas Young Noun 1. Thomas Young - British physicist and Egyptologist; he revived the wave theory of light and proposed a three-component theory of color vision; he also played an important role in deciphering the hieroglyphics on the Rosetta Stone (1773-1829)
Young , who in 1802 used photons to demonstrate interference.
In Mlynek's atom interferometer, a supersonic beam of helium atoms passes through a 2-micrometer-wide opening in a thin gold foil. This "spreads" the atom into a wider wave before it travels through two smaller slits. While passing through the two smaller slits, the waves scatter again and eventually recombine into the original atom's single wave. Mlynek and co-worker Olivier Carnal carnal adjective Referring to the flesh, to baser instincts, often referring to sexual “knowledge” suggest the device can resolve a phase shift equal to 0.053 of the atom's wavelength. Since an atom's wavelength is known, the researchers can translate that phase shift into the minute distance by which the wave paths differed.
Pritchard's device--which uses a thin silicon-nitride membrane with a series of extremely fine slits cut into it for a diffraction grating--is even more sophisticated. A beam of sodium atoms must travel through three of these diffraction gratings before yielding an interference pattern. This instrument can resolve a phase shift of 0.016 wavelength, a significant improvement over Mlynek's device, Pritchard says. Moreover, the use of sodium allows additional precision, since sodium atoms are heavier than helium atoms and therefore have a shorter de Broglie wavelength De Broglie wavelength
The wavelength γ = h/p associated with a beam of particles (or with a single particle) of momentum p; h = 6.626 × 1034 joule-second is Planck's constant. .
That precision wasn't easy to achieve. To preserve the instrument's sensitivity, the MIT MIT - Massachusetts Institute of Technology researchers had to incorporate a number of features merely to eliminate vibrations and maintain the alignment of the three diffraction gratings. They even included a laser interferometers to continuously monitor the gratings' alignment.
Physicists have long used atoms--in the form of solid objects such as diffraction gratings, mirrors and lenses--to manipulate light. Some of the new interferometers do just the opposite: Light -- in the form of laser pulses -- manipulates atoms. In the July 8 PHYSICAL REVIEW LETTERS, two groups describe atom interferometers that accomplish this reversal.
In one such device, a calcium atomic beam Atomic beam or atom laser is special case of particle beam; it is the collimated flux (beam) of neutral atoms. The imaging systems using the slow atomic beams can use the Fresnel zone plate (Fresnel diffraction lens) of a Fresnel diffraction mirror as focusing element. is divided into four wave paths by two laser beams perpendicular to the atoms. A second pair of lasers, aimed in the opposite direction of the first pair of laser beams, redirects the waves to either of two detection areas, where the atoms are counted. Both detectors reveal interference patterns, although chance determines which detector will tally a given atom.
This complex instrument is sensitive to rotational changes, report Jurgen Helmcke of the Federal Agency for Technical and Scientific Research in Braunschweig, Germany, and his colleagues. By rotating their instrument on a turntable at different speeds, the researchers can influence the paths of the wave components, thereby shifting the resulting interference patterns. The laser gyroscopes on many of today's airplanes work by a similar principle, but theory indicates that an atom-based system would offer 10 billion times as much sensitivity, useful for general-relatively experiments.
At Stanford University Stanford University, at Stanford, Calif.; coeducational; chartered 1885, opened 1891 as Leland Stanford Junior Univ. (still the legal name). The original campus was designed by Frederick Law Olmsted. David Starr Jordan was its first president. , Steven Chu Steven Chu (Chinese: 朱棣文; Pinyin: Zhū Dìwén), born 1948 in St. Louis, Missouri, is an American experimental physicist. and his colleagues manipulate atoms of a slightly different sort. While other research teams work with fast-moving atomic beams, Chu slowly pumps laser-cooled atoms through his interferometer interferometer: see interference under Interference as a Scientific Tool. See also virtual telescope.
An instrument that measures the wavelengths of light and distances. with an "atomic fountain" (SN:8/19/89, p.117). The languid atoms spend up to 0.5 second within the device--an important consideration in measurements of minute effects. "The sensitivity [of an interferometer] increases when you use slow atoms," Chu explains.
Pritchard agrees and says he plans to try slower atoms in his own devices. He adds, however, that the "brightness" of such sources needs improvement. Chu's fountain can deliver atoms much more slowly than an atomic beam, he says, but it cannot yet match the beam's intensity--the number of atoms delivered.
In Chu's interferometer, lasers not only precool pre·cool
tr.v. pre·cooled, pre·cool·ing, pre·cools
To reduce the temperature of (produce or meat, for example) by artificial means before packaging or shipping. atoms but also lie at the heart of the device. Two lasers--one on each side of the atom's path--provide an initial pulse that splits the atom into a super-position of two different energy states. The higher energy state, recoiling from the laser pulse, moves away from the lower energy state so that the atom appears to be in two places at once. A second pulse reverses the action, causing the atom to reconverge. A third laser pulse ultimately reads the interference pattern.
For another experiment, Chu directed the lasers along, rather than across, the path of the crawling atoms. As a result, an atom's components actually travel the same path at slightly different speeds, so that they move apart from each other in space. This setup should allow the most precise measurement yet of a single atom's gravitational acceleration, potentially achieving a resolution of 1 part in 10 billion, the Stanford researchers assert.
With further refinement, atom interferometers could compete with the laser technology now used in gyroscopes, says Pritchard. But these new devices will shine their brightest in probing the minute details of physics, he maintains. For experimental physicists, improving measurements by a single decimal place can represent a life's goal--a goal now achievable with atom interferometers.
This added precision should help to test the predictions of general relatively and might finally lay to rest the controversial issue of a fifth force, physicists say. It might also dispel any remaining doubts about the charge neutrality of atoms. To confirm atomic neutrality, experimenters would apply an electric field to only one wave component of an atom. If the atom is not neutral, the field will create a discernible change in the interference pattern.
While the new interferometers provide a powerful tool for unveiling atomic properties, "we're not going to find that quantum mechanics is wrong," cautions Pritchard. That's fortunate--because without the perplexing per·plex
tr.v. per·plexed, per·plex·ing, per·plex·es
1. To confuse or trouble with uncertainty or doubt. See Synonyms at puzzle.
2. To make confusedly intricate; complicate. theory, physicists could never have performed their latest show-stopping trick.