A Positron Named Priscilla Scientific Discovery at the Frontier (1994) / Chapter Skim
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2 A Positron Named Priscilla: Trapping and Manipulating Atoms
2 A Positron Named Priscilla: Trapping and Manipulating Atoms
Pages 34-59
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From page 34... ...
A new instrument, the scanning tunneling microscope, already has picked up and dropped single atoms into specific locations. It has even dragged such atoms across an underlying surface.
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From page 35... ...
It does not resemble a cobblestoned pavement, with individual atoms as the stones. It, too, is fuzzy, as if the cobblestones had an overlay of cotton.
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From page 36... ...
The sensor demands attention in its own right. It features a needle with a very sharp tip, able to probe individual atoms.
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From page 37... ...
A feedback loop, measuring this current, keeps the needle tip at a desired height above the surface (see Figures 2.1 and 2.2~. Gerd Bionic and Enrich Rohrer, at the IBM Zurich Research Laboratory In Switzerland, built the first scaring h~nnel~ng microscope ~~~ as - In ~ ~ ,~~ PIEZ~LEC~IC SOB I - { I _~LECTROSTATIC ~t ~ WAS ~ or: DE i/ ELECTRIC 1 VOLTAGE SOURCE FOR ( PlEZOEUECTR~ SACKS \ \ FIGURE 2.2 Example of a scanning tunneling microscope.
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From page 38... ...
A common procedure involved a feedback control that would measure the tunneling current as the needle scanned over the surface, automatically adjusting the height of the needle to keep that current constant. The varying voltage fed to the vertical piezoelectric element, to adjust the needle height, then would correspond to the surface details seen while scanning.
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From page 39... ...
From that, we learn to identify what they look like." An important set of experiments have involved xenon atoms on a surface of nickel. Xenon is an inert gas; it does not readily take part in chemical reactions.
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From page 40... ...
"We find that the ability to slide a xenon atom over a nickel surface is independent of both the sign and the magnitude of the electric field, the voltage, and the current," Eigler writes. "It does, however, critically depend upon the separation between the tip and the atom." This leads to a simple procedure for repositioning such atoms, one by one: Locate them using imaging; lower the tip to attract an atom of interest; move the tip as desired; and then raise the tip.
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From page 41... ...
This corresponds to just twice the spacing of atoms in the underlying nickel surface. From this we deduce that such linear chains of xenon atoms order commensurately with the underlying nickel lattice.
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From page 42... ...
When the atom jumps to the tip, the system switches into high conductance. In a representative experiment these two states, respectively, passed a current of 1.2 x 10-8 and 9 x 10-8 amperes, repeatedly turning the high current on and off as the single xenon atom jumped back and forth between tip and surface in response to the voltage pulses (see Figure 2.6~.
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From page 43... ...
Here, too, the scanning tunneling microscope offers help. Chris Lutz, a colleague of Eigler at IBM's Almaden center, has used this instrument to move atoms of platinum on a surface of platinum.
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From page 44... ...
Many catalysts provide such surfaces within the petrochemical industry, and chemists hope to study such reactions at the level of individual atoms and molecules by using the scanning tunneling microscope. As a prelude to such work, Eigler has tried to produce a molecule of carbon dioxide by coaxing carbon monoxide into combination with an atom of oxygen.
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From page 45... ...
A POSITRON NAMED PRISCILLA As an example of what can be achieved, Hans Dehmelt of the University of Washington has carried out exquisitely precise studies of individual electrons. Electrons carry an electric charge, which allows them to respond readily to electric fields; they also respond easily to a magnetic field.
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From page 46... ...
This follows from a principle of quantum mechanics: Processes that take place in diminishingly small regions require correspondingly higher energies.
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From page 47... ...
The basic technique for the study of atoms, in use for over a century, has been spectroscopy The detailed observation of spectral lines. The common sodium light fixture, seen along highways, has a close relation to the usual apparatus.
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From page 48... ...
The usual procedure tunes the laser to a frequency just below that of a spectral line and points the laser directly into the oncoming beam. Due to the Doppler shift, atoms in flight see a laser wavelength close to that of their spectral line and indeed absorb the photons quite readily.
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From page 49... ...
It relies on the Zeeman effect, whereby a magnetic field causes a spectral line to split in two. As the field increases, the two halves of the line move farther apart.
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From page 50... ...
Indeed, Hans Dehmelt, coils then trap the atoms, who has pursued his lengthy involvement with Priscilla the Positron, has which have nearly zero similarly trapped a barium ion named Astrid. The opportunity to trap velocity From Cooling ions electrically, in turn, means that one can first trap them and then cool anc/ TDoppht /n/g Atomc~sM bay them, which can be easier than the cool-first, trap-later procedure with Metcalf Copyright ~ atoms A third advantage is that ions that resist laser cooling can never 1987 by Scientific theless reach low temperatures by storing them in a trap along with American, Inc.
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From page 51... ...
Perhaps the simplest trap involves three sets of laser beams, oriented respectively to define x, y, and z axes and intersecting within a small region of space. These lasers are tuned to just below the frequency of a strong spectral line.
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From page 52... ...
In practice, one locates this focus within the intersection of the sets of beams that form optical molasses. The lasers then feature two different frequencies: There are molasses beams that are close to a spectral line along with the focused beam that is well away from such a line.
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From page 53... ...
Use of the focused beam introduces rapidly varying oscillations in their electron densities. The magnetic trap, in turn, changes the spectral lines through the Zeeman effect.
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From page 54... ...
.~ ~ ~ _- ~ _ -If - _ 1 1 is. MOLASSES BEAMS COOLATOMS /\ /: ~ COILS GENERATE MAGNETIC FIELD BEAM INJECTS ATOMS relatively dense beam of very slow atoms that can feed a continuous atomic fountain.
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From page 55... ...
, while another, based on the hyperfine splitting of certain spectral lines of atoms, would be determined by a combination of QED and nuclear forces. If the strength of the nuclear forces changed relative to the electromagnetic forces, the two very precise clocks would begin to "tick" at different rates.
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From page 56... ...
APPLICATIONS IN BIOLOGY The direct manipulation of atoms and other elementary particles, with the scanning tunneling microscope, with lasers, and by using electric and magnetic fields, thus opens a host of prospects. These include fundamental tests of basic features in physical theories, new states of matter, studies of chemical reactions at the atomic level, new instruments for the precise measurement of time and of gravity fields, and even a new approach in searching for oil.
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From page 57... ...
The basic idea, that of using a single laser beam for this purpose, is that of Arthur Ashkin, of AT&T Bell Laboratories. Chu and his colleagues call it "optical tweezers" and have been using it in studies of DNA.
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From page 58... ...
It is not yet possible to consult a HewlettPackard catalog for an atomic clock based on these principles or a gravity meter based on interferometry of ultracold atoms. Still less have such instruments served to make fundamentally new measurements or observations, which would stand as important contributions in their own right.
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From page 59... ...
A POSITRON NAMED PRISCILIA Itano, W
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