SIGHTINGS
Capturing Quantum Corrals
Felice Frankel
In this observer's opinion, the "orange and blue quantum
corrals" shown in full on pages 262-263 have become
contemporary scientific visual icons, in the same category as
NASA's "earthrise" image or Doc Edgerton's splashing
milk drop. Don Eigler and Dominique Brodbeck were instrumental
in developing these images at IBM. Here they discuss the
process. The reader should take note that an important visual
component of both images is the representation of the electron
eigenstates—wavelike structures—within the corrals.
The drama of the atomic peaks might distract our eyes, but with
such amazing science, does it really matter?
F. F. Don, tell us how you collected the data and
then processed that information, turning it into an image.
D. E. We used a scanning tunneling microscope, in
which a metal needle under computer control is made to move along
the contours of the surface being imaged. The height of the needle
at each location on a square grid of points is recorded as a number
in the computer. This sequence of numbers representing the height of
the surface at each point on the grid is then rendered by the
computer to look like a three-dimensional solid.
The orange corral was something that I did specifically
for the cover of Science. I wanted to create an image with
as dramatic a perspective as possible to entice the editors into
using the image on the cover of the magazine. I chose the colors,
"lighting conditions" and point of view of the observer to
suit the purpose of the moment. We do rendering of the data during
data acquisition, but it is much more rudimentary: just black and
white and usually a top-down view of the data, without the benefit
of 3-D perspective. During construction of the corral we were
looking at data that very much appeared like the images at right.
When Dominique Brodbeck created the blue corral image,
we had a more educational or scientific purpose in mind, and so the
perspective is one in which you get a much better idea of the shape
of the corral and the shape of the waves in the interior of the
corral. Nonetheless, Dominique's image appeared on the cover of
Physics Today (November 1993).
But for the Science cover, in a sense, I
started with an empty canvas (a black computer screen) and a concept
about what I wanted to achieve. I began to apply paint. It was a
matter of displaying the corral in 3-D and then searching for the
combination of perspective, lighting, surface properties and color
that communicated what I wanted to communicate. So the final image
came to reveal itself in steps as I worked with computer knob-things.
I wanted others to share in my sense of being an
intimate observer of the atoms and the quantum states of the corral.
That is why I put the point of view so low and close to the ring of
atoms. But it wasn't quite as casual as the preceding statement
might imply. I was searching for something and found it.
F. F. Do you remember your response when you
finally finished the process?
D. E. There definitely was a response on my part
when I looked at the completed structure for the first time. I don't
have an exact recollection, but I‘m fairly confident it was
something like "Cool! There is quantum mechanics!" The
symmetry and simplicity of the waves in the interior of the corral
have a special meaning to a physicist. These things were entirely
predictable, and we routinely work them out in our problem sets when
we take courses in quantum mechanics. We knew of them in a purely
cerebral way. But here they are, alive to our eyes and responsive to
our hands ... quantum mechanics made visceral! The realization that
you can build a quantum state of your own design, see the result,
and change and shape the quantum state like a lump of clay ... well
that's just candy to a physicist. The corrals evoke a
delectable intimacy between us and the quantum world.
F. F. Dominique, describe to us the differences
between the two images.
D. B. Both images try to visually separate the ring
of atoms from the wavy surface, but there is a significant
difference in how this is achieved. In the orange corral, the
separation is achieved by using a clever combination of
different-colored light sources (turquoise from the left, red from
the front directly into the camera) and surface-reflection
properties. This is quite tricky and takes a lot of trial and error
in practice. I guess this was only really feasible because Don's
workstation is equipped with a special dial-and-knob box that maps
all these parameters to hardware controls and allows a very direct
way of interaction. (It would actually be interesting to study how
the haptic experience influences the interaction.) Plus the
rendering is fast enough to show the changes in real time. I think
interaction is key here.
For the blue corral, on the other hand, the separation
effect was achieved by coloring the surface by height, using a color
map that assigns a single color to a particular height above ground.
There are far fewer degrees of freedom to handle compared to the
technique described above. The lighting setup then just uses a white
light coming from behind left, at a polar angle that maximizes the
sense of shape of the subtle wave pattern outside of the corral.
From an image creator's point of view, what I always
liked about working with STM data was the fact that the surfaces are
very similar to landscapes, and that you can apply the same design
guidelines and intuition as you do in landscape photography.
F. F. Don, can you briefly give the history of this technology?
D. E. It happened in five distinct steps.
Step 1: Invention of the scanning tunneling microscope and
its ability to create atomic-resolution images (Gerd Binnig and
Heinrich Rohrer, the 1986 Nobel Prize in Physics with Ernst Ruska)
Step 2: The design, construction and operation of a
low-temperature (4 degrees Kelvin) ultra-high-vacuum STM that would
allow the study of atomically clean, well-characterized surfaces at
temperatures low enough to stop the thermal diffusion of atoms on a
surface (Eigler)
Step 3: The demonstration of the ability to manipulate
individual atoms with atomic precision using a low-temperature STM
(Eigler and Schweizer 1990, Nature 344:524-526)
Step 4: The discovery that the standing waves due to
surface-state electrons could be imaged with the STM (Crommie, Lutz
and Eigler 1993, Nature 363:524-527)
Step 5: Confinement of the surface-state electrons into
quantum corrals made from individually placed atoms (Crommie, Lutz
and Eigler, the article that accompanied the quantum corral image)
The steps are sequential and linked in the sense that
each step was not possible without the previous step, and each step
was motivated by the previous one.