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| Physics Update
- April 2000 |
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| Chemistry
in a Bose–Einstein condensate (BEC).
Physicists at the University of Texas at Austin, led
by Daniel Heinzen, created diatomic molecules (dimers) using
stimulated free–bound transitions in a BEC of individual rubidium-87
atoms. They illuminated the BEC with two laser fields that had
frequencies a mere 636 MHz apart, a difference equal to the
dimer’s binding energy. In this situation, a pair of nearby
87Rb atoms simultaneously absorb a photon from one laser field,
then emit a photon into the other field, binding to each other
in the process. Unlike molecular recombination in three-body
collisions, the 87Rb2 dimer was formed essentially at rest.
The lack of kinetic energy made high-precision spectroscopy
possible, and a line width of a scant 1.5 kHz—10 000 times narrower
than for previous experiments in laser-cooled gases—was measured.
Such high resolution, in turn, allowed the group to measure
molecule– condensate interactions for the first time. (R. Wynar
et al., Science 287, 1016, 2000.) |
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| The
limits of control have been derived. MIT researchers
Seth Lloyd and Hugo Touchette combined statistical mechanics,
thermodynamics, and information theory to examine the complementary
roles of information and uncertainty in control processes. From
the perspective of thermodynamics, controlling a system means
reducing its disorder, or entropy; that reduction also removes
some of our uncertainty about the system and therefore increases
our information about it. The two theorists analyzed an arbitrary
system coupled to an uncontrollable environment. Such a system—which
can be closed, open, linear, nonlinear, chaotic, quantum, or
more complex—is monitored by an appropriate measurement apparatus
and acted on by a controlling device. The controller itself
can be either open-loop (acting independently of the state of
the system) or closed-loop (based on some information gathered
about the system). Lloyd and Touchette established a formalism
for looking at the general control problem, and showed that
the amount of entropy that a controller can remove from a dynamical
system has an upper bound. They believe that their statistical
approach is particularly suited for controlling chaotic dynamics
and quantum systems. (H. Touchette, S. Lloyd, Phys. Rev.
Lett. 84, 1156, 2000.) |
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 Magnetic
mirage in a quantum corral. The scanning tunneling
microscope enables researchers to push individual atoms around
on a surface and to image them. Shown here is an elliptical
quantum corral made of 36 cobalt atoms carefully positioned
on a copper surface. An extra magnetic cobalt atom is at one
of the two foci of the ellipse, where its magnetic moment interacts
with the confined surface electron waves, and is seen as the
Kondo effect (the purple peak). Remarkably, the same Kondo effect
is seen at the other focus, where no magnetic impurity exists.
Nonmagnetic atoms, or atoms placed off of a focus, produced
no such phantoms. The IBM—Almaden physicists speculate that
it may be possible to perform “remote spectroscopy” on such
a mirage rather than on a real atom or molecule (thus avoiding
atomic perturbations). (H. C. Manoharan, C. P. Lutz, D. M. Eigler,
Nature 403, 512, 2000.) |
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Previous
Physics Updates:
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| Neutral
atoms on a curvy track. Physicists at the University
of Colorado at Boulder and the nearby National Institute of
Standards and Technology facility sent laser-cooled rubidium
atoms into a 10 cm long, 100 mm wide channel between two current-carrying
wires attached to a glass substrate. The atoms were attracted
to the low magnetic field along the channel’s center. The “atom
waveguide” followed a path similar to that of a pedestrian avoiding
a lamppost, curving out, around, and back to the original trajectory.
All three curves had a 15 cm radius of curvature, and as many
as two million atoms per second were sent through the course.
Part of a growing toolbox of atom optics components, the new
waveguide may find use in atom interferometers and in other
forms of high-precision metrology. (D. Mueller et al., Phys.
Rev. Lett. 83, 5194, 1999.) |
| A
d-wave p-squid
(superconducting quantum interference device) has been built
by a group led by Jochen Mannhart at Augsburg University in
Germany, together with Chang Tsuei of IBM—Yorktown Heights.
The working fluid of superconductors consists of Cooper pairs
of electrons or holes that form a macroscopic quantum state
with specific symmetry properties. For example, most low-temperature
superconductors have a spherical, or “s-wave,” symmetry—if you
imagine one electron at the origin of some coordinate system,
the likelihood of finding the paired electron is pretty much
the same in all directions. In high-temperature superconductors,
the symmetry is thought to resemble a four-leaf clover, and
is referred to as d-wave. A fundamental consequence of d-wave
symmetry is a phase-change of p between
neighboring lobes of the clover in the quantum wavefunction
describing the Cooper pair. The new device, dubbed the p-SQUID,
uses one standard Josephson junction and one so-called p-junction,
and it might prove useful for novel superconducting electronics
or even superconducting qubits in a future quantum computer.
(R. R. Schulz et al., Appl. Phys. Lett. 76, 912,
2000.) |
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©
2000 American Institute of Physics
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