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| Physics Update
- May 2000 |
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| Entanglement
of four particles
has been experimentally achieved by researchers at the National
Institute of Standards and Technology in Boulder. Entanglement
refers to a special linking that persists between the wavefunctions
of particles (such as photons or ions) even when the particles
are physically separated or otherwise isolated from one another.
The NIST group used beryllium ions and, following a proposal
put forth last year by Klaus Mølmer and Anders Sørensen (University
of Aarhus, Denmark), first prepared the ions in their spin-down
state and nearly in the ground state of their collective motion.
Then, with a single, carefully tuned and timed laser pulse,
the four ions were driven through their coupled motion into
an entangled superposition of being all spin-down and all spin-up.
In principle, according to NIST’s Chris Monroe, the technique
can be used to entangle many more than four particles, with
evident usefulness for quantum information technologies. (C.
A. Sackett et al., Nature 404, 256, 2000. |
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| Early
cancer detection,
using backscattered light and an optical fiber probe, has been
developed. Led by Michael Feld, an MIT group developed a method
for detecting precancerous (dysplastic) tissue in the epithelium,
the layer of tissue lining the inner surfaces of the body, where
more than 85% of all cancers originate. Dysplasia involves characteristic
changes in the cell nuclei—they become enlarged, crowded, and
more varied in size and shape, and they contain more chromatin
(genetic material). As announced by Rajan Gurjar at the APS
March Meeting held in Minneapolis, the MIT researchers use a
narrow fiber-optic probe both to shine white light onto epithelial
tissue, and to collect the light that is backscattered by the
cell nuclei—light scattering spectroscopy applied to small spheres.
The spectral and polarization properties of the returned light
then provide information on the nuclei’s size distribution,
location, and (through the index of refraction) chromatin content.
The researchers have successfully identified precancerous colon
and esophageal tissue in clinical tests, and believe that their
technique will reach the commercial market in the next few years. |
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| Carbon-60
can superconduct—even
without chemical doping. When alkali metals are intercalated
into normally insulating C60, they add charges that turn the
material into a conductor and, at low enough temperatures, a
superconductor. But now, it seems, enough charges can be attracted
to the surface layers of a C60 crystal in a field-effect device
to turn the material into a superconductor without chemical
doping. That’s what Bertram Batlogg of Bell Labs, Lucent Technologies
reported at the Sixth International Conference on the Materials
and Mechanisms of Superconductivity, held this February in Houston,
Texas. Batlogg, Hendrik Schön, and Christian Kloc made a field-effect
transistor (FET) with a high-mobility single crystal of C60
and found that it became superconducting below 11 K. In an FET,
a sample of C60 is coated with an insulating layer and a gate
is placed on top. A voltage applied to the gate attracts charges
to the interface between the C60 and the insulator. With the
FET arrangement, the Bell Labs researchers estimate that they
have induced a surface charge of three electrons for each C60
molecule in the topmost layer. If only one or a few layers is
involved in conduction, then C60 may be a two- dimensional superconductor,
but it seems to exhibit behavior similar to the bulk superconductivity
seen in chemically doped A3C60, where A is an alkali atom. The
Bell Labs team suggests that the FET might be used as a superconductor-to-insulator
switch. |
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Previous
Physics Updates:
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| Highly
optimized tolerance (HOT)
has recently been proposed as a mechanism for generating complexity
and power laws. One characteristic of many natural and man-made
systems is power-law statistics. That is, the likelihood of
an event (such as a forest fire, power outage, or web file transfer)
occurring decreases as some power of the event’s “size.” In
addition, many systems are complex—made up of a plethora of
components whose individual properties are of little use in
predicting the behavior of the entire system. Interactions or
phenomena at many size scales (from very small to very large)
contribute to the overall state of these systems. One theory
that tries to explain all this is self-organized criticality
(SOC). Now, Jean Carlson of the University of California, Santa
Barbara, and John Doyle of Caltech have proposed an alternative
that not only generates power laws, but, they believe, provides
better insight into systems that are tuned—through either natural
selection or engineering design—for robust performance in uncertain
environments. They describe their model as “robust, yet fragile,”
in the sense that perturbations not accounted for in the design
(or evolution) may have especially far-reaching consequences.
For example, organisms and ecosystems are remarkably robust
with regard to large variations in temperature, moisture, nutrients,
and predation, but can be catastrophically sensitive to unexpected
perturbations, such as a novel virus or a crashing asteroid.
Carlson and Doyle say that, unlike SOC, the HOT theory accounts
for the “profound tradeoffs in robustness and uncertainty” that
drive complex systems. They believe that fundamental limitations
exist that could turn out to be as important as other conservation
principles in physics. They have been exploring the application
of their theory to a number of biological and engineering problems
with the help of experts in those fields. (J. M. Carlson, J.
Doyle, Phys. Rev. Lett. 84, 2529, 2000. Also see their
full paper in Phys. Rev. E 60, 1412, 1999.) |
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©
2000 American Institute of Physics
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