Superconducting single-photon optical detector has been demonstrated
by a Russian-US collaboration. The researchers deposited ultrathin (5
nm), 0.2-micron-wide strips of superconducting niobium nitride on a sapphire
substrate. When a photon struck the film, a hotspot of excited electrons
was created that disrupted the superconducting current and generated a
detectable voltage spike. Rapid diffusion of hot electrons and cooling
by liquid helium healed the hotspot within about 30 ps, restoring the
superconductivity and allowing for a gigahertz repetition rate. The detector
is already finding uses in checking integrated circuits, where it can
record individual infrared photons that are released when a transistor
switches on or off. The researchers say that it might also be used in
practical quantum cryptography and as an efficient detector of optical
signals for wireless communications in space. (G. N. Gol'tsman et al.,
Appl. Phys. Lett. 79, 705, 2001.) --jrr
Insect senses suggest novel neural networks. Animals gather information
about their environments as sensory neurons generate minute electrical
signals in response to chemicals, light, sounds, and other stimuli. A
new model of neural networks, based on recent studies of insect olfactory
systems, suggests that neurons can be linked in a way that allows them
to identify many more stimuli than was previously possible. Researchers
from the Institute for Nonlinear Science at the University of California,
San Diego, propose that one neuron is able to delay the firing of another
neuron. This inhibitory capability means that a given stimulus leads to
a specific, robust, and reproducible time sequence of neural activity.
The researchers used observed features of a locust's antenna lobe, exposed
to fragrances such as cherry and mint, to guide development of the model,
which they call competitive networks, or winnerless competition (WLC).
They found that, with a network built of N neurons, their model
could identify roughly (N - 1)! different stimuli. That is, an
11-neuron WLC network should be able to identify more than 3 million items.
The WLC model helps explain why a rose, by any other name, would smell
as sweet--but wouldn't smell like an onion. Ultimately, the WLC model
may lead to high-capacity, potent computing networks that resemble an
insect antenna or a human nose more than a desktop PC. (M. Rabinovich
et al., Phys. Rev. Lett. 87, 068102, 2001.) --jrr
Evidence for the onset of quark effects in a nuclear reaction
has been observed. In low-energy processes, a nucleus is well described
by its constituent nucleons--neutrons and protons--and the mesons that
hold them together. When a very-high-energy particle strikes a nucleus,
however, it penetrates the nucleus so deeply that the reaction can be
described only in terms of quarks and gluons. Now, the several-GeV middle
ground is being explored. In experiments at Jefferson Lab in Virginia,
a multi-institutional collaboration used photons with energies up to 5.5
GeV to break up deuterium nuclei, and studied the angular distribution
of the resulting protons. When the emitted proton had a transverse momentum
of at least 1 GeV/c, the data were best described by quark-counting
rules. Protons with less transverse momentum were well described by the
nucleon-meson picture. The deduced distance scale for the interaction
at the crossover energy is about 0.1 fm, larger than many current theoretical
expectations for the onset of quark-counting-rule behavior. (E. C. Schulte
et al., Phys. Rev. Lett. 87, 102302, 2001.) --bps
A bronze-age belt from Syria has been studied with scanning Auger
microscopy. Researchers use the SAM technique to identify elements near
the surface of a sample by monitoring the kinetics of electrons that were
knocked out of the atoms by an incoming electron beam. In this way, a
collaboration of researchers from Italy, the US, and the UK has characterized
the ancient artifact's microchemistry with a precedent-setting spatial
resolution as good as 15 nm. Although bronze is an alloy of copper and
tin, the group found that the belt's original patina contained neither
of those elements at the surface; rather, the patina arose from silicates
and other compounds at the burial site. The researchers even spotted alloy
inhomogeneities introduced by the smith during the belt's manufacture.
Turn to page 32 for more on physics and archaeology.
(E. Paparazzo et al., J. Vac. Sci. Technol. A 19, 1126,
2001.) --pfs
How constant is the fine-structure constant? The most prominent
dimensionless parameter in nature,α = e2/4πε0ħc ≈ 1/137, is fundamental to understanding atomic structure. Modern
attempts at unified theories allow α and the
other fundamental coupling constants to change with time. Since 1999,
John Webb and his colleagues at the University of New South Wales in Australia
have been reporting astronomical evidence purporting to show that α
has changed slightly over cosmological time. They measure α
at different redshifts z by looking for deviant spacings between
absorption lines of quasar spectra due to intervening gas clouds along
the line of sight. Now, using data from the high-resolution spectrometer
at the 10-m Keck I telescope, they report that, during the cosmological
epoch from z = 0.5 to 3.5, the average value of α
was almost a part in 105 less than it is now. It was a four-standard-deviation
effect. A result so potentially important cries out for confirmation--or
refutation. (J. K. Webb et al., Phys. Rev. Lett. 87, 091301,
2001.) --bms
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