PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 633 April 16, 2003   by Phillip F. Schewe, Ben Stein, and James
Riordon

THE FIRST SINGLE-MOLECULE, SINGLE-BASE-RESOLUTION DNA SEQUENCING has been
carried out by a Caltech group.    In this new approach, the bases forming
the backbone of the typical DNA molecule are viewed one by one in the act of
replicating.  To be more exact, a DNA polymerase molecule, acting as a
genetic xerox machine, copies a single strand of DNA by adding complementary
base units to it; the "fuel" for this process, the base molecules being
added, were fluorescently labeled beforehand (by attaching site-specific,
light-producing fluorophore molecules), so the DNA sequence could be
observed by microscope observations (schematic setup figure at
www.aip.org/mgr/png ).   Sequencing single-molecule DNA strands is
intrinsically difficult because of the high linear data storage density: the
bases are only about 3.4 angstroms apart along the DNA helix. Past efforts
to sequence bases through their fluorescence have been complicated by
background noise, a problem avoided by the Caltech scientists through
careful use of two laser pulses, one for producing pinpoint fluorescence and
another for nulling or "bleaching" the fluorescence in order to prepare for
the next base identification.
Stephen Quake (quake{at}caltech.edu) and his colleagues can currently identify
no more than about 6 bases in a row, so this research is still at the
proof-of-principle stage.   However, within about two years or so, Quake
believes, his process should be a factor of ten faster than standard
gel-electrophoresis techniques used to sequence DNA molecules on a wholesale
level, and several orders of magnitude cheaper.  (Braslavsky et al., Proc.
Natl. Acad. Sci., 1 April 2003.)

CHARGE SYMMETRY BREAKING has been observed in two experiments reported at
the recent American Physical Society meeting in Philadelphia.  In the 1930s,
physicist Werner Heisenberg proposed that the neutron and proton are simply
slightly different manifestations of the same particle, called the
"nucleon." Modern nuclear physics endorses this view: plenty of nuclear
reactions proceed exactly the same way if a proton takes the place of a
neutron, or vice versa. However, this close similarity breaks down in some
cases, leading to a situation known as "charge symmetry breaking" (CSB). In
separate experiments at the Indiana University Cyclotron Facility (IUCF) and
the TRIUMF cyclotron in Canada, researchers have made groundbreaking new
measurements of CSB (which, incidentally, is a nuclear-physics phenomenon
completely different from charge [C] conjugation in particle physics).  Such
CSB measurements can provide deep insights into why nature gave the neutron
and proton slightly different masses. At an even more fundamental level, the
CSB measurements can potentially yield more precise values of the mass
difference between the up and down quarks that make up protons and neutrons.
 Nuclear theorists are busily analyzing these new experimental results to
put tighter constraints on the up-down mass difference.
At the APS meeting, Ed Stephenson of Indiana University
(stephens{at}iucf.indiana.edu) announced the first unambiguous
identification of a rare process: the fusion of two nuclei of heavy hydrogen
to form a nucleus of helium and an uncharged pion, one of the subatomic
particles responsible for the strong force that binds nuclei together. This
process would not exist at all were it not that nature allowed a small
violation of charge symmetry. Over a two-month period, researchers observed
this rare reaction several dozen times, giving physicists enough data to
test theories of charge-symmetry breaking.
Representing a collaboration at TRIUMF, Allena Opper of Ohio University
(opper{at}ohiou.edu) discussed the detection of CSB in another nuclear
reaction: the fusion of a proton and neutron, which produces a charged pion
as one of its products.  Viewed from a perspective or ("reference
frame") at
which the proton and neutron meet at the center, the reaction, repeated man
times, produces a small excess of pions (0.17%) in a preferred direction.
Such an asymmetry is a hallmark of CSB. Taken together, these new CSB
results promise a wealth of information on such things as the slightly
different electromagnetic fields inside each nucleon. As it turns out, such
fields may contribute to the proton-neutron mass difference, as they carry
energy which convert into a small amount of mass.

TUNABLE PHOTONIC CRYSTALS. Photonic crystals affect the flow of photons in
much the same way that electronic devices affect the flow of electrons. Most
photonic crystals, however, have specific properties that cannot be varied
once the crystals are made. A few types of photonic crystals, such as fluid
suspensions of colloidal silica, can be modified on the fly, but the time
required to change configurations is inconveniently long. Researchers at
Brown University  have now made photonic crystals that can be modified in
milliseconds. The tunable photonic crystals consist of a class of materials
known as holographic-polymer dispersed liquid crystals (H-PDLCs). Complex
structures are defined in the material by exposing it to an interference
pattern produced by a set of four laser beams. Liquid crystal droplets form
in regions where the laser light interferes coherently; these droplets
constitute a photonic crystal. An electric field applied to the suspension
of liquid crystals modifies the refraction index of the droplets, which
changes the spectrum of light that the photonic crystals transmits. The new
photonic crystals are easily constructed on a wide range of scales, which
allows them to affect a wide spectrum of light, and can replicate
sophisticated structures including diamond lattices as well as anisotropic
lattices that affect light differently depending on the direction of
propagation through the crystal. Potential applications of the tunable
photonic crystals include filters to selectively block certain light
frequencies. With further improvement, they may also lead other optical
devices such as to novel lasers and optical waveguides. Jun Qi of Brown
University (jun_qi{at}brown.edu, 401-863-3078) described the tunable photonic
crystals in a paper he presented recently at the Optical Fiber and
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