[tt] Physics News Update 838

[Eugen Leitl]

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From: physnews at aip.org
Date: Fri, 7 Sep 2007 11:03:26 -0400
To: eugen at LEITL.ORG
Subject: Physics News Update 838
Reply-to: physnews at aip.org


PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 838   September 7, 2007 by Phillip F. Schewe, Ben Stein
www.aip.org/pnu
        	
ACOUSTIC QUANTUM DOTS.  A new experiment at the Cavendish Lab at the
University of Cambridge is the first to controllably shuttle
electrons around a chip and observe their quantum properties.  A
quantum dot restricts electrons to a region of space in a
semiconductor so tiny as to be essentially zero-dimensional.  This
in turn enforces a quantum regime; the electron may only have
certain discrete energies, which can be useful, depending on the
circumstances, for producing laser light or for use in detectors and
maybe even future computers.
A quantum dot is usually made not by carving the semiconductor into
a tiny grain but rather by imposing restrictions on the electron*s
possible motions by the application of voltages to nearby
electrodes.  This would be a static quantum dot.  It is also
possible to make dynamic quantum dots-that is, moving dots that are
created by the passage of surface acoustic waves (SAWs) moving
through a narrow channel across the plane of a specially designed
circuit chip (see figure at http://www.aip.org/png/2007/289.htm).
The acoustic wave itself is generated by applying microwaves to
interleaved fingered electrodes atop a piezoelectric material like
GaAs. The applied  electric fields between finger-electrodes induce
a sound wave to propagate  along the surface of the material.
These acoustic waves have the ability to scoop electrons and
chauffeur them along the surface.
The tiny region confining the electron even as it moves is in effect
a quantum dot.  Such acoustic-based dynamic quantum dots have made
before, but according to Cambridge researcher Michael Astley
(mra28 at cam.ac.uk), this is the first time the tunneling of the
electrons (even single electrons) into and out of the quantum dots
has been observed.  This is an important part of the whole
electron-shuttling process since one wants control over the electron
motions and spins.  If, moreover, electrons in two very close
acoustic wave channels could be entangled, then this would present
the chance to make a sort of flying qubit, which could be at the
heart of a quantum computer.  (Astley et al., Physical Review
Letters, upcoming article; lab website at
http://www.sp.phy.cam.ac.uk/SPWeb/research/SAWQC/research.html#SAWQC)

CURLY HAIR GETS LESS TANGLED THAN STRAIGHT HAIR.  The hair on
people*s heads (typically 100,000-150,000 hairs per head) comes in
lots of shades, degrees of oiliness, and amounts of curliness.
Jean-Baptiste Masson, who works at the Laboratory for Optics and
Biosciences of the Ecole Polytechnique in France set out to study
the problem scientifically.  On the experimental front, he consulted
hairdressers and got them to count tangles in people*s hair.  On the
theoretical front, he devised a geometrical model of hair, hoping to
explain the results mathematically.  Tangles, defined as groupings
of hair that resist combing, proved to be almost twice as prevalent
with straight hair than with curly hair.  Masson
(jean-baptiste.masson at polytechnique.fr) explains this by saying that
although straight hairs interact with each other less frequently the
interaction is at great angles, and it is the relative angle between
hairs that causes tangles.  This in turn is a consequence of the
surface properties of the hairs.  One possible application of this
work on hair, Masson says, is in designing velcro-like products.
For instance, the velcro properties could be changed by adding extra
scales to the soft part of the velcro elements or by making the
tension o
f the strings higher-the equivalent of making the strands
straighter.  Masson, whose main field of research is biophysics,
expects his geometrical modeling might also be useful in the study
of polymers and other filamentary materials in the biological
world.  (American Journal of Physics, August 2007)

***********
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