PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 720 February 17, 2005
by Phillip F. Schewe, Ben Stein
        
QUANTUM-DOT PHOTON DETECTORS.  Physicists at Toshiba Research Europe and
the University of Cambridge have developed a device that can efficiently
detect single photons, an achievement that should assist researchers in a
number of diagnostic fields, such as medical imaging, chemical analysis,
and environmental monitoring.  The device depends on a quantum dot, a tiny
semiconductor island that, owing to its essentially zero-dimensional
physical extent (a disk 30 nm wide and 8 nm tall), forces electrons to
possess only certain discrete energies.  Indeed, quantum dots are sometimes
referred to as artificial atoms because of their small size and quantized
electron energy states.  This quantum dot is encased inside another
semiconductor structure called a resonant tunneling diode.  In the diode
two conducting gallium-arsenide layers are separated by an insulating
aluminum-arsenide layer.  If the GaAs layers have the right voltage
alignment a current can tunnel from the one layer to the other.  If
misaligned, little current flows.  Here's where the quantum dot comes in. 
The layers can be purposely slightly misaligned in such a way that capture
by the dot of a "hole" excited in the diode by an incident photon
can re-align the two GaAs layers, allowing the tunneling current to resume.
 In other words, the arrival of a photon in the dot results in the
switch-on of the diode.  This form of single-photon detection gets around
the frequent false detections arising from the avalanche of electrons
needed in the common amplified-photoelectron approach to photon detection. 
Right now, the device correctly detects single photons at a rate of 12%,
but this should shortly rise to 65%, Toshiba physicist Andrew Shields
(andrew.shields{at}crl.toshiba.co.uk,
44-1223-436900, www.QUANTUM.TOSHIBA.CO.UK) believes.  At that level the
dot-diode detector could speed up bit rates used in quantum cryptography
and other forms of quantum information processing. (Blakesley et al.,
Physical Review Letters, 18 February 2004)
        
BUBBLES REDUCE DRAG.  Physicists in the lab have now confirmed under
controlled conditions what shipbuilders have known for some time, that a
shot of bubbles can help reduce the drag encountered by a ship moving
through water.  Detlef Lohse and his colleagues at the University of Twente
in the Netherlands start with one of the classic fluid dynamics
experiments, a Taylor-Couette cell, consisting of a bath of fluid held
between two concentric cylinders, the inner of which rotates.  The drag
effect of the fluid on this inner cylinder can be measured with great
precision.  By introducing a stream of bubbles at the base of the cell, the
drag could be reduced by as much as 20%.  Conversely, by introducing a
stream of buoyant particles at the bottom, the drag was enhanced.    In
Japan, the largest shipbuilding nation in the world, the subject of bubble
drag reduction is very hot.  (Van den Berg et al., Physical Review Letters,
4 February 2005; contact Detlef Lohse, d.lohse{at}tnw.utwente.nl,
31-53-489-8076; http://www.tn.utwente.nl/pof/; see also http://www.fom.nl ;
for related Japanese result, see http://www.nmri.go.jp/index_e.html)

EVIDENCE FOR QUANTIZED DISPLACEMENT in nanomechanical oscillators.
Physicists at Boston University have performed an experiment in which tiny
silicon paddles, sprouting from a central stick of silicon like the vanes
from a heat sink, seem to oscillate together in a peculiar manner: the
paddles can travel out to certain displacements but not to others.  The
setup for this experiment consists of a lithographically prepared structure
looking like a double-sided comb (see picture at
http://nano.bu.edu/antenna-large.jpg ).  Next, a gold-film electrode is
deposited on top of the spine.  Then a current is sent through the film and
an external magnetic field is applied.  This sets the structure to
vibrating at frequencies as high as one gigahertz. This makes the structure
the fastest man-made oscillator.  (Atoms and molecules can vibrate faster
than this, but not any chunk of matter, until now.)  At relatively warm
temperatures, this rig, small as it is, behaves according to the dictates
of classical physics.  The larger the driving force (set up by the magnetic
field and the current moving through the gold electrode) the greater the
excursion of the paddles.  This is no more than Hooke's law. At millikelvin
temperatures, however, quantum mechanics takes over from classical
mechanics.  In principle, the energies of the oscillating paddles are
quantized, and this in turn should show up as a propensity of the paddles
(500 nm long and 200 nm wide) to displace only by discrete amounts.  The
Boston University experiment sees signs of exactly this sort of behavior. 
(Gaidarzhy et al., Physical Review Letters, 28 January 2005; contact
Pritiraj Mohanty, 617-353-9297, mohanty{at}buphy.bu.edu; lab website,
http://nano.bu.edu/quantum-motion.html )

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