Science > Physics > PHYSICS NEWS UPDATE -- Number 720 February 17, 2005 by Phillip F.Schewe, Ben Stein
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Science > Physics |
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"Sam Wormley" |
| Date: |
17 Feb 2005 10:33:58 AM |
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PHYSICS NEWS UPDATE -- Number 720 February 17, 2005 by Phillip F.Schewe, Ben Stein |
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@crl.toshiba.co.uk,
44-1223-436900, http://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@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@buphy.bu.edu; lab website,
http://nano.bu.edu/quantum-motion.html )
***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
from physics meetings, physics journals, newspapers and
magazines, and other news sources. It is provided free of charge
as a way of broadly disseminating information about physics and
physicists. For that reason, you are free to post it, if you like,
where others can read it, providing only that you credit AIP.
Physics News Update appears approximately once a week.
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| User: "Uncle Al" |
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| Title: Re: PHYSICS NEWS UPDATE -- Number 720 February 17, 2005 by PhillipF.Schewe, Ben Stein |
17 Feb 2005 12:26:47 PM |
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Sam Wormley wrote:
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 720 February 17, 2005 by Phillip F. Schewe, Ben Stein
[snip]
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@buphy.bu.edu; lab website,
http://nano.bu.edu/quantum-motion.html )
Sweet! Now we want to see a graph of behavior vs. temperture to espy
the crossover.
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz.pdf
.
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