Text 335, 76 rader
Skriven 2005-02-17 17:53:27 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 720
===============
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, 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 )

---
 * Origin: Big Bang (1:106/2000.7)