Number 302, January 8, 1997 by Phillip F. Schewe and Ben Stein
SYMPATHETIC COOLING, a process by which particles of one
type cool particles of another type, has been demonstrated for the
first time with neutral atoms. Using a combination of lasers and
magnetic fields, Christopher Myatt and his colleagues at NIST
and the University of Colorado (303-492-2548) trapped a group
of rubidium-87 atoms each having one of two possible values for
spin, a quantity that describes how a particle responds to a
magnetic field. Atoms with one spin value are less tightly bound
in the trap's magnetic fields and can be used to cool atoms with
the other spin value since the weakly confined atoms could more
easily escape the trap and carry away the energy given up by the
second species during collisions. Applying this technique to the
two rubidium spin species, the researchers have created, for the
first time, two overlapping clouds of Bose-Einstein condensates,
the new state of matter in which a group of atoms falls into
exactly the same quantum state. They also observed that the
BECs of the two rubidium species repelled each other.
Sympathetic cooling may help enable Bose-Einstein condensation
for rare isotopes, and may greatly facilitate comparative studies
between fermions and bosons. (C.J. Myatt et al., upcoming
article in Physical Review Letters.)
A NEW ELECTROLUMINESCENT DEVICE USES ONE-
TENTH THE VOLTAGE of previous devices. Head-mounted
displays (small enough to fit into a visor) in automobile, aircraft,
and microsurgery environments won't be practical until the
conversion of electricity into tiny parcels of light can be done
using small currents and voltages. At the heart of a thin-film
electroluminescent (TFEL) device is a host material such as ZnS
doped with luminescing centers such as Mn atoms. On either
side of this material are insulating layers which serve as suppliers
of electrons. High electric fields, supplied by a voltage applied
across the whole sandwich, launch electrons into the ZnS where
they strike a manganese atom, which emits a photon. A new
TFEL concept developed at Georgia Tech (Christopher Summers,
chris.summers@gtri.gatech.edu) employs much thinner insulating
layers, which permits the electrons to reach their necessary
velocity using much less voltage: 15-25 V instead of the
customary 150-200 V. The efficiency of the new device is still
low and the cost of growing the crystalline insulating layers is
comparatively high, but the lower-voltage requirements, and the
smaller circuitry this will permit, may make the approach
worthwhile. (C.J. Summers et al., upcoming article in Applied
Physics Lett.)
DIGITAL MIRRORS .Texas Instruments has developed a digital
micromirror device (DMD), basically a planar array of thousands
of tiny, independently-steerable mirrors. Each pixel in the device
consists of a mirror (only 16 microns across) mounted on a
hinged platform. A signal sent to an electrode makes the mirror
tilt forward or backward; a beam of light aimed at the pixel is
thereby reflected toward a viewing screen or scattered into
oblivion. This compact, fully digital form of optical switch is
not yet available in a commercial product, but it may have
advantages over liquid crystals in large projection display
systems. (Physics World, December 1996.)
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