Number 698, August 26, 2004
by Phil Schewe and Ben Stein
The World's Smallest Atomic Clock
The world's smallest atomic clock, about the size of a rice grain,
is built around a microcell about 1 mm3 in volume filled with cesium
atoms. It draws only about 30 mA of current from a 2.5 V battery. Atomic
clocks are the best timekeepers because they are able to convert the
high-precision information contained in the light emitted by alkali
atoms (the light emerging from an atomic transition from one energy
level to another can be measured to an uncertainty of better than a
part in a billion) into a usable standard for defining the second.
The new miniature clock has a precision of 3.5 x 10-10.
What this means is that events can be timed with an uncertainty of about
one part in 3 billion. Scientists at NIST in Boulder, Colorado make
atomic clocks that are far more precise---the F-1 clock is good to about
one part in 10 trillion---but this requires a huge table-top’s worth
of equipment. The mini version being reported now should eventually
reach a stability of about 10-11, some 10,000 times better
than any quartz oscillator clock of equivalent size and power.
How will this new cheap, tiny, low-power, high-precision MEMS clock
be used? In satellites, GPS receivers, networked computer CPU’s, possibly
in cell phones. (Knappe
et al., Applied Physics Letters, 30 August 2004; contact
John Kitching, kitching@boulder.nist.gov, 303-497-3328; for an explanation
of precision and accuracy, see NIST
Time & Frequency glossary.)
Optical Funnel for Focusing Cold Atoms
A new experiment at the Tokyo Institute of Technology uses evanescent
light to focus cold atoms and output as a beam. Evanescent light is
the faint optical field (a sort of aura of light stuck on a material)
that is found on the material surface when a laser beam reflects away
from the material via "total internal reflection." In this case, the
focusing effect occurs when a hollow laser beam moving upwards splays
outward around a funnel-shaped piece of glass. The light, shone downward
and covering the inner edge of this funnel, helps to repel and cool
a blob of atoms held and chilled in a magneto-optical trap (MOT) and
falling slightly under the force of gravity.
Evanescent light has been used before to guide atoms through a hollow
optical fiber (see Update
272), but in the Tokyo work there are new features: high flux intensity,
low temperature, and small beam diameter. The funnel focuses an atom
swarm about 2 mm wide is forced to collimate down to the size of the
funnel's exit hole, which in the experiment was 200 microns, for a net
focusing factor of 100 (see figure).
Furthermore, a micron-sized hole is now being tested, which should result
in a focusing factor of a million, and a beam flux intensity of some
1015 atoms/cm2-s.
Akifumi Takamiazwa (Akifumi.Takamizawa@physik.uni-muenchen.de) says
that he and his colleagues hope to make a nanometer-sized funnel as
small as atomic de Broglie wavelength and use it eventually for single-atom
manipulation, perhaps for processes in which one atom can transfer one
bit of information. (Takamizawa et al., Applied
Physics Letters, 6 September 2004; also see http://uuu.ae.titech.ac.jp/research-e.html
and http://www.coe21-pni.titech.ac.jp/eng/task/index.htm)
Superprotonic Transitions
Electrons are the charge carriers in most electronic transactions.
Sometimes, in semiconductors, holes, the moving voids recently vacated
by an electron, constitute a usable current flow.
But positive ions can also act as an important current. Lead-acid batteries
in cars are a prominent application of this principle.
A particularly interesting phenomenon in this regard is the “superprotonic”
transition, an effect discovered in the 1980s by Russian scientists,
in which the proton conductivity jumps by several orders of magnitude
at a certain temperature, when a structural rearrangement of some of
the molecular oxyamion groups (such as SO4) occurs.
Sossina M. Haile and her colleagues at Caltech (smhaile@caltech.edu,
626-395-2958) have performed new experiments which have expanded the
roster of superprotonic materials, or cleared up past mysteries. For
example, they have cleared up any doubt that the solid-form acid CsH2PO4,
whose chemistry and conducting properties are especially promising as
a candidate for the electrolyte in fuel cells, can undergo the superprotonic
transition.
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