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MEMS..in
depth
MEMS... im detail |
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NIST
unveils chip-scale atomic clock
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NIST
physicist John Kitching displays the heart of the
world's smallest atomic clock. This "physics
package" is about the size of a grain of rice.
© Geoffrey Wheeler
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The
heart of a minuscule atomic clock---believed to be
100 times smaller than any other atomic clock---has
been demonstrated by scientists at the Commerce Department's
National Institute of Standards and Technology (NIST),
opening the door to atomically precise timekeeping
in portable, battery-powered devices for secure wireless
communications, more precise navigation and other
applications.
Described in the Aug. 30, 2004, issue of Applied Physics
Letters, the clock's inner workings are about the
size of a grain of rice (1.5 millimeters on a side
and 4 millimeters high), consume less than 75 thousandths
of a watt (enabling the clock to be operated on batteries)
and are stable to one part in 10 billion, equivalent
to gaining or losing just one second every 300 years.
In addition, this "physics package" could
be fabricated and assembled on semiconductor wafers
using existing techniques for making micro-electro-mechanical
systems (MEMS), offering the potential for low-cost
mass production of an atomic clock about the size
of a computer chip and permitting easy integration
with other electronics. Eventually, the physics package
will be integrated with an external oscillator and
control circuitry into a finished clock about 1 cubic
centimeter in size.
"The real power of our technique is that we're
able to run the clock on so little electrical power
that it could be battery operated and that it's small
enough to be easily incorporated into a cell phone
or some other kind of handheld device," says
physicist John Kitching, principal investigator for
the project. "And nothing else like it even comes
close as far as being mass producible."
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The
"physics package" of the chip-scale atomic
clock includes (from the bottom) a laser, a lens,
an optical attenuator to reduce the laser power,
a waveplate that changes the polarization of the
light, a cell containing a vapor of cesium atoms,
and (on top) a photodiode to detect the laser light
transmitted through the cell. The tiny gold wires
provide electrical connections to the electronics
for the clock.
NIST Photo
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The
mini-clock is comparable in size and long-term stability
to temperature-compensated quartz crystal oscillators,
currently used in portable devices. NIST scientists
expect to improve the clock's long-term stability and
reduce its power consumption to the point where the
device could substantially improve the performance of
many commercial and military systems that require precision
time keeping.
The chip-scale clock is the latest advance in time keeping
at NIST, which for decades has been a world leader in
the development of new technologies for measuring time
and frequency. Atomic clocks long have provided the
most accurate realizations of both of these quantities
but also have traditionally been large--up to two meters
in height--as well as power-hungry and expensive to
build.
The new clock is based on the same general idea as other
atomic clocks such as the NIST-F1 fountain clock--measuring
time by the natural vibrations of cesium atoms, at 9.2
billion "ticks" per second--but uses a different
design. In the chip-scale clock, cesium vapor is confined
in a sealed cell and probed with light from an equally
small infrared laser, which generates two electromagnetic
fields. The difference in frequency of these two fields
is tuned until it equals the difference between two
energy levels of the atoms. The atoms then enter a "dark
state" in which they stop absorbing and emitting
light; this point defines the natural resonance frequency
of cesium. An external oscillator, such as quartz crystal
like those found in wristwatches, then can be stabilized
against this standard.
The chip-scale clock is less accurate than larger atomic
clocks such as fountain clocks. However, the clock's
small size, low power dissipation and potentially low
cost make it ideal for a variety of commercial and military
applications. Compared to quartz crystal oscillators,
the most precise time and frequency references of equivalent
size and power, chip-scale atomic clocks potentially
offer a 1,000-fold improvement in long-term timing precision.
Chip-scale atomic clocks have many potential uses. In
wireless communications devices, these clocks could
improve network synchronization and channel selection
to enhance security and anti-jamming capabilities. In
Global Positioning System (GPS) receivers, small clocks
could improve the precision of satellite-based navigation
systems such as those used in commercial and military
vehicles and emergency response networks. In addition,
as atomic clocks get smaller and cheaper and use less
power, they could replace quartz crystal oscillators
in many common products such as computers, offering
several orders of magnitude better time keeping.
The integrated design described in the paper also could
be modified to make millimeter-scale magnetic field
sensors based on atoms as well as a variety of other
miniaturized spectroscopic tools and devices.
The work was supported by NIST and the Defense Advanced
Research Projects Agency (DARPA).
For more information on the chip-scale clock, go to
http://www.boulder.nist.gov/timefreq/ofm/smallclock/.
As a non-regulatory agency of the U.S. Department of
Commerce's Technology Administration, NIST develops
and promotes measurement, standards and technology to
enhance productivity, facilitate trade and improve the
quality of life.
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www.nano-tsunami.com
This
story has been adapted from a news release -
Diese Meldung basiert auf einer Pressemitteilung
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Deze
tekst is gebaseerd op een nieuwsbericht -
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MEMS
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