Atomic clock to sync handhelds
By Eric Smalley, Technology
keep extremely accurate time by measuring the vibration of atoms. They have
been around for 50-odd years, but they aren't in widespread commercial use
because they tend to be large and draw a lot of power.
Researchers from the National Institute of Standards and Technology
(NIST) have devised the principal component of an extremely tiny atomic
clock that will draw very little power.
The physics package, or atomic works, of the clock is 9.5 cubic
millimeters, or about the size of a grain of rice, and keeps time accurately
enough that it is likely to be off by no more than 25 microseconds per day,
which translates to no more than a second per 126 years, said Leo Hollberg,
a group leader in the Time and Frequency Division at the National Institute
of Standards and Technology (NIST).
The atomic works can be fabricated on standard computer chips using
existing manufacturing methods for making microelectromechanical systems.
This makes it potentially easy to mass produce and integrate with other
The chip-scale atomic clock consumes 73 thousandths of a watt, which
is comparable to common quartz crystal oscillators. The device is the key
step toward integrating battery-operated atomic clocks into handheld devices
like Global Positioning System (GPS) systems, radios and cellphones.
"Until now, atomic clocks were too big and required too much power
to even be considered for battery-powered instruments," said Hollberg. "One
application that seems likely in the near future is in GPS receivers, where
precise timing can enhance the performance... [and] reduce vulnerability
to jamming," he said.
The smallest previously developed atomic works is about one cubic
centimeter in size and uses several watts of power, according to Hollberg.
Precise timing in a small package also promises to improve secure
communications protocols and support higher data transfer rates, said Hollberg.
"As data rates increase, better timing resolution and synchronization are
required," he said.
The researchers' atomic works consist of a cesium vapor cell, a
semiconductor laser, a photodetector, a lens, and filters.
The device measures the vibration frequency of cesium atoms, which
is 9,192,631,770 cycles per second, and uses the measurement to calibrate
a quartz crystal electronic oscillator.
To take the cesium frequency measurement, the clock's laser excites
cesium atoms to the appropriate energy state for resonating with a microwave
field. A microwave field, tuned by an electronic oscillator, cycles through
a narrow range of frequencies close to the cesium atom's resonant frequency.
As the microwaves resonate with the cesium atoms, the atoms fluoresce, or
release energy in the form of photons, which are detected by the device's
photodetector. When the microwave field is tuned to the atoms' precise resonant
frequency, the atoms reach their maximum fluorescence level.
The measurement of the peak fluorescence calibrates the oscillator,
and this feedback loop locks the microwave field to the resonant frequency.
The researchers are working on making the clock accurate to one
microsecond per day, or one second in 3,171 years, dropping its power consumption
to 30 milliwatts, and putting it in a package that, including an oscillator
and control electronics, would measure one cubic centimeter, said Hollberg.
"Compared to the smallest commercial atomic clocks this would be a volume
reduction by a factor of about 50 times, and a reduction in power consumption
by about 100 times," he said.
Such a clock would be about 1,000 times more accurate than the high
quality quartz crystal oscillators used in many types of instruments today,
NIST's most accurate atomic clock, NIST-F1, takes more than 30 million
years to gain or lose a second. The device, which is the primary clock for
official U.S. time, occupies most of a room.
It will be about five years before the chip-scale atomic clock is
ready for commercialization, Hollberg said.
Hollberg's research colleagues were Svenja Knappe, John Kitching,
Li-Anne Liew and John Moreland from the National Institute of Standards
and Technology, and Vishal Shah and Peter D. D. Schwindt from the National
Institute of Standards and Technology and the University of Colorado at
Boulder. The work appeared in the August 30, 2004 issue of Applied Physics
Letters. The research was funded by the Defense Advanced Research Projects
Agency (DARPA) and the National Institute of Standards and Technology.
Timeline: 5 years
TRN Categories: Applied Technology; Physics
Story Type: News
Related Elements: Technical paper, "A Microfabricated Atomic
Clock," Applied Physics Letters, August 30, 2004
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