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.”
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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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
To
receive a high resolution version of these images
contact
Gail Porter, gail.porter@nist.gov,
(301) 975-3392. |
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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on the image to get the high
resolution version.
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