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Good Timing For Nanoscale Atomic Clocks
by David Pescovitz
spectrum is a dwindling natural resource. By some estimates in less
than a decade there will be no more frequencies left for the next-generation
of palmtop computers and handheld communicators. But according to
mechanical engineering professor Albert P. Pisano, director of Berkeley's
Electronic Research Laboratory, outfitting every wireless device
from a next-generation palmtop computer to a basic FM radio
with a nano-mechanical clock that's accurate down to ten
quandrillionths of a second per day could reopen the radio spectrum
for tomorrow's new business.
slide compares the components of a conventional atomic clock
with the design for the Integrated Nano Mechanical Regulated
Atomic Clock. (Click for larger
Image courtesy Al Pisano
"Nanotechnology is going to revolutionize how you divide the frequency
spectrum and what you use it for," says Pisano, who several months
ago with Berkeley professors Liwei Lin and Luke P. Lee, Cornell
University professor Amit Lal, and industrial partner Frequency
Electronics, Inc. launched the Integrated Nano Mechanical Regulated
Atomic Clock project.
"Now, FM stations are .2 megahertz apart," Pisano adds. "But what
if they could be .02 megahertz apart?"
Of course, atomic
clocks which calculate the passage of time (not the time
of day) based on the resonant frequency of specific kinds of atoms
are nothing new. In most homes, atomic clocks have eliminated
the frustration of VCRs that annoyingly blink "12:00" by setting
themselves via an onboard radio receiver that "sets" itself based
on a radio signal from a centrally-located atomic clock maintained
by NIST (US National Institute for Standards and Technology) in
Fort Collins, Colorado. Traditional atomic clocks like the ones
that tell your VCR the time and are used in myriad scientific applications
are table-top rigs of power-hungry lasers, mirrors, and high-frequency
electronic circuitry that cost upwards of $1,000. Less accurate
atomic clocks that regulate data flow for the Internet are shoe-box
sized devices that consume 150 watts of power and cost $2000. Pisano
and his team hope to shrink the package down to one-centimeter cubed,
reduce the power consumed down to 50 milliwatts, and cut the cost
to possibly $100.
Al Pisano is also a director of the Berkeley Sensor and Actuator Center. (Click for larger
Peg Skorpinski photo
With atomic clocks the size of sugar cubes, Pisano says, next-generation
wireless devices can share radio frequencies based on time.
"Currently, signals are divided into different wavelengths," he
says. "But there's a limit to how close you can pack those wavelengths
together. To this "wavelength division" multiplexing you can now
add economical time-division multiplexing. You can pack far more
data into a spectrum if you not only spread it across frequencies
but also across time."
Pisano explains, is not so different than two people communicating
on walkie-talkies. Each user takes a turn talking, or transmitting
over the specified frequency. But with onboard atomic clocks, devices
could take turns that might last only 250 nanoseconds to so. That's
where the Integrated Nano Mechanical Regulated Atomic Clock comes
in. To prevent transmissions from stepping on each other's toes,
the devices need to track the passage of time with far more accuracy
than provided by classic crystal oscillator-based clocks like those
on your wall or wrist. Chip-scale atomic clocks are of great interest
to the military as well, potentially enabling "jam resistance and
strong-encryption in data communication...and missile and munitions
guidance," according to a project overview from the Defense Advanced
Research Projects Agency that sponsors Pisano and his team's research.
of the Nano Mechanical Atomic Clock will constructed at
UC Berkeley's state-of-the-art Microfabrication Laboratory.
(Click for larger image.)
Bart Nagel photo
Pisano's approach exploits much the same physics as full-size atomic
clocks but, he says, "we've taken everything that makes an atomic
clock large and require a lot of power and thrown it out."
So far, the group has developed a preliminary design and begun work
on several of the individual components necessary for a fully-fledged,
centimeter-cubed atomic clock. A "modestly-functioning" prototype
is still three years away, Pisano says.
The Integrated Nano Mechanical Regulated Atomic Clock will work
by using photons to pump the atoms in rubidium vapor to a higher
energy state. Once the atoms are pumped to a higher physical layer
inside the device, they're disturbed by radio frequency microwaves
generated by an oscillator so that they drop down again to a lower
layer. A tiny laser determines the opacity of that layer caused
by the quantity of atoms there and adjusts the frequency of the
oscillator to depopulate the higher layer as efficiently as possible.
"Basically, I have a bottle full of molecules that soak up light
if I shake the molecules at the right frequency," Pisano says. "Once
I know that the frequency is correct, I can compute how long it
takes for exactly one second to pass."
There are several technical and scientific challenges that the research
team will face, but one of the researchers' biggest hurdles will
be reducing the power consumption of the device so it's not dwarfed
by its batteries. The most practical way to ease power consumption,
Pisano explains, is by using the atomic clock as a periodic reference
for a standard crystal oscillator. Once a day or so, the atomic
clock could be "woken up" to recalibrate the crystal oscillator.
"We're essentially making a fat atomic wristwatch," Pisano says.
Pisano's Home Page
Electronics Research Laboratory
DARPA Chip-Scale Atomic
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