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NIST
physicist Scott Diddams adjusts a femtosecond laser
system that is an important component of next-generation
atomic clocks based on optical rather than microwave
frequencies.
©2004 Bruce Erik Steffine
For
a high resolution copy of this image contact Gail
Porter.
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Three
of the world’s premier measurement laboratories—including
the Commerce Department’s National Institute of Standards
and Technology (NIST)—have lined up the “hash
marks” from four of the world’s best optical frequency
rulers and declared that they match. The experiments, reported
in the March 19, 2004, issue of the journal Science,1
are a significant step toward next-generation “atomic
clocks” based on optical rather than microwave frequencies.
Such clocks are expected to be as much as 100 times more accurate
than today’s best timekeeping systems.
Applications
for ultra-precise timekeeping include navigation, telecommunications
and basic scientific research.
Optical
“rulers” are lasers that emit pulses of light
lasting just 10 femtoseconds (10 quadrillionths of a second,
or 10 millionths of a billionth of a second). The experiments
demonstrated that femtosecond laser devices could be used
to reproducibly generate and accurately control the frequency
of electromagnetic fields—a critical step in taking
the measurement of time beyond its current accuracy level
of about 0.1 nanosecond per day (i.e., losing or gaining no
more than about 0.1 billionths of a second per day).
These devices are
called “frequency combs” because a graph of the
oscillating electromagnetic waves looks like the teeth of
a hair comb. The output of these frequency combs can be used
as a ruler for measuring time and frequency. For instance,
a femtosecond frequency comb can reproducibly divide an interval
of an hour into 10 quintillion (one followed by 19 zeroes)
segments of equal time. The combs also could be used in making
ultra-precise length measurements.
Until
a few years ago, the femtosecond laser devices were the missing
link in the engineering of optical atomic clocks. The world’s
current best atomic clocks, such as the NIST-F1 laser-cooled
cesium fountain clock, are based on microwave vibrations in
atoms with a frequency of about 9 billion cycles per second.
While this is very fast, electronic systems can accurately
count these vibrations.
But no
electronic systems exist that can directly count the optical
oscillations in atoms such as calcium and mercury at about
500,000 billion cycles per second. A frequency comb, functioning
like the electronics in a conventional clock, would be used
to divide the very fast oscillations of future optical clocks
into lower frequencies that can be linked to microwave standards
such as NIST-F1 and compared to distribution systems such
as the Global Positioning System (GPS) and broadcasts from
NIST’s radio stations.
“These
lasers are the gears of our next-generation clocks,”
says NIST physicist Scott Diddams, a co-author of the Science
paper. “Our experiments made certain that the gears
will run smoothly.”
NIST physicists
and collaborators compared the operation of four femtosecond
laser systems of different designs—two systems built
at NIST, one by the Bureau International des Poids et Mesures
in France, and one by East China Normal University in Shanghai.
The team
compared the devices in pairs, with reference to a third device
arbitrarily chosen as a standard, on six days over a period
of several months. The teeth of two combs were lined up and
then a radio frequency “beating” technique—the
optical equivalent of using a tuning fork to determine how
closely a piano key is tuned to the correct note – was
used to check the exactness of the match.
The NIST
experiments are the first to compare the operation of multiple
femtosecond frequency combs—thereby demonstrating reproducibility—and
to verify that both the starting position of a comb and the
spacing between the teeth can be
controlled precisely.
The lasers
used in the experiments emit light across a broad frequency
range, from the visible to near-infrared parts of the spectrum.
This versatility enables scientists to design frequency combs
with teeth that match various optical frequency standards
now under development, which, in turn, allows much better
performance and increases the likelihood of practical applications
resulting from the technology. One key application would be
optical clocks much more accurate than today’s best
clocks, such as NIST-F1.
Femtosecond
frequency combs could be used to make more accurate optical
clocks that could help answer research questions such as whether
fundamental physical constants—essential to many practical
calculations made in science—have changed very slightly
over billions of years. NIST scientists already have used
optical clocks to set limits on changes in one fundamental
constant (the “fine-structure” constant that describes
the strength of the electromagnetic force). 2 Further
studies of this type could help develop a better understanding
of the fundamental laws of nature.
Scientists
from OFS Laboratories in New Jersey collaborated on the frequency
comb comparisons with the team from NIST and the French and
Chinese institutions.
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.
1
Ma, L.S.; Bi, Z.; Bartels, A.; Robertsson, L.; Zucco, M.;
Windeler, R.S.; Wilpers, G.; Oates, C.; Hollberg, L.; and
Diddams, S.A. “Optical Frequency Synthesis and Comparison
with Uncertainty at the 10-19 Level.” Science
303, 5665: 1843-1845 (March 19, 2004).
2
Bize, S.; Diddams, S.A.; Tanaka, U.; Tanner, C.E.; Oskay,
W.H.; Drullinger, R.E.; Parker, T.E.; Heavner, T.P.; Jefferts,
S.R.; Hollberg, L.; Itano, W.M.; and Bergquist, J.C. “Testing
the Stability of Fundamental Constants with the 199Hg+
Single-Ion Optical Clock,” Physical Review Letters
90, 15:150802 (April 18, 2003).
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