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KF5JRV > TECH 10.04.16 15:04l 79 Lines 4437 Bytes #999 (0) @ WW
BID : 1326_KF5JRV
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Subj: Optical Lattice Clock
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Sent: 160410/1256Z 1326@KF5JRV.#NWAR.AR.USA.NA BPQK1.4.65
Heralding a new age of terrific timekeeping, a research group led by a
National Institute of Standards and Technology (NIST) physicist has
unveiled an experimental strontium atomic clock that has set new world
records for both precision and stability—key metrics for the performance
of a clock.
The clock is in a laboratory at JILA, a joint institute of NIST and
the University of Colorado Boulder.
Described in a new paper in Nature,* the JILA strontium lattice clock
is about 50 percent more precise than the record holder of the past few
years, NIST’s quantum logic clock.** Precision refers to how closely
the clock approaches the true resonant frequency at which its reference
atoms oscillate between two electronic energy levels. The new strontium
clock is so precise it would neither gain nor lose one second in about
5 billion years, if it could operate that long. (This time period is
longer than the age of the Earth, an estimated 4.5 billion years old.)
The strontium clock’s stability—the extent to which each tick matches
the duration of every other tick—is about the same as NIST’s ytterbium
atomic clock, another world leader in stability unveiled in August,
2013.*** Stability determines in part how long an atomic clock must
run to achieve its best performance through continual averaging. The
strontium and ytterbium lattice clocks are so stable that in just a
few seconds of averaging they outperform other types of atomic clocks
that have been averaged for hours or days.
“We already have plans to push the performance even more,ö NIST/JILA
Fellow and group leader Jun Ye says. “So in this sense, even this new
Nature paper represents only a ‘mid-term’ report. You can expect more
new breakthroughs in our clocks in the next 5 to 10 years.ö
The current international definition of units of time requires the use
of cesium-based atomic clocks, such as the current U.S. civilian time
standard clock, the NIST-F1 cesium fountain clock. Hence only cesium
clocks are accurate by definition, even though the strontium clock has
better precision. The strontium lattice clock and some other experimental
clocks operate at optical frequencies, much higher than the microwave
frequencies used in cesium clocks. Thanks to the work at NIST, JILA
and other research organizations across the world, the strontium
lattice clock and other experimental clocks may someday be chosen as
new timekeeping standards by the international community.
The strontium clock is the first to hold world records for both precision
and stability since the 1990s, when cesium fountain atomic clocks were
introduced. In the past decade, the rapid advances in experimental atomic
clocks at NIST and other laboratories around the world have surprised
even some of the scientists leading the research. NIST, which operates
the NIST-F1 time standard, pursues multiple clock technologies because
scientific research can take unpredictable turns, and because different
types of atomic clocks are better suited for different practical
applications.
In JILA’s world-leading clock, a few thousand atoms of strontium are
held in a column of about 100 pancake-shaped traps called an optical
lattice formed by intense laser light. JILA scientists detect strontium’s
“ticksö (430 trillion per second) by bathing the atoms in very stable
red laser light at the exact frequency that prompts the switch between
energy levels.
To check the performance, the JILA team compared two versions of the
strontium clock, one built in 2005 and the other just last year. Both
clocks have set previous records of various types. In the latest work,
the two clocks fully agreed with each other within their reported
precision—demonstrating the ability to make a duplicate copy and maintain
the performance level. This is an advantage for clock comparisons to
lay the groundwork for the eventual selection of a next-generation
time standard.
Recent technical advances enabling the strontium clocks’ record
performance include the development of ultrastable lasers and precise
measurements of key effects—atom collisions and environmental
heating—that cause tiny changes in the clock’s ticking rate.
Next-generation atomic clocks have already contributed to scientific
research and are expected to lead to the development of novel
technologies such as super-sensors for quantities such as gravity and
temperature.
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