A ’70s song by the late singer Jim Croce begins, “If I could save time in a
bottle…” And when it comes to atomic clocks-those ultra-precise standard-keepers to
which other precision timekeeping devices are setósome do just that. Atoms of an
element are often held in a glass vacuum chamber whose walls are coated to prevent the
atoms’ collision with the walls from altering their internal compositions. Inevitably,
however, such collisions still distort the atoms and make them ‘tick’ differently,
causing the clocks to run fast or slow.
Now a team of physicists and engineers at NASA’s Jet Propulsion Laboratory,
Pasadena, Calif., has developed an improved way to release the time genie from its
bottle, so to speak. Building upon more than a decade of work on a frequency standard
called the linear ion trap, the JPL Frequency Standards Laboratory team has developed
and installed a new trapped ion atomic clock for the U.S. Naval Observatory in
Washington that essentially eliminates these walls. These recent JPL innovations are
expected to provide 20 times improved stability over previous trapped ion clocks. The
result is a clock that’s effective stability is equivalent to about one minute in 10 billion
yearsóthe approximate age of the universe.
The instrument, based on mercury ions, will be measured with a large ensemble
of atomic clocks operated to form a very stable, continuous timescale at the U.S. Naval
Observatory, which serves as the center of all U.S. Department of Defense timekeeping
and supports the needs of the Global Positioning System, or GPS. During this
evaluation, the ion clock will also be used as a frequency reference for transcontinental
time and frequency transfer comparisons to be performed between the Observatory and
the only other ion clock of its kind, located at JPL.
“These trapped ion atomic clocks are designed for long-term stability,
continuous operation and high reliability,” said Dr. Robert Tjoelker, supervisor of
JPL’s Frequency and Timing Advanced Instrumentation Development Group. “Long-
term timekeeping is an ideal application for the technology.”
In the linear ion trap frequency standard, mercury ions–atoms with an electron
removed– collide not with a wall but with an applied electric force field. The field
completely surrounds the ions, forming a container called an ion trap. “Atomic ions
colliding with this sort of ‘wall’ are disturbed about 10,000 times less than in glass cell-
based atomic clocks,” said Dr. John Prestage of the JPL Quantum Sciences and
Technology Group. Because the mercury ions have a positive charge, they can be held
with oscillating electric fields in a container produced with metallic electrodes inside an
ultra-high vacuum system, and made into a clock.
Like all clocks, atomic clocks measure frequency of a recurring event to keep
time. A wonder of quantum mechanics that govern the world of atoms is that every
isolated atom in the universe is exactly the same as every other atom of the same
element and containing the same number of neutrons. Atomic clocks have unique
measurement capability because every atom or ion in the clock is quantum-
mechanically identical to every other one. Therefore, by measuring the transition of
atoms as they move back and forth between two energy levels, atomic clocks provide
an absolute reference for frequency and time. Their success is such that time and
frequency are today measured with far higher accuracy than any other physical
quantity.
One use of the time scale maintained at the U.S. Naval Observatory is to
monitor onboard GPS space clocks and reset them periodically to keep the GPS radio
navigation system working so well. These onboard clocks aren’t as accurate as the
ground clock ensemble maintained at the Observatory.
NASA uses atomic clocks to provide reliable and consistent navigation for
interplanetary space travel, where fractional disparities in clock tick rates can
dramatically affect the navigation of spacecraft. Trapped ion clock technology
currently operates in NASA’s Deep Space Network and is also being developed for
small, low-mass and low-power space flight applications.
The U.S. Naval Observatory performs an essential scientific role for the United
States, Navy and Department of Defense. Its mission includes determining positions
and motions of the Earth, Sun, Moon, planets, stars and other celestial objects,
providing astronomical data; determining precise time; measuring Earth’s rotation; and
maintaining the Master Clock for the U.S. Department of Defense. Observatory
astronomers formulate the theories and conduct the relevant research necessary to
improve these mission goals. This astronomical and timing data, essential for accurate
navigation and support of communications on Earth and in space, is vital to the Navy
and Department of Defense and is used extensively by other government agencies and
the public at large.
JPL is NASA’s lead center for frequency and time and is responsible for
technology development, generation, and distribution of ultra-stable reference
frequencies and synchronized timing signals for the Deep Space Network. NASA’s
Office of Space Flight, Washington, D.C., supports JPL’s linear ion trap frequency
standard research.
JPL is a division of the California Institute of Technology in Pasadena.
