September 23, 2002
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 set-some 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 with an effective stability equivalent to about one minute in 10 billion years -- the 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.
Contacts: JPL/Alan Buis (818) 354-0474
U.S. Naval Observatory/Geoff Chester (202) 762-1438