Atomic clocks have broad commercial and scientific applications.  The most visible application is the Global Positioning System (GPS).  Better clocks are needed for the synchronization of high-speed communication networks, interplanetary navigation, and tests of general relativity.

By using lasers to cool atoms to 1 microKelvin, all of the dominant error sources in room-temperature atomic clocks (such as Doppler shifts) can be reduced by a factor of 1,000 or more.  However, at 1 microK, the atoms are moving so slowly that their wave nature leads to cross sections as large as the square of the deBroglie wavelength, about a million square Angstroms!  The collisions cause a frequency shift in laser-cooled clocks that is the largest source of error.  We have demonstrated that using Rb reduces this largest error source by a factor of 50.  With new techniques, Rb clocks achieve accuracies approaching 1 part in 10¹⁷.  Currently, we're studying collisions of juggling atoms to dramatically increase the stability of fountain clocks.
More on the juggling Rb clock

In addition to our laboratory work, we have a project to build laser-cooled atomic clock to fly on the International Space Station.  RACE, the Rubidium Atomic Clock Experiment, takes advantage of the small collision shift of Rb and aims for the highest clock performance.
More on RACE

Future clock are likely to be based on optical frequencies.  Here the oscillators will be ultra-stable lasers.  Ultrastable lasers, nearly as good as the best microwave clocks, have already been demonstrated.  But to make a clock, you must be able to count the cycles of the oscillator and no electronics operate at optical frequencies of 5x10¹⁴ cycles per second.  Using a (mode-locked) femtosecond duration pulsed laser, others have shown how to divide the optical frequency by a factor of 1 million so that electronics can count the cycles.  Our work is focussing on demonstrating new ideas to make ultrastable lasers.

Finally, we are studying the basic quantum mechanics and unique aspects of low energy scattering with atoms.  These experiments are also performed in laser-cooled atomic fountains.  We've juggled Cs atoms in a fountain to study these collisions.  Future experiments with the juggling fountain will precisely probe the interatomic interactions and probe scattering effects and potentially test fundamental theories.  Juggling is now being used in the clock experiments above.
More on Quantum Scattering of Cs

E-mail: kgibble@phys.psu.edu

The new millennium begins when? USA Today.  You can see some news highlights on our Rb clock from the Hartford Courant, Physics News Update, & Science and, on the juggling Cs fountain, at PRFocus, & Physics World (Feb '99, p. 5).

Microgravity Program, the , the , and   University.
This work was initiated with support from a
Precision Measurement Grant and an NSF National Young Investigator award.