NIST develops phase-modulation servos for atomic clocks. (News Briefs).

NIST scientists in Boulder, in collaboration with he Jet Propulsion Laboratory (JPL), have developed an improved modulation method for laser-cooled atomic clocks, providing for a high level of immunity to vibrations and substantial reduction of a number of systematic frequency shifts that can affect these clocks. The concept involves phase modulation of the interrogating microwave field, rather than the traditional frequency modulation used in most atomic clocks. In this new scheme, the phase of the microwave field in the first portion of the Ramsey cavity is fixed and the phase in the second Ramsey region is varied relative to the phase in first region. The advantage of the method is that the frequency of the microwave field can be kept continuously on the center of the resonance, rather than being stepped from one side of the resonance to the other, as is done using frequency modulation. At the peak of the resonance the clock is substantially less sensitive to vibration and to systematic effects than it is when the system resides most of the time on the steepest portion of the resonance curve.

The concept was initially developed to address the vibration sensitivity of the laser-cooled clock on the Primary Atomic Reference Clock in Space. In this collaborative program, involving NIST, JPL, the University of Colorado, and the Harvard-Smithsonian Center for Astrophysics, a laser-cooled cesium clock will be put aboard the International Space Station in 2005 to perform certain tests on gravitational theory and to improve upon the realization of the second. The modulation concept was tested on the NIST cesium fountain clock, NIST-F1, and found to work so well that it has become the preferred mode of operation. It has also been picked up by others and is being used on other fountain clocks around the world.

CONTACTS: Tom Parker, [phone]; tparker @boulder.nist.gov.