Cycle Time Reduction in Trapped Mercury Ion Atomic Frequency Standards

The use of the mercury ion isotope 201Hg+ was examined for an atomic clock. Taking advantage of the faster optical pumping time in 201Hg+ reduces both the state preparation and the state readout times, thereby decreasing the overall cycle time of the clock and reducing the impact of medium-term LO noise on the performance of the frequency standard. The spectral overlap between the plasma discharge lamp used for 201Hg+ state preparation and readout is much larger than that of the lamp used for the more conventional 199Hg+. There has been little study of 201Hg+ for clock applications (in fact, all trapped ion clock work in mercury has been with 199Hg+); however, recently the optical pumping time in 201Hg+ has been measured and found to be 0.45 second, or about three times faster than in 199Hg+ due largely to the better spectral overlap. This can be used to reduce the overall clock cycle time by over 2 seconds, or up to a factor of 2 improvement.