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.
The use of the 201Hg+ for an atomic clock is totally new. Most attempts to reduce the impact of LO noise have focused on reducing the interrogation time. In the trapped ion frequency standards built so far at JPL, the optical pumping time is already at its minimum so that no enhancement can be had by shortening it. However, by using 201Hg+, this is no longer the case. Furthermore, integrity monitoring, the mechanism that determines whether the clock is functioning normally, cannot happen faster than the clock cycle time. Therefore, a shorter cycle time will enable quicker detection of failure modes and recovery from them.