Ion Trap as a Frequency Standard Measurement of Ba+ HFS Frequency Fluctuations

The 8-GHz "field independent" F = 1, mF = 0 to F = 2, mF = 0 ground level Zeeman transition frequency in 137Ba+ ions has been compared with a commercial Cs beam standard. About 10+ ions were stored in a RF quadrupole trap for several hours and cooled by He buffer gas. Hyperfine pumping was achieved by a pulsed tunable dye laser and the transition between the Zeeman sublevels was induced by a microwave signal locked to the primary frequency standard. The transition was observed via changes in the ionic fluorescence intensity. The observed frequency fluctuations ??v/v of < 2 ?? 10-12 over a 5-h measurement period are compatible with the specified fluctuations of the reference. Possible sources of frequency shifts were. analyzed and found to be controllable within the precision set by the atomic frequency reference.