Physics of the Mercury Trapped Ion Frequency Standard

Frequency Standards Laboratory

The LITS atomic clock, developed by the Time and Frequency Systems Research Group at JPL, incorporates a pencil-shaped line of mercury ions which is suspended in space by radio-frequency fields. This isolates the ions from any contact with walls or other containment devices that could cause a variation of their internal frequencies. Microwave absorption by the mercury ions (at a 40.5 GHz hyperfine transition in ¹⁹⁹Hg⁺) is sensed by ultraviolet fluorescence, and any variations of the fluorescence are used to stabilize the microwave frequency. By continuously correcting the microwave frequency in this way, ultra-high frequency stability of better than 1x10^-15 can be achieved.

The Linear Ion Trap (LIT) structure, developed to increase the previously low signal-to-noise ratio associated with trapped ion frequency standards, holds 20 times the number of ions as conventional traps. The combination of the large number of ions and very high line Q results in a frequency standard with a stability that is nearly equal to that hydrogen masers for averaging times to 10,000 seconds - beyond which the mercury-ion standard exceeds the stability of any other frequency standard. This frequency-standard technology gives more stability per dollar and per mass than hydrogen masers and is certain to improve as development continues.

Two properties of atomic transitions are exploited to produce an ultrastable frequency. First, the atoms as resonators are more reproducible than any other cavity or crystal resonator. Second, the atomic transistions are far more immune to changes in the environment than any other oscillator. The atomic resonance is as narrow as 17 mHz at the 40-GHz transition. This is the highest line quality factor (Q) ever measured in a microwave atomic transition - 10,000 times greater than the line Q for current cesium-beam clocks.

Microwaves near 40.5 GHz enter the trap region from the left. State selection light from the ²⁰²Hg discharge lamp enters from the right, focused onto the trapped ions, and collected in a Pyrex horn. Fluorescence from the trapped ions is collected in a direction normal to the page.

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