Towards a Microwave-Laser Double Resonance Passive Frequency Standard Using Trapped Ytterbium Ions.

This thesis represents the research done in determining the feasibility of using ytterbium as the active ion in a trapped ion frequency standard. The ground state of ^{171}Yb^+ is split into two hyperfine levels (F = 0,1) separated by 12.64 GHz, the population of which can be manipulated by optically pumping the ions through an intermediary excited state (^2P_{1/2} or ^2P_{3/2 }) using laser systems operating at 369 nm or 329 nm respectively. Once an imbalance in the population of the hyperfine levels has been achieved, the influence of microwave radiation at 12.64 GHz can be observed. The extent the microwaves redistribute the ion population in the hyperfine levels depends on how closely the frequency of the microwaves match the true splitting between the hyperfine levels. The major obstacle in the development of a frequency standard based on the ytterbium ion is the existence of population trapping states. These metastable states prevent the ions from making the desired clock transition by retaining the ions in an energy eigenstate for a time corresponding to the lifetime of the eigenstate. Population trapping has been associated with metastable D and F-states whose energies are less than that of the excited P-state used for optical pumping. Several solutions to this problem were proposed and tested. The first solution involved using additional lasers to optically pump the ions out of the metastable D-states before population trapping occurred. This procedure met with some limited success but was abandoned for a more efficient technique of quenching the trap state with a buffer gas. Several different buffer gases were tested and we discovered that molecular nitrogen was extremely effective. A low pressure (rm P<=2.66 times10^-4Pa) quenching rate of (3.78 +/- 0.99times10^4)/sec/Pa was measured. This was the first effective demonstration of avoiding population trapping in excited ytterbium. Once the problem of population trapping was solved we were able to conduct microwave-optical double resonance spectroscopy on isotopically enriched ^{171}Yb^+. Using the method of Ramsey separated oscillatory fields we obtained 25 mHz wide Ramsey fringes with a signal-to-noise ratio of 350:1 in the shot noise limit. This yields a potential stability of {~}1.8times10^ {14}/sqrt{tau} and thus demonstrates the feasibility of using ^{171}Yb^+ as the active ion in a trapped ion frequency standard.