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A sharp resonance line that appears in three-photon transitions between the 1 S 0 and 3 P 0 states of alkaline earth and Yb atoms is proposed as an optical frequency standard. This proposal permits the use of the even isotopes, in which the clock transition is narrower than in proposed clocks using the odd isotopes and the energy interval is not affected by external magnetic fields or the polarization of trapping light. The method has the unique feature that the width and rate of the clock transition can be continuously adjusted from the M Hz level to sub-mHz without loss of signal amplitude by varying the intensities of the three optical beams. Doppler and recoil effects can be eliminated by proper alignment of the three optical beams or by point confinement in a lattice trap. The three beams can be mixed to produce the optical frequency corresponding to the 3 P 0 -1 S 0 clock interval.

We have employed the method of shelving to measure Zeeman resonances of a single trapped Ba + ion in low magnetic fields, and have observed radio frequency transitions with linewidths of 5 Hz, limited by magnetic-field noise. We have also observed the shift in the Zeeman frequency when the ion is illuminated by off-resonant light. A simultaneous measurement of such light shifts in two atomic states of Ba + will permit a precise determination of the ratio of transition matrix elements. Furthermore, using this method with an ion in a standing lightwave, a proposed parity nonconservation (PNC) measurement with Ba + or Ra + could be realized. In this paper we review methods and the status of these experiments and discuss the remaining challenges of the PNC experiment.

The extremely long lifetimes of the metastable D states of the Ba 1 ion make it difficult to measure them accurately due to their high sensitivity to collisional effects. Thus a discrepancy exists between theories and experiments for the Ba 1 D 3͞2 state. We have measured the D 3͞2 lifetime under a nearly ideal condition, i.e., single trapped Ba 1 ions in ultrahigh vacuum. The obtained value of 79.8 6 4.6 s resolves the existing discrepancy. The collisional quenching is found insignificant in our experiment, but there are indications of a non-negligible fine-structure mixing effect. [S0031-9007(97)