External-field shifts of the 199Hg+ optical frequency standard

Frequency shifts of the ^199Hg^+ 5d^10 6s ^2S_1/2 (F=0, M_F=0) to 5d^9 6s^2 ^2D_5/2 (F=2, M_F=0) electric-quadrupole transition at 282 nm due to external fields are calculated, based on a combination of measured atomic parameters and ab initio calculations. This transition is under investigation as an optical frequency standard. The perturbations calculated are the quadratic Zeeman shift, the scalar and tensor quadratic Stark shifts, and the interaction between an external electric field gradient and the atomic quadrupole moment. The quadrupole shift is likely to be the most difficult to evaluate in a frequency standard and may have a magnitude of about 1 Hz for a single ion in a Paul trap.Comment: 9 pages, RevTeX 4, submitted to J. Research of the National Institute of Standards and Technolog

Quantum Zeno effect in the decay onto an unstable level

Under certain assumptions it is shown that the decay of level 2 of a three-level system onto level 1 is slowed down because of the further decay of level 1 onto level 0. It is argued that this phenomenon may be interpreted as a consequence of the quantum Zeno effect. The reason why this may be possible is that the second decay (or accompanying photon radiation) may be considered as a sign of the transition 2 -> 1 so that during the first transition the system is under continuous observation.Comment: 9 pages, LATE