A narrow reference transition in a single trapped ion can be observed with high efficiency using the quantum jump, or electron shelving, technique.
A typical term scheme for a trapped ion optical frequency standard is shown above. The ion is laser-cooled using a strong (allowed) transition to a short-lived excited state and this results in a fluorescence rate which is high enough to monitor the single ion using photon counting techniques.
If another laser is used to drive a weak ('forbidden') transition, which might typically have an upper state lifetime of around 1 s, then sudden changes of fluorescence are observed as the ion is removed from the cooling cycle. If only one ion is in the trap, then the fluorescence is either in a 'high' or a 'low' state, and a typical plot of fluorescence with time is shown below. The abrupt changes in fluorescence as the ion moves into and out of the long-lived state are referred to as quantum jumps.
If more than one ion is in the trap, then the fluorescence level with time changes sharply between one of several levels, depending on how many ions are fluorescing. This provides a useful diagnostic for determining that there is indeed only one ion in the trap.
- H. Dehmelt, IEEE Trans. Instrum. Meas. 31, 83 (1982)