One of the NPL strontium ion traps

A narrow forbidden transition in a single ion confined in an electromagnetic trap is close to ideal as a frequency standard. By laser cooling, the ion can be confined to within a wavelength of light, ensuring that the transition is virtually free from Doppler frequency shifts. Since there is only one ion held in the trap, in a vacuum, the transition is also free from frequency shifts caused by collisions. Although electric and magnetic fields are present, these can be minimised and measured and upper limits calculated for the resulting frequency shift. Finally, the ion can be interrogated for long periods of time and observed with high efficiency and signal-to-noise using the quantum jump technique.

To achieve this ideal situation, an ion is produced by ionising an atom produced from an oven and then held in an electromagnetic trap. When the ion is formed, it is well above room temperature and must be laser cooled to confine it to the centre of the trap. This is achieved using a laser which is tuned slightly below the centre frequency of a strong (allowed) transition. A  typical term scheme is shown below. However, with most ions that are used as optical frequency standards, an ion in the upper state of the cooling transition can decay either down to the ground state or to another longer-lived state. This could eliminate fluorescence and hence cooling for periods of typically up to a second. To avoid this problem, a second (repumping) laser is used to drive the ion back up to the upper state of the cooling transition.

Typical term scheme for a trapped ion optical frequency standard

Since the upper level of the transition used as the optical frequency standard has a long lifetime (around 1 s or even longer), a very narrow linewidth laser must be used to probe the transition. To avoid broadening the transition via the ac Stark shift, it is important that the probe and cooling lasers are not on at the same time. A typical ion interrogation cycle therefore starts with the ion fluorescing and the probe laser off. The cooling laser is then switched off and the probe laser switched on for a short period, typically 30 ms to 300 ms. This switching is achieved using a combination of computer-controlled acousto-optic modulators and mechanical shutters. When the probe laser is turned off and the cooling laser is switched back on, we see whether or not the ion is fluorescing. If it is not, then a quantum jump event is recorded. A number of cycles (typically 40) are repeated before changing the probe laser frequency. A histogram of the number of quantum jumps as a function of frequency is recorded in this way, and used to find the centre frequency of the transition.

Optical frequency standards based on forbidden transitions in single laser-cooled trapped ions are being developed at a number of laboratories worldwide. At NPL we are studying standards based on strontium and ytterbium ions. The experience gained in our ion trapping research is also being used in quantum information processing experiments.