At NPL we are working on a variety of frequency standards based on cold atoms. Reducing the temperature of the atoms or ion used for frequency metrology allows for long measurement cycles and greatly reduces velocity-related shifts of the clock transition frequency. To cool the atoms or ions, we use Doppler cooling methods, which exploit laser-atom interactions to reduce the temperature. This works for all types of atom-based clocks being used and developed at NPL, including microwave atomic fountain standards, trapped ion optical frequency standards, and neutral atom optical frequency standards.

Lattice clocks, which hold the clock atom in a dipole lattice trap during measurement, represent a potential leap forward for frequency standards research, and are candidates for a future redefinition of the second. At present there are two competing methods for frequency metrology at the highest level: single trapped ions and large ensembles of neutral atoms, both using narrow transitions that can be excited by optical frequencies. Ion-based standards are currently 'in the lead', as single trapped ions are extremely insensitive to external perturbations.Neutral-atom standards, however, come a close second. What they lose in larger systematic shifts of the transition frequency (mostly collision shifts and velocity effects due to the fact that the atoms cannot be held in their trap during the clock measurement) they gain in statistics. With a system consisting N atoms (where N is often greater than 10 million), it is as if we are able to do N single-atom experiments simultaneously, leading to an improvement by a factor of the square root of N.

The concept of a neutral atom lattice clock^[1] is a way to map the environmental insensitivity of the ion standards onto a system made up of millions of atoms. Interrogating the clock transition while the atoms are held in a trap made from a laser beam would normally cause large shifts in the energy levels of the atoms due to the Stark effect. However Katori et al. showed that there are certain calculable 'magic' frequencies for which the ground and excited energy levels of the clock transition shift by precisely the same amount, leaving unchanged the energy difference that we are trying to measure. The potential for this type of clock is so great that every major national metrology laboratory in the world is now working towards a neutral-atom lattice clock. At NPL we are developing a lattice clock based on neutral strontium atoms.

1. H. Katori et al., Phys. Rev. Lett. 91, 173005 (2003).