At NPL we are designing and implementing an optical lattice clock based on the ¹S₀ → ³P₀ transition at 698 nm in neutral strontium. There are many reasons we have chosen neutral strontium for our clock. First, it has both even and odd isotopes with reasonable abundance, the best of each being ⁸⁸Sr at 82% and ⁸⁷Sr at 7%, for which one can obtain enhanced samples. In order to get the atoms cold enough for loading into an optical lattice, strontium has both a broad (fast cycling) transition that is good for Doppler cooling - can reach sub-Doppler temperatures in odd isotope atoms - and a narrow (slow cycling, but not too slow) transition to allow for sub-Doppler cooling in the even isotopes. The clock transitions are quite promising, being in the red and easily accessible to standard diode laser technology. The odd isotope has a clock transition with a linewidth of approximately 1 mHz, allowed weakly through mixing of the hyperfine states. The same transition in the even isotope is doubly forbidden, but can be accessed using some recently discovered schemes involving applied magnetic or electric fields.

Energy level diagram for neutral strontium

Doppler cooling transition shown in blue (461 nm),
narrow-line cooling transition (689 nm) and clock transition (698 nm) shown in red

Our compact and diode-laser-based system includes a novel permanent magnet Zeeman slower, which eliminates the need for current driven magnetic field coils and the associated vibration-inducing water cooling and large power supplies. We will also be developing a lattice laser system that will create an optical lattice trap in which the neutral Strontium atoms will be held during the clock transition measurement. As part of our clock system development we are working on new methods for measuring and reducing the blackbody radiation (BBR) induced frequency shift of the clock transition. As an initial step we are incorporating a 2D MOT system to cool and compress transversely the Zeeman-slowed beam and also transfer the atoms to a measurement and lattice chamber away from the magnets of the Zeeman slower and the intense beam of thermal atoms and BBR coming from the Sr oven. A modular system is being designed to allow for the investigation of BBR systematic effects in a variety of experimental configurations. We are also developing a sub-Hz linewidth laser, which is needed to probe the 698 nm clock transition. The laser system is similar to that used in the NPL strontium ion optical frequency standard at 674 nm, where Hz level linewidths have already been demonstrated on ~3 s timescales. Further improvements to the laser linewidth and stability are expected via the use of novel ultra-stable cavity design geometries.