Clocks with trapped neutral atoms in optical lattices

Unlike ions, neutral atoms cannot be manipulated simply with a static or relatively low frequency oscillating electromagnetic field. However, they can be trapped in light potential wells created by a laser standing wave. These regularly spaced wells then constitute an optical lattice allowing several thousand atoms trapping, giving hope for a significant improvement in stability compared to a single ion clock. However, for a long time, the clock frequency shift due to this trapping light was considered as an inescapable obstacle to the high accuracy optical lattice clocks development. But a solution has been found with the trapping  laser wavelength adjustment to a “magic value” such that the clock transition energy levels of the trapped atoms are modified in the same way due to the laser light: the clock frequency is then not affected in first approximation.

Light potential well in a laser standing wave’s optical lattive
Strontium optical lattice clock. We observe the fluorescence of the trapped strontium atoms

Numerous optical lattice clocks are currently being developed in laboratories, using different atoms. It is with strontium atom (Sr) that the best frequency uncertainty – record of all categories – has been obtained to date: 2. 10-18.

In France, several clocks with strontium and mercury optical lattices have been produced. Mercury has the advantage of having six usable isotopes because they are abundant, including two fermions and four bosons. Another advantage is due to the value at least ten times lower for one of the dominant systematic effects (displacement due to ambient thermal radiation) compared to the other atoms.