Atomic frequency standards (or clocks) precisely measure atomic structure, usually the ground-state hyperfine splitting of an alkali metal such as Rb, Cs, or K. Conventional atomic clocks, as found in laboratory standards and GPS satellites, use a vapor cell to contain the alkali metal. Our research focuses on the physics of vapor cells and the physics limiting the performance of vapor-cell atomic clocks.

Vapor cells are a wonderful tool to study atomic and molecular collisions, since a small amount of inert buffer gas is typically present in order to slow diffusion of the alkali-metal atoms to the cell walls. Collisions between alkali-metal atoms and the buffer gas lead to clock frequency shifts ("the pressure shift") and broadening effects. Through precision measurements of nonlinearity in the pressure shifts of different buffer gases, we have been able to study short-lived van der Waals molecules in vapor cells [1].

Frequency standards experience many other frequency shifting and broadening mechanisms. One particularly important effect is the "light shift", which is the clock frequency shift due to the light that is needed for optical pumping of the alkali-metal atoms. The light shift leads to drift and noise in the clock signal, since it changes in time, and is particularly troublesome for clocks that use lasers for optical pumping. In our research, we study effects that are much smaller than the light shift, and so it is crucial to remove it from our measurements. As a consequence, we discovered a new, simple method to suppress the light shift in laser-pumped clocks which takes advantage of how the light shift differs spatially throughout a vapor cell [2].

A research atomic clock. It's actually not very good as an atomic clock compared to frequency standards that can be purchased commercially. It is good for science, though!