Mesoscopic physics in vapor-cell atomic clocks

In the early 1980s, it was common to hear that Rb atomic clocks were limited to Sy(τ) = 10^-11/τ^1/2 at best, and that their ultimate frequency stability could never be better than low-parts of 10^-13. Twenty-first century Rb clocks put the lie to those pronouncements. Today, it is not uncommon for vapor-cell clocks to exhibit short-term stabilities at low-parts of 10^-13/τ^1/2, and ultimate stability levels in parts of 10^-15. Here, we discuss what it will take to improve those clocks further (e.g., getting the long-term stability to break into the 10^-16 range). Specifically, it is our contention that next steps will require a better understanding of mesoscopic physics in vapor-phase systems: atomic phenomena occurring on a scale between the microscopic level of quantum dynamics and the macroscopic level of vapor temperature and pressure. Though at present we have some understanding of mesoscopic physics and its effects, much more needs to be understood. Further, while past and present advances have been driven by “one-off” phenomenological discoveries, it is our opinion that rapid advancement will be best facilitated by a more overarching vision of mesoscopic vapor-phase physics, which at present is lacking.