Solid-State Electrochemical Bidirectional Alkali Vapor Source for Cold Atom Microsystems
Authors: Kang Songbai , Mott Russell P. , Gilmore Kevin A. , Sorenson Logan D. , Rakher Matthew T. , Donley Elizabeth A. , Kitching John , Roper Christopher S. 
Published: February 2020

Cold atom microsystems have the potential to bring the precision measurement capabilities of cold atom clocks, gyroscopes, and accelerometers to portable systems and platforms (1). Trapping cold atoms in a vapor cell requires controlling alkali vapor pressure, which is a function of temperature. Using temperature control to control alkali vapor pressure is common in the lab, but requires too much power for portable microsystems. Techniques such as light-induced atomic desorption, pulsed alkali dispensers, and double MOT chambers have also been used to control alkali vapor density in the lab, but none is rapid, bidirectional, low-power, and miniaturizable as needed for portable cold atom microsystems. Solid ion conductors have been used with widely spaced Pt electrodes as alkali sources, but operate at high voltages (≥50V) and elevated temperatures (≥120°C) (2), thus requiring more power that allowable for portable microsystems applications.
We present a low-power bidirectional alkali vapor source based on solid-state electrochemistry. The device comprises a solid electrolyte, beta”-alumina, in between two electrodes. One electrode contacts the alkali vapor inside the vapor cell and consists of a uniform 10nm glassy carbon layer covered with widely spaced Pt lines. The other electrode, which serves as a solid-state alkali reservoir, consists of widely space Pt lines and graphite particles.
The thin glassy carbon layer getters rubidium until it becomes saturated. When a positive voltage is applied across the device electrodes, alkali atoms in the glassy carbon layer are oxidized and alkali ions transport into the solid electrolyte. This reduces the rubidium concentration in the glassy carbon and, rubidium vapor gettering resumes, causes rubidium vapor pressure reduction. Alkali ions are reduced at the other electrode and alkali atoms are intercalated in the graphite in the solid state alkali reservoir to maintain ion balance in the solid electrolyte. When a negative voltage is applied the process reverses. Alkali ions are reduced at the glassy carbon electrode, thus quickly saturating the glassy carbon with alkali atoms to stop rubidium gettering and further providing additional rubidium to evaporate and increase the rubidium vapor pressure in the vapor cell. Reversible actuation with this device has been demonstrated with voltages as low as 5 V and peak power as low as 3.4 mW. Furthermore, we will present the use of this device in a feedback loop to control Rb vapor density. The ability to electrically control alkali vapor pressure at low power is an enabling advance for cold atom microsystems.
This material is based upon work supported by the Defense Advanced Research Projects Agency (DARPA) and Space and Naval Warfare Systems Center Pacific (SSC Pacific) under Contract No. N66001-15-C-4027. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of DARPA or SSC Pacific.
1. J. Kitching, S. Knappe and E. A. Donley, IEEE Sensors J, 11, 1749 (2011).
2. J. J. Bernstein, A. Whale, J. Brown, C. Johnson, E. Cook, L. Calvez, X. Zhang and S. W. Martin, in Solid-State Sensors, Actuators and Microsystems Workshop, p. 180, Hilton Head Island, South Carolina (2016).