Accurate measurement of the loss rate of cold atoms due to background gas collisions for the quantum-based cold atom vacuum standard

We present experimental measurements of thermalized collisional rate coefficients of ultra-cold ⁷Li and ⁸⁷Rb colliding with room-temperature He, Ne, N₂, Ar, Kr, and Xe. In our experiments, a vacuum metrology standard---a combined flowmeter and dynamic expansion system---is used to set a known number density for the room-temperature gas in the vicinity of magnetically trapped ultracold ⁷Li or ⁸⁷Rb clouds. Each collision with a background atom or molecule removes a ⁷Li or ⁸⁷Rb atom from its trap and the change in the atom loss rate with background gas density is used to determine the thermalized loss rate coefficients with fractional standard uncertainties that are better than 1.6 % for ⁷Li and 2.7 % for ⁸⁷Rb. We find consistency between the measurements and recent quantum-scattering calculations of the loss rate coefficients [J. Klos and E. Tiesinga, J. Chem. Phys. 158 014308 (2023)] except for the loss rate coefficient for ⁸⁷Rb colliding with Ar. Nevertheless, the agreement between theory and experiment for all other studied systems provides validation that a quantum-based cold-atom vacuum standard (CAVS) serves as both a sensor and primary standard for vacuum. Future work includes validating the range of operation of the CAVS and extending the list of background gases to other common gases found in vacuum systems, including CO, CO₂, H₂, and O₂.