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

We present the measurements of thermalized collisional rate coefficients for ultra-cold Li-7 and Rb-87 colliding with room-temperature He, Ne, N-2, Ar, Kr, and Xe. In our experiments, a combined flowmeter and dynamic expansion system, a vacuum metrology standard, is used to set a known number density for the room-temperature background gas in the vicinity of the magnetically trapped Li-7 or Rb-87 clouds. Each collision with a background atom or molecule removes a Li-7 or Rb-87 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 better than 1.6% for Li-7 and 2.7% for Rb-87. We find consistency-a degree of equivalence of less than one-between the measurements and recent quantum-scattering calculations of the loss rate coefficients [K & lstrok;os and Tiesinga, J. Chem. Phys. 158, 014308 (2023)], with the exception of the loss rate coefficient for both Li-7 and Rb-87 colliding with Ar. Nevertheless, the agreement between theory and experiment for all other studied systems provides validation that a quantum-based measurement of vacuum pressure using cold atoms also serves as a primary standard for vacuum pressure, which we refer to as the cold-atom vacuum standard.