Precision measurement of the Rb-H₂ cross-section for the cold atom vacuum standard

We have created a self-defining ultra-high vacuum (UHV) standard based on measuring the loss rate of Rb atoms from a magnetic trap due to collisions with background particles of density n, Γ=n <σv> [New J. Phys. 21 102001 (2019), Metrologia 57 025015 (2020), Metrologia 58 022101 (2021)]. The loss rate coefficient characterizing the collision, <σv>, varies with trap depth, U, and follows a universal law governing quantum diffractive collisions between particles. The value of the loss rate coefficient at zero trap depth can be determined from a fit to the universal function and the precision is dictated by the range of trap depths accessible experimentally. As H₂ is the predominant background constituent in the UHV, we have determined the value of the loss rate coefficient for Rb-H₂ collisions with an uncertainty of 3% by trapping Rb atoms in the |F = 1, m_F = -1> ground hyperfine state. Here, we have doubled the achievable trap depths by preparing the Rb atoms in the |F = 2, m_F = 2> hyperfine ground state, which has reduced the uncertainty of the measured loss rate coefficient for Rb-H₂ collisions down to the 1% level. This method can be generalized to measure the loss rate coefficients for Rb with other gases to further improve the precision of the cold atom vacuum standard.