AMO
In the literature: open
Atom-interferometric test of universality of free fall at 1e-15
In plain words
Two kinds of atoms dropped together should fall at exactly the same rate. Atom interferometers have checked this to about one part in $10^{12}$, while a satellite with solid masses reached one part in $10^{15}$.
Precise statement
Dual-species atom interferometry (Rb-85/Rb-87, Asenbaum et al 2020) bounds the Eotvos parameter $\eta$ at about 1e-12; MICROSCOPE bounds $\eta(\mathrm{Ti}, \mathrm{Pt})$ at about 1e-15 (2022). Determine $\eta$ for a pair of quantum test masses with uncertainty at or below 1e-15, for example in a $100\,\mathrm{m}$ class tower or in space.
What would settle it
A dual-species interferometer result for $\eta$ with total uncertainty below 1e-15 (MAGIS-100, AION, STE-QUEST class).