A gravimetric support network for Very Long Baseline Atom Interferometry

With the introduction of portable atom interferometers (AI), a genuinely independent method for the determination of g is available for the first time since the introduction of laser interferometer based in struments. Current AIs offer several advantages and already reach the accuracy of classical sensors. Additionally, a small number of stationary experiments were implemented for research in fundamental physics and geodesy. These instruments, extending the free fall distance of atoms to several meters, allow for longer evolution times of the wave function, thereby increasing the sensitivity of the AI compared to decimetres in portable devices. The construction of an AI with a 9 m interaction zone is currently being completed at Leibniz University Hannover.

The knowled ge of g and its gradient is required for the evaluation of systematic effects and uncertainties in AI experiments. Therefore, a gravimetric control network connected to one absolute gravimeter pier was established and repeatedly observed during the construction of the Very Long Baseline Atom Interferometry facility (VLBAI). Before the installation of the instrument, this network included the central axis of the VLBAI and one vertical off-axis parallel profile. The latter profile can also be observed during operation of the VLBAI. The effect of local gravity changes, e. g., hydrology, is comparable to 1 nm/s² on both axes. The gravimetric measurements serve as a reference during initial tests of the VLBAI. Repeated observations in the future will be used to characterize the effect of local hydrology and other mass variations along the vertical axis. A model of the research building and groundwater level monitoring supplements the gravimetric network. As the VLBAI is capable of measuring g and its vertical gradient with higher accuracy (<1 nm/s²) than classical instruments, the model will be used to transfer g to a gravimetry laboratory for gravimeter comparisons.

We present our strategy for gravimetric control of the VLBAI. This will provide a reference at first and will later be used to establish the VLBAI as a reference for gravimeter comparisons. The results of the first gravimetric campaigns and the comparison with the model of the VLBAI environment show an agreement within the instrumental uncertainties of the relative gravimeters used.