Fig. 1: Hourglass gradiometer.
a, Hourglass gradiometer using two counter-oriented single-beam MOTs, realized using mirror assemblies (blue). The initial atom clouds (green) fall under local gravitational acceleration, g, before being subjected to light pulses separated by time T to realize the atom interferometers (purple). The beam delivery is indicated with arrows (see Methods for details). The cooling beams (red) are deflected by the in-vacuum mirrors (blue) to provide cooling in all directions, with the central portion of each input beam passing through the aperture between the mirrors to provide the final cooling beam for the opposite MOT. The atom interferometry beams (yellow arrows) have a smaller beam waist, such that they pass through the mirror aperture without significant clipping. Each interferometer is operated simultaneously, with a vertical baseline separation of 1 m. b, Temporal variation of atom cloud temperatures from each trapping region (top panel), measured using time of flight41, and the relative change of the 1 m cloud separation baseline over time (bottom panel) (solid lines: averaged data at a bin size of 50 measurements at 4 s per measurement; shaded regions: σ range of the averaged data), determined from time of arrival. c, Measurement of the gravity gradient variation caused by movement of a test mass between two positions—either close to the sensor (open points) or displaced from the sensor (filled points). Each measurement number represents a specific position of the test mass, with the odd measurement numbers having the mass close to the sensor. Each data point is formed from the average of eight gravity gradient measurements, with each of those containing 25 shots from the atom interferometer each taking 1.5 s. The error bar for each data point is the standard error of the eight gravity gradient readings. The test mass was moved approximately every 20 min, with a variation of ±3.5 min, and its position was repeatable to approximately 1 cm. The modelled projection of the change in gravity gradient signal, ΔGzz, is shown in red.
