Atom Interferometer Gravity Gradiometers in the Laboratory

Short answer: Atom interferometers use cold atoms in free fall as test masses, and two of them at different heights form a gravity gradiometer that measures how gravity changes over a short distance. A 2019 paper reported a proof-of-principle measurement of the Earth's gravity gradient with a single cloud of rubidium-87 atoms. A 2022 preprint reported a compact dual-interferometer instrument with a resolution of 0.86 E and a measured local gradient of 3114(53) E.

What does a gravity gradiometer measure?

A gravity gradiometer measures the difference in gravitational acceleration between two points. Using the same atoms or two atom clouds makes common vibrations cancel in part, which is why the approach is of interest for sensing. The unit used in the 2022 abstract is the E, short for Eotvos, and the abstract gives its measured results in that unit.

What did the 2019 single-source experiment show?

The 2019 Physical Review A abstract reports a proof-of-principle direct measurement of Earth's gravity gradient using an atom-interferometer gradiometer with a single proof mass of cold rubidium-87 atoms. The gradiometer is in a double-loop configuration, which gives a gravity-gradient dependent phase shift insensitive to dc acceleration and constant rotation rate. The device can run as a gravimeter or a gradiometer by adding an extra Raman pi pulse. The authors measured the gradient first with a vibration isolation platform and then without seismic isolation, using the correlation between the interferometer signal and the vibration signal from an auxiliary classical accelerometer, because a double-loop geometry lacks common-mode vibration rejection. They say the simplicity of the setup, a single atomic source and one detection, and its immunity to rotation-induced contrast loss make it a possible candidate for onboard gravity-gradient measurements. That is a stated possibility and not a demonstrated use.

What did the 2022 preprint report?

The 2022 arXiv preprint, which has not necessarily been peer reviewed, presents a compact gravity gradiometer based on dual rubidium-85 atom interferometers using stimulated Raman transitions. The baseline is 44.5 cm, the interrogation time is 130 ms, and the sensor head has a volume of less than 100 litres. The authors optimize parameters for short-term sensitivity and use a rejection algorithm based on inverting the Raman wave vector to improve long-term stability. After an averaging time of 17000 s, they report a phase resolution of 104 microradians, corresponding to a gravity gradient resolution of 0.86 E, and describe it as the sub-E atom gravity gradiometer with the highest compactness to date, as far as they know. After evaluating and correcting system errors from light shift, residual Zeeman shift, the Coriolis effect and self-attraction, the instrument serves as an absolute gradiometer, and the local gradient is measured to be 3114(53) E.

How to read this

The two experiments use different isotopes and configurations, so their results are not directly comparable. The compactness and resolution claims are the authors' own. This post is based on abstracts and visible summary text and full texts were not checked. A third listed source on space-based gradiometry was left out because it concerns a funded technology programme rather than a measured result. This post makes no claim about products.

FAQ

What is an E?

The unit of gravity gradient used in the 2022 abstract. The preprint reports a resolution of 0.86 E after 17000 s averaging.

Why use a double-loop configuration?

According to the 2019 abstract it gives a phase shift that depends on the gradient but not on dc acceleration or constant rotation rate.

How compact was the 2022 instrument?

A sensor head of less than 100 litres with a 44.5 cm baseline.

Is it ready for field use?

The 2019 authors call their setup a possible candidate for onboard measurements. Neither abstract reports field deployment.