A transportable atom interferometer-based gravity gradiometer has been developed at JPL to carry out measurements of Earth’s gravity field at ever finer spatial resolutions, and to facilitate high-resolution monitoring of temporal variations in the gravity field from ground- and flight-based platforms. Existing satellite-based gravity missions such as CHAMP and GRACE measure the gravity field via precise monitoring of the motion of the satellites; i.e. the satellites themselves function as test masses. JPL’s quantum gravity gradiometer employs a quantum phase measurement technique, similar to that employed in atomic clocks, made possible by recent advances in laser cooling and manipulation of atoms. This measurement technique is based on atom-wave interferometry, and individual laser-cooled atoms are used as drag-free test masses.

Figure 1. As shown in the schematic of the Atom Interferometer-Based Gravity Gradiometer, the dual atom interferometers measure the gravity gradient over the measurement baseline d. The platform accelerations ap are effectively cancelled in the differential measurement. (T is the time between pulses, keff is the effective Raman laser wave number, and g is gravitational acceleration.)
The quantum gravity gradiometer employs two identical atom interferometers as precision accelerometers to measure the difference in gravitational acceleration between two points (Figure 1). By using the same lasers for the manipulation of atoms in both interferometers, the accelerometers have a common reference frame and non-inertial accelerations are effectively rejected as common-mode noise in the differential measurement of the gravity gradient. As a result, the dual atom interferometer-based gravity gradiometer allows gravity measurements on a moving platform, while achieving the same long-term stability of the best atomic clocks.

In the laboratory-based prototype (Figure 2), the cesium atoms used in each atom interferometer are initially collected and cooled in two separate magneto-optic traps (MOTs). Each MOT, consisting of three orthogonal pairs of counter-propagating laser beams centered on a quadrupole magnetic field, collects up to 109 atoms. These atoms are then launched vertically as in an “atom fountain” by switching off the magnetic field and introducing a slight frequency shift between pairs of lasers to create a moving rest frame for the trapped atoms. While still in this moving-frame molasses, the laser frequencies are further detuned from the atomic resonance (while maintaining this relative frequency shift) to cool the atom cloud’s temperature to 2 μK or below, corresponding to an rms velocity of less than 2 cm/s. After launch, the cold atoms undergo further state and velocity selection to prepare for atom interferometry. The atom interferometers are then realized using laser-induced stimulated Raman transitions to perform the necessary manipulations of each atom, and the resulting interferometer phase is measured using laser-induced fluorescence for state-normalized detection. More than 20 laser beams with independent controls of frequency, phase, and intensity are required for this measurement sequence.

Figure 2. The photograph shows the Quantum Gravity Gradiometer in the laboratory. The magnetic shields around the lower atom interferometer have been removed for clarity.
This instrument can facilitate the study of Earth’s gravitational field from surface and air vehicles, as well as from space by allowing gravity mapping from a low-cost, single spacecraft mission. In addition, the operation of atom interferometer-based instruments in space offers greater sensitivity than is possible in terrestrial instruments due to the much longer interrogation times available in the microgravity environment. A space-based quantum gravity gradiometer has the potential to achieve sensitivities similar to the GRACE mission at long spatial wavelengths, and will also have resolution similar to GOCE for measurement at shorter length scales.

This work was done by Nan Yu, Robert J. Thompson, James R. Kellogg, David C. Aveline, Lute Maleki, and James M. Kohel of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. NPO-46280

Topics:
Cartography Gravity Imaging and visualization Lasers Microgravity Optics Physical Sciences Satellites Soils Spacecraft Vehicles Weather and climate
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A Transportable Gravity Gradiometer Based on Atom Interferometry

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NASA Tech Briefs Magazine

This article first appeared in the May, 2010 issue of NASA Tech Briefs Magazine (Vol. 34 No. 5).

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Overview

The document discusses the development of a transportable gravity gradiometer based on atom interferometry, spearheaded by NASA's Jet Propulsion Laboratory (JPL). This innovative instrument aims to enhance the measurement of gravitational fields, which is crucial for various scientific and engineering applications, including Earth science and planetary exploration.

The gravity gradiometer utilizes light-pulse atom interferometers, which leverage quantum interference techniques to achieve high sensitivity and stability compared to traditional mechanical devices. The document outlines the operational principles of the instrument, emphasizing its ability to measure gravity gradients with high precision. The technology is designed to operate effectively in microgravity environments, where the atoms used as test masses can be nearly drag-free, allowing for longer interrogation times and improved measurement accuracy.

Key features of the gravity gradiometer include its compact and robust design, making it suitable for transportable platforms. The instrument aims to provide state-of-the-art performance in terms of atom shot-noise limited signal-to-noise ratio (SNR) and gravity gradient sensitivity. The document highlights the successful demonstration of the atom interferometer in laboratory settings, achieving significant sensitivity levels and showcasing the potential for future space applications.

The document also discusses the challenges faced during development, such as environmental noise and mechanical vibrations, which can affect measurement accuracy. Solutions like active phase feed-forward correction are proposed to mitigate these issues, ensuring reliable performance in various operational conditions.

In addition to its technical details, the document emphasizes the broader implications of this technology for satellite-based mapping of gravitational fields on Earth and other planets. The ability to monitor time-varying gravitational fields from a single satellite platform could revolutionize our understanding of geophysical processes and contribute to advancements in fields such as climate science, resource management, and natural disaster monitoring.

Overall, the document serves as a comprehensive overview of the ongoing efforts to develop a high-sensitivity, transportable gravity gradiometer, highlighting its potential impact on scientific research and practical applications in space exploration and Earth sciences.