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arXiv:physics/0105088 · Physical Review A

Sensitive absolute-gravity gradiometry using atom interferometry

J. M. McGuirk, G. T. Foster, J. B. Fixler, M. J. Snadden, Mark A. Kasevich

Published Feb 8, 2002• 585 citations
arXivHTMLPDF

Abstract

We report the demonstration of a sensitive absolute-gravity gradiometer based on light-pulse atom-interference techniques. The gradiometer consists of two absolute accelerometers operated in a differential mode. We report a differential acceleration sensitivity of $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}g/{\mathrm{Hz}}^{1/2}$ and an inferred differential acceleration accuracy of less than ${10}^{\ensuremath{-}9}g.$ This corresponds to a gravity-gradient sensitivity of $4E/{\mathrm{Hz}}^{1/2}$ ${(1E=10}^{\ensuremath{-}9}{\mathrm{s}}^{\mathrm{\ensuremath{-}}2})$ and an accuracy of better than $1E$ for a 10-m separation between accelerometers. We demonstrate that the instrument can be used to detect nearby masses in a vibrationally noisy environment and characterize instrument sensitivity to spurious acceleration and rotation noise.

DOI
10.1103/physreva.65.033608

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We report the demonstration of a sensitive absolute-gravity gradiometer based on light-pulse atom-interference techniques. The gradiometer consists of two absolute accelerometers operated in a differential mode. We report a differential acceleration sensitivity of 4\ifmmode×\else\texttimes\fi10\ensuremath−9g/Hz1/24\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}g/{\mathrm{Hz}}^{1/2}4\ifmmode×\else\texttimes\fi10\ensuremath−9g/Hz1/2 and an inferred differential acceleration accuracy of less than 10\ensuremath−9g.{10}^{\ensuremath{-}9}g.10\ensuremath−9g. This corresponds to a gravity-gradient sensitivity of 4E/Hz1/24E/{\mathrm{Hz}}^{1/2}4E/Hz1/2 (1E=10\ensuremath−9s\ensuremath−2){(1E=10}^{\ensuremath{-}9}{\mathrm{s}}^{\mathrm{\ensuremath{-}}2})(1E=10\ensuremath−9s\ensuremath−2) and an accuracy of better than 1E1E1E for a 10-m separation between accelerometers. We demonstrate that the instrument can be used to detect nearby masses in a vibrationally noisy environment and characterize instrument sensitivity to spurious acceleration and rotation noise.
We here present a high sensitivity gravity-gradiometer based on atom interferometry. In our apparatus, two clouds of laser-cooled rubidium atoms are launched in fountain configuration and interrogated by a Raman interferometry sequence to probe the gradient of gravity field. We recently implemented a high-flux atomic source and a newly designed Raman lasers system in the instrument set-up. We discuss the applications towards a precise determination of the Newtonian gravitational constant G. The long-term stability of the instrument and the signal-to-noise ratio demonstrated here open interesting perspectives for pushing the measurement precision below the 100 ppm level.
As part of a program to determine the gravitational constant G using multiple independent methods in the same laboratory, an atom gravity gradiometer is being developed. The gradiometer is designed with two magneto-optical traps to ensure both the fast simultaneous launch of two atomic clouds and an optimized configuration of source masses. Here, the design of the G measurement by atom interferometry is detailed, and the experimental setup of the atom gravity gradiometer is reported. A preliminary sensitivity of 3 × 10−9 g/Hz to differential gravity acceleration is obtained, which corresponds to 99 E/Hz (1 E = 10−9 s−2) for the gradiometer with a baseline of 0.3 m. This provides access to measuring G at the level of less than 200 parts per million in the first experimental stage.
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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 atomwave interferometry, and individual laser-cooled atoms are used as drag-free test masses. 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 10(exp 9) 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 K 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. 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.
Atom interferometry offers new perspectives for geophysics and inertial sensing. We present the industrial prototype of a quantum-based instrument: a compact, transportable, differential quantum gravimeter capable of measuring simultaneously the absolute values of both gravitational acceleration ggg and its vertical gradient ${\mathrm{\ensuremath{\Gamma}}}_{zz}$. While the sensitivity to ggg is competitive with the best industrial gravimeters, the sensitivity on ${\mathrm{\ensuremath{\Gamma}}}_{zz}$ reaches the limit set by quantum projection noise---leading to a long-term stability of 0.1E0.1\phantom{\rule{0.28em}{0ex}}\mathrm{E}0.1E ($1\phantom{\rule{0.28em}{0ex}}\mathrm{E}=1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{\ensuremath{-}2}$). This dual-purpose instrument constitutes the industrial integration of cold atom sensors for practical applications. It paves the way for different applications in geophysics, civil engineering, and gravity-aided navigation, where accurate mapping of the gravitational field plays an important role.