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arXiv:2201.03345 · Physical Review A

Compact differential gravimeter at the quantum projection-noise limit

Camille Janvier, Vincent Ménoret, Bruno Desruelle, Sébastien Merlet, Arnaud Landragin, Franck Pereira dos Santos

Published Feb 7, 2022• 90 citations
arXivHTMLPDF

Abstract

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 $g$ and its vertical gradient ${\mathrm{\ensuremath{\Gamma}}}_{zz}$. While the sensitivity to $g$ 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 $0.1\phantom{\rule{0.28em}{0ex}}\mathrm{E}$ ($1\phantom{\rule{0.28em}{0ex}}\mathrm{E}=1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{\ensuremath{-}2}$). 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.

DOI
10.1103/physreva.105.022801

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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 \ensuremathΓzz{\mathrm{\ensuremath{\Gamma}}}_{zz}\ensuremathΓzz​. While the sensitivity to ggg is competitive with the best industrial gravimeters, the sensitivity on \ensuremathΓzz{\mathrm{\ensuremath{\Gamma}}}_{zz}\ensuremathΓzz​ 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 (1E=1\ifmmode×\else\texttimes\fi10\ensuremath−9s\ensuremath−21\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}1E=1\ifmmode×\else\texttimes\fi10\ensuremath−9s\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.
Atomic gravimetry measures the gravitional acceleration of an ensemble of identical neutral atoms through the atom interferometry based on Raman two-photon transition, where the Raman lasers are used as “optical ruler" and the identical neutral atoms play the role of test mass and quantum storage of gravity field information. As the novel gravimetry system suffers no mechanical wear and works at a higher repetition rate, atom gravimetry can reach a much better short-term sensitivity than the classical falling-corner-cube counterpart, and the accuracy is reasonably well. Since the very first in-laboratory device built over 30 years ago, atomic gravimetry has matured to a versatile tool that offers both precise and accurate absolute gravity measurement to fundamental research and metrology, such as testing the weak equivalence principle or linking the mass and weight of the test mass in Watt (Kibble) balance. As the current rapid development of compact cold-atom gravimeters, atomic gravimetry is moving out of the laboratory to be used as a quantum enhancement of classical devices in dynamical measurement, such as geophysics survey, inertial navigation and space mission. We review the roadmap for the development of cold-atom absolute gravimetry, highlight recent advances in the dynamical measurement using compact cold atom gravimeters and discuss the associated technological challenges. In the future, with the maturity of its application in dynamical measurement or civil engineering and the innovation in atom interferometry theory, atomic gravimetry could play a more and more important role in our life.
We evaluate the sensitivity of a dual cloud atom interferometer to the measurement of vertical gravity gradient. We study the influence of most relevant experimental parameters on noise and long term drifts. Results are also applied to the case of doubly differential measurements of the gravitational signal from local source masses. We achieve a short-term sensitivity of $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ g/Hz\sqrt{\text{Hz}}Hz​ to differential gravity acceleration, limited by the quantum projection noise of the instrument. Active control of the most critical parameters allows us to reach a resolution of $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ g after 8000 s on the measurement of differential gravity acceleration. The long-term stability is compatible with a measurement of the gravitational constant GGG at the level of ${10}^{\ensuremath{-}4}$ after an integration time of about 100 h.
Inertial sensors based on cold atoms and light-pulse interferometry exhibit state-of-the-art sensitivity and ultra-low measurement bias that could revolutionize a variety of fields including geophysics and seismology, gravitational wave detection and fundamental tests of gravity, and inertial navigation. In the latter case, cold-atom interferometers are widely considered as breakthrough technology for future autonomous INSs. Nowadays, absolute quantum inertial sensors are available as commercial and field-deployable devices. Yet, their current size and complexity has not yet reached a technology readiness level compatible with mobile applications or plug-and-play operation by non-specialists. These systems are in desperate need of further development and miniaturization. We will present progress beyond these first experiments while exploring new methods relying on 3D matter-wave manipulation that can lead to interferometer geometries that are simultaneously sensitive to accelerations and rotations in 3D and can discern their vector components within a single measurement. These novel innovations produce additional sensitivity to rotations and gravity gradients which has not yet been exploited experimentally and could be a game changing innovation for future atomic sensors. The ability to measure the full acceleration and rotation vectors with a compact, high-precision, low-bias inertial sensor could strongly impact the fields of inertial navigation, gravity gradiometry, and gyroscopy.
Atom Interferometry methods open new prospectives for absolute acceleration measurements; the quantum features of matter and the employment of atomic test masses with manipulation technique of internal and external degrees of freedom lead to a new generation of quantum sensors. In this thesis I present the development of a prototype for a transportable sensor at system and subsystem level. An instrument with such characteristics can find applications in many felds. The work has been developed in the framework of two European projects respectively oriented to the realization of an atom interferometry based absolute gravimeter for geophysical and metrological applications and to the realization of a Space Atom Interferometer for tests in micro-gravity environment. In order to focus on the possible scientific landscape I briefy illustrate in the introduction the existing devices and their characteristic performances, pointing out how the atom interferometry may represent a valid alternative leading to a new generation of quantum devices. The study, the development and the realization stages for our instrument are presented either for the laser system realization and for the the implementation of the whole apparatus. This work leads to the development of a very compact atomic physics interferometric system responding to the scientific requirements that represent our target. From the laser system side it has been realized also a complete and compact apparatus organized in a modular framework with stand alone components characterized by high working performances. This laser subsystem also includes laser sources in an external cavity configuration specially oriented to transportable apparatuses; they are characterized by an intrinsical passive stability, robustness and very narrow emission linewidth. The experience with neutral atom manipulation and with atom cooling and trapping technique has been integrated with a direct implementation, characterization and optimization of a Two Dimensional Magneto Optical Trap as a cold atoms source for an operating Atom Interferometer. Such kind of implementation represents a fundamental topic for a transportable sensor realization.
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.