We propose a design for an atom-interferometric setup involving multiple 2D and 3D interferometric components with multiple atomic clouds for measuring the full tensor gravity gradient and discuss its benefits, limitations, and applications.
Quantum interferometer gravity gradiometer for 3D mapping is a project for developing the technology of atom interferometer-based gravity sensor in space. The atom interferometer utilizes atomic particles as free fall test masses to measure inertial forces with unprecedented sensitivity and precision. It also allows measurements of the gravity gradient tensor components for 3D mapping of subsurface mass distribution. The overall approach is based on recent advances of laser cooling and manipulation of atoms in atomic and optical physics. Atom interferometers have been demonstrated in research laboratories for gravity and gravity gradient measurements. In this approach, atoms are first laser cooled to micro-kelvin temperatures. Then they are allowed to freefall in vacuum as true drag-free test masses. During the free fall, a sequence of laser pulses is used to split and recombine the atom waves to realize the interferometric measurements. We have demonstrated atom interferometer operation in the Phase I period, and we are implementing the second generation for a complete gradiometer demonstration unit in the laboratory. Along with this development, we are developing technologies at component levels that will be more suited for realization of a space instrument. We will present an update of these developments and discuss the future directions of the quantum gravity gradiometer project.
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.
Gravity gradiometers based on atom-wave interferometry hold the promise for increased sensitivity, and suitability for space applications. This capability will significantly advance our knowledge of solid Earth and the oceans, as well as other solar system planets and their moons. The quantum interferometer gravity gradiometer is based on atom-wave interferometry, and is intrinsically more sensitive than laser interferometers, because of the finite mass of the atom and the shorter de Broglie wavelengths associated with atom waves. Recent advances in laser cooling and trapping have made quantum interferometer applications practical. In this paper the principle of the operation of the quantum gravity gradiometer will first be described, together with the progress in its development to date. Estimates for the performance, and projections for the sensitivity will also be presented, together with potential applications to gravity science. Over the past few years advanced in laser cooling has brought about a significant progress towards realization of ultra-high sensitivity gravity gradiometers based on atom wave (matter wave) interferometry. These instruments represent a powerful potential for sensitive measurements of the components of the gravity gradient tensor from space to provide access to three-dimensional maps of geophysical structures deep below the surface. The exquisite sensitivity associated with atom-wave interferometry holds a great promise for new gravity mapping and monitoring capabilities - higher measurement sensitivity, finer spatial resolution, and, most importantly, temporal monitoring. This implies measurement opportunities for the Earth Observing System to develop comprehensive models for the understanding of the planetary inner structure and dynamics, changes in ice sheets and ocean currents, changes in underground water storage, and weather, earthquake, and volcano prediction capabilities. In this paper, we will review briefly the principles of the quantum gravity gradiometer and its advantages in the space environment. We will then describe our laboratory experiment and report some of the component development work towards a space-borne system. Gravity gradiometers based on atom interferometers are currently under development at the Jet propulsion Laboratory under NASA sponsorship.
We report on the characterization of a dual cloud atom interferometer for gravity gradient measurements using third-order Bragg diffraction as atom optical elements. We study the dependence of the contrast and the gradiometer phase angle against the relevant experimental parameters and characterize the instrument sensitivity. We achieve a sensitivity to gravity gradient measurements of $2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\phantom{\rule{4pt}{0ex}}{\mathrm{s}}^{\ensuremath{-}2}$ (26 E) after 2000s of integration time.
We propose a method for using a single-axis atom interferometric gravity gradiometer to measure off-diagonal elements of the gravity gradient tensor. By tilting the gradiometer, the measured gradient becomes a linear combination of different components of the gravity gradient tensor, and through multiple measurements at different tilts the separate tensor components can be inferred. We present a theoretical and numerical investigation of this technique, both for terrestrial surveys where the tilt is statically set by the user and for surveys where a strapdown sensor is dynamically tilted by the motion of the platform. We show that the gradiometer's sensitivity to the vertical gravity gradient is only slightly reduced by this method while allowing for more gradiometer information to be obtained. Major sources of error and loss of sensitivity on dynamic platforms are shown to be mitigated using an optical-gimbal technique employing commercially-available fibre-optic gyroscopes and tip-tilt mirrors.