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Construction of a Rubidium Fountain Atomic Interferometer for Gravity Gradiometry

JN Tinsley

Published Jan 1, 2019• 2 citations
Publisher

Abstract

Atomic interferometry is a highly precise metrological technique of interest to both fundamental and applied physics. One field in which atomic interferometry is of particular interest is gravimetry, where instruments based upon the controlled manipulation of atomic states have superseded the performance of classical devices. In such devices, a standing wave derived from laser beams with well-defined phase difference are used to impart momentum to atoms undergoing free fall, generating spatial superpositions along the axis of gravity, before returning the atoms to an overlapped final state for recombination. This thesis presents the design and experimental progress towards a gravity gradiometer based upon the interference of rubidium-87 atoms, cooled and launched in a fountain configuration. The fountain is leveraged from a decommissioned frequency standard, which has been re-purposed and redesigned for use as a combined absolute gravimeter and gravity gradiometer package. It is capable of loading 100 billion atoms in 100 ms and then cooling and launching these atoms in a moving molasses configuration, with a final temperature of 5 μK. Furthermore, the process can be quickly repeated, with a second cloud subsequently launched, reaching its apogee at the same time as the first cloud, allowing for the possibility of simultaneous measurement at different heights and, in the future, the inference of the gravity gradient. Control of the fountain is provided by a stable and miniaturised optical setup, allowing for use with a future transportable or field-based device. A Raman beam system to perform the interferometry sequence has been constructed and characterised, likewise designed in a minaturised and stable manner. The Raman beams simultaneously generated Ramsey interference fringes on two independent clouds of atoms -- one launched to a height of 0.7 m and the other to 1 m. Such fringes were clearly observable after 200 ms of evolution time. Mach-Zehnder interference fringes were also demonstrated for dropped and launched atoms, providing the capability for gravimetric measurements to be made once the system has been fully characterised. The principle causes of noise in the Mach-Zehnder cases were identified as arriving from a lack of atoms and the phase noise of the Raman beams. To overcome these challenges, a new Raman beam system has been preliminary designed and tested and a new iteration of the fountain, moving towards quantum degeneracy, has been planned.

DOI
10.17638/03046742

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Atomic interferometry is a highly precise metrological technique of interest to both fundamental and applied physics. One field in which atomic interferometry is of particular interest is gravimetry, where instruments based upon the controlled manipulation of atomic states have superseded the performance of classical devices. In such devices, a standing wave derived from laser beams with well-defined phase difference are used to impart momentum to atoms undergoing free fall, generating spatial superpositions along the axis of gravity, before returning the atoms to an overlapped final state for recombination. This thesis presents the design and experimental progress towards a gravity gradiometer based upon the interference of rubidium-87 atoms, cooled and launched in a fountain configuration. The fountain is leveraged from a decommissioned frequency standard, which has been re-purposed and redesigned for use as a combined absolute gravimeter and gravity gradiometer package. It is capable of loading 100 billion atoms in 100 ms and then cooling and launching these atoms in a moving molasses configuration, with a final temperature of 5 μK. Furthermore, the process can be quickly repeated, with a second cloud subsequently launched, reaching its apogee at the same time as the first cloud, allowing for the possibility of simultaneous measurement at different heights and, in the future, the inference of the gravity gradient. Control of the fountain is provided by a stable and miniaturised optical setup, allowing for use with a future transportable or field-based device. A Raman beam system to perform the interferometry sequence has been constructed and characterised, likewise designed in a minaturised and stable manner. The Raman beams simultaneously generated Ramsey interference fringes on two independent clouds of atoms -- one launched to a height of 0.7 m and the other to 1 m. Such fringes were clearly observable after 200 ms of evolution time. Mach-Zehnder interference fringes were also demonstrated for dropped and launched atoms, providing the capability for gravimetric measurements to be made once the system has been fully characterised. The principle causes of noise in the Mach-Zehnder cases were identified as arriving from a lack of atoms and the phase noise of the Raman beams. To overcome these challenges, a new Raman beam system has been preliminary designed and tested and a new iteration of the fountain, moving towards quantum degeneracy, has been planned.
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.
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.
The work presented in this thesis focusses on the development of a transportable atom-interferometry experiment and a compact fibre laser system towards precision measurements of gravitational acceleration. Interference fringes are shown with clouds of cold 87Rb atoms using co-propagating laser beams to drive stimulated Raman transitions. This is demonstrated both inside and outside of laboratory environments for which an integrated and transportable experiment is constructed. Further improvements are presented that enable the generation of clouds containing 1.7 · 108 atoms at a rate of 2.5 Hz and having a temperature of (7 ± 1) µK. This is largely due to the development of a compact laser system based on all-fibre coupled components. It is demonstrated that the laser system designed here can achieve fast frequency sweeps over 1.8 GHz within 2 ms, making it widely applicable in compact atom-interferometry experiments with rubidium atoms. This is shown by creating a Mach–Zehnder type interferometer with counter-propagating Raman beams, thus enabling measurements of gravitational acceleration. Since the laser system uses only two lasers and one fibre amplifier, a significant reduction in size is achieved, as well as a decrease in the total power consumption of the overall experiment by a third to (162 ± 7) W.
This work describes the technology developed for the transportable gravity gradiometer, ''GGtop", constructed at the University of Birmingham. The device aims to simultaneously interrogate two physically separated, free-falling cold atom clouds using the technique of Raman interferometry to perform differential gravity measurements. Such a technique will suppress common-mode noise sources, such as vibrations, that otherwise limit classical gravimeters and cold atom interferometers based on single-cloud measurements. The technique is expected to improve speed and sensitivity for field measurements greatly. \nUsing a combination of commercially available components and novel designs, intended to enhance portability and robustness, allowed for demonstration of atomic interference with the apparatus via Ramsey's method of separated oscillatory fields. The achieved fringe contrast of ~2%, defined as the difference in the number of atoms detected in IF = 2>, was limited by drifts stemming from some of the novel designs which prompted continued optimisation of the underpinning subsystems. To address performance issues parts of the experiment were redesigned with the goal of improving reliability at the expense of some portability. Using the retrofitted experiment, interference was once again achieved with fitted fringe spacing of 134.7±2.0 uS in good agreement with the 133.9 uS defined by the experimental control. A factor of 10 improvement in contrast was found with the central fringe demonstrating 18% of the atoms detected in the IF = 2> state when normalised to the total 3D MOT number. The 3.71 ± 0.01 kHz fitted linewidth of the central fringe gives a frequency uncertainty of 5.43 ± 0.01 x 10 - 7. This result leaves the experiment in a good position to begin making measurements of gravity.
Atom interferometry is a maturing quantum technology which provides one of the most precise approaches to explore fundamental physics, e.g. to search for dark matter, and one day to observe gravitational waves in the frequency range from 10 mHz - 10 Hz. Standard atom interferometers use Bragg or Raman two-photon transitions for the coherent manipulation of matter waves. In a Michelson configuration for atom-aided gravitational waves detection, two orthogonal arms are built to reject laser noise, in the same way as optical interferometers. Atom interferometers utilizing an optical clock transition for coherent manipulation are less affected by the technical noise of the laser, and as a result that single baseline detectors become feasible.At LP2N, we are building a vertical gravity-gradiometer based on atom interferometry using strontium atoms probed on such a clock transition. The final experimental setup will implement a 6-meter tall atomic fountain to realize gradiometry with two or more separated atomic clouds.In my dissertation, I present the realization of the experimental setup in all its sub-systems, and the first experiments with atoms at millikelvin temperature obtained with the first cooling stage with blue light. The laser system for the second red cooling phase and for the coherent manipulation is ready; meanwhile, we realized a dual-frequency Zeeman slower that improves the atomic flux by a factor of six. We also present a study of an original scheme to achieve pulsed and fast light-atom interaction in a narrow-linewidth cavity by exploiting the Stark shift as a high bandwidth switch; this technique could open to the use of narrow-linewidth cavities – and notably their high power buildup - in several contexts, like atom interferometry and quantum information.
We describe two experiments where atom interferometry is applied for precision measurements of gravitational effects. In the first, we measure the Newtonian gravitational constant G using an atom interferometry gravity-gradiometer which combines a rubidium fountain, a juggling scheme for fast launch of two atomic clouds, and Raman interferometry. We show that the sensor is able to detect the gravitational field produced by source masses and G is measured with better than 10-2 accuracy. In the second experiment, using ultra-cold strontium atoms in a vertical optical lattice and observing persistent Bloch oscillations for several seconds, we measure gravity acceleration with micromet-ric spatial resolution. We discuss the prospects for the study of gravitational forces at short distances and show that unexplored regions can be investigated in the search for deviations from Newtonian gravity.