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Towards a cold atom gravimeter

Andrew Chew

Published Jan 1, 2013• 0 citations
OpenAlex

Abstract

This thesis describes the construction of a portable cold atom gravimeter with the aim to be employed in remote locations. Two species of alkali atoms would be eventually used, namely Rubidium-87 and Cesium-133. The advantage of such a system is shorter duty cycle, and shorter dead time. In this thesis we focus on the experimental realization of a Rubidium-87 gravimeter, using the Ramsey-Bord´e interferometry method first described by C. Bord´e, and employed by Kasevich et. al. Approximately 10^8 atoms are laser cooled and trapped to a MOT and further cooled by a optical molasses to a temperature of 12μK. Two counterpropagating Raman beams separated by a frequency matching the ground state hyperfine splitting will be used to excite stimulated Raman transitions to place the atoms into a superposition of the two ground metastable states. This thesis describes in detail the vacuum system and the laser systems built to cool and trap the atoms and to produce the Raman beams to perform the Ramsey sequence. The thesis covers also the relevant theory related to our experiment.

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This thesis presents work performed to obtain an optical mask for conducting matter-wave interfer-ometry experiments with ultra-cold rubidium-85 atoms. The optical mask is essentially an absorptive diffraction grating made of light, and it can imprint a periodic density pattern on a cloud of atoms with a period of half the wavelength of the light used. The mask has an analogous effect to quickly passing a diffraction grating through the cloud, removing atoms located at the nodes of the grating (though the optical mask depumps atoms to a different hyperfine ground state rather than actually removes them). The mask should be useful in performing precision measurements of physical constants such as the fine-structure constant α, and acceleration due to gravity g. The thesis briefly expounds major historical developments in atom interferometry and laser cooling. Prerequisites for an optical mask include a functioning magneto-optical trap, and the ability to perform polarisation-gradient cooling on trapped atoms. The theory of these two techniques is reviewed and the principle of the the optical mask is explained. The experimental apparatus that was constructed to realise the optical mask is described, and technical developments made along the way are presented. Finally, aspects of the ex-periment relevant to the optical mask are characterised. The experiment can reliably produce samples of cold atoms at temperatures of around 10 µK, and can use the cold atoms as an optical frequency reference accurate to ∼ 1 MHz. Clear evidence that the cloud of atoms is being density-modulated by the optical mask is presented. Improvements required to make the experiment into a fully-functioning interferometer are also discussed. i ii

•Jan 1, 2014•1 citation
This thesis describes the construction of a portable cold atom gravimeter with the aim to be employed in remote locations. Two species of alkali atoms would be eventually used, namely Rubidium-87 and Cesium-133. The advantage of such a system is shorter duty cycle, and shorter dead time. In this thesis we focus on the experimental realization of a Rubidium-87 gravimeter, using the Ramsey-Bord´e interferometry method first described by C. Bord´e, and employed by Kasevich et. al. Approximately 10^8 atoms are laser cooled and trapped to a MOT and further cooled by a optical molasses to a temperature of 12μK. Two counterpropagating Raman beams separated by a frequency matching the ground state hyperfine splitting will be used to excite stimulated Raman transitions to place the atoms into a superposition of the two ground metastable states. This thesis describes in detail the vacuum system and the laser systems built to cool and trap the atoms and to produce the Raman beams to perform the Ramsey sequence. The thesis covers also the relevant theory related to our experiment.
This thesis details the construction of a portable platform for cold atom experiments, as part of the European iSense project. This culminated in the demonstration of a working portable cold atom interferometer to the praise of the EC and reviewers during the final project meeting. Reductions in the size, weight and power consumption of all the crucial components for a cold atom system such as lasers, optics, magnetic field generation and electronics have been realised. These novel components have been integrated into a portable device that has been transported and operated by just two people. The completed device weighs 63 kg and uses 240 W power in a volume of just 61.1 litres. The system has generated a cloud of (6.7±0.7)x 106^66 rubidium-87 atoms at a temperature of 4.4 μK in 2 seconds using a mirror-MOT setup. The device has successfully performed atom interferometry in the form of a Ramsey interferometer both at the University of Birmingham, UK and also at an office in Brussels, BE after a journey of 570 km. A measurement of the hyperfine splitting of the ground state of rubidium-87 was performed and gives an uncertainty of δ\deltaδω\omegaω/ω\omegaω = 5.2 × 10−^-−7^77. The theoretical limit of the sensitivity to gravity of the iSense system is 3.9 μGal/√Hz.
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.
A compact laser system for a portable 87Rb atom interferometry gravimeter has been demonstrated in this work. This laser system is based on frequency doubling of a single seed laser at the wavelength of 1560 nm. The frequency of the seed laser is controlled by a digital unit with an analog feedback circuit. By using this frequency control method, the frequency of the laser system can be shifted over 1 GHz. Based on this method, the Raman frequency can be locked on the F = 3 → F′ = 4 transition of 85Rb atoms. Moreover, the Raman sideband and the repumping laser are generated by a phase modulator, and it can generate different laser frequencies to meet the requirements of a typical atom interferometer. Additional sidebands in the Raman beam produced from the phase modulator are optimized and reduced, allowing us to observe atom interference with a free evolution time of 320 ms. The control unit including the laser system has been integrated into a box with a volume of 1.5 m × 0.6 m × 0.6 m, and the weight of which is only 150 kg. Using this compact optical scheme, our atomic gravimeter has achieved a sensitivity of 53 μGal/Hz1/2 and a resolution of better than 1 μGal (1 μGal = 1 × 10−8 m/s2) in an integration time of 3000 s.
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 thesis presents work performed to obtain an optical mask for conducting matter-wave interfer-ometry experiments with ultra-cold rubidium-85 atoms. The optical mask is essentially an absorptive diffraction grating made of light, and it can imprint a periodic density pattern on a cloud of atoms with a period of half the wavelength of the light used. The mask has an analogous effect to quickly passing a diffraction grating through the cloud, removing atoms located at the nodes of the grating (though the optical mask depumps atoms to a different hyperfine ground state rather than actually removes them). The mask should be useful in performing precision measurements of physical constants such as the fine-structure constant α, and acceleration due to gravity g. The thesis briefly expounds major historical developments in atom interferometry and laser cooling. Prerequisites for an optical mask include a functioning magneto-optical trap, and the ability to perform polarisation-gradient cooling on trapped atoms. The theory of these two techniques is reviewed and the principle of the the optical mask is explained. The experimental apparatus that was constructed to realise the optical mask is described, and technical developments made along the way are presented. Finally, aspects of the ex-periment relevant to the optical mask are characterised. The experiment can reliably produce samples of cold atoms at temperatures of around 10 µK, and can use the cold atoms as an optical frequency reference accurate to ∼ 1 MHz. Clear evidence that the cloud of atoms is being density-modulated by the optical mask is presented. Improvements required to make the experiment into a fully-functioning interferometer are also discussed. i ii