This thesis investigates the operation of atom interferometers in dynamic inertial conditions. As such, the cross-coupling effect of an accelerating sensor head on the output of an interferometer has been simulated, showing a contrast drop to 59% for T = 80 ms and 49% for T = 40 ms if the centre-of-mass of the atoms is within the waist radius of the Raman laser beam. Two ways to combat dynamic conditions are presented. Firstly, a model that is able to simulate phase shear fringes using fluorescence coupled with camera imaging is presented, helping to pave way towards extracting contrast and interferometer phase in one experimental shot. Secondly, a preliminary investigation of multi-modal measurements of gravity, magnetic field and magnetic field gradient is presented. Atom shot noise uncertainties for a T = 50 ms interferometer with 10\(^6\) atoms are estimated as 3 \(\times\) 10\(^-\)\(^8\) ms\(^-\)\(^2\) per shot for gravity and 4 \(\times\) 10\(^-\)\(^6\) Gm\(^-\)\(^1\) per shot for magnetic field gradient. Experimentally, gravity gradiometer based on atom interferometry has been built. It is capable of producing over 10\(^8\) atoms in the MOT, with cloud temperatures of 7.3 \(\pm\) 0.2 \(\mu\)K for the top chamber and 7.4 \(\pm\) 1.7 \(\mu\)K for the bottom chamber following molasses and state preparation. Interferometry is demonstrated, able to obtain short T gravity fringes without vibration isolation platform. Implementation steps required for phase shear readout are investigated, implementing a beam tilting Raman telescope, camera imaging and building an analysis technique using principal component analysis. The system is integrated with a classical accelerometer and tiltmeter (as well as a linear rail) and is able to obtain Rabi oscillations while moving.
This thesis investigates the operation of atom interferometers in dynamic inertial conditions. As such, the cross-coupling effect of an accelerating sensor head on the output of an interferometer has been simulated, showing a contrast drop to 59% for T = 80 ms and 49% for T = 40 ms if the centre-of-mass of the atoms is within the waist radius of the Raman laser beam. Two ways to combat dynamic conditions are presented. Firstly, a model that is able to simulate phase shear fringes using fluorescence coupled with camera imaging is presented, helping to pave way towards extracting contrast and interferometer phase in one experimental shot. Secondly, a preliminary investigation of multi-modal measurements of gravity, magnetic field and magnetic field gradient is presented. Atom shot noise uncertainties for a T = 50 ms interferometer with 10(^6) atoms are estimated as 3 (\times) 10(^-)(^8) ms(^-)(^2) per shot for gravity and 4 (\times) 10(^-)(^6) Gm(^-)(^1) per shot for magnetic field gradient. Experimentally, gravity gradiometer based on atom interferometry has been built. It is capable of producing over 10(^8) atoms in the MOT, with cloud temperatures of 7.3 (\pm) 0.2 (\mu)K for the top chamber and 7.4 (\pm) 1.7 (\mu)K for the bottom chamber following molasses and state preparation. Interferometry is demonstrated, able to obtain short T gravity fringes without vibration isolation platform. Implementation steps required for phase shear readout are investigated, implementing a beam tilting Raman telescope, camera imaging and building an analysis technique using principal component analysis. The system is integrated with a classical accelerometer and tiltmeter (as well as a linear rail) and is able to obtain Rabi oscillations while moving.
The presented thesis describes the setting up of an inertial sensitive atom interferometer for navigation using Rubidium 87 atoms, which is able to perform horizontal acceleration measurement. An ultra-high vacuum interferometer chamber is designed and built. The cold atoms are prepared in a compact 2D MOT setup and loaded into the main MOT. These atoms are then cooled down to few K and prepared in a magnetically insensitive ground state using a sequence of optical and microwave pulses, after which atoms are selected in a narrow band of the velocity. A three-pulse Mach-Zehnder ( pi/2 -pi-pi/2) interferometer using stimulated Raman transitions is used to measure the horizontal acceleration. To improve the sensitivity to acceleration, the fringe contrast of the interferometer requires to be optimised. By investigating the intensity profile of the Raman beams using atoms, a procedure is developed to operate the interferometer in an improved condition, which is able to enhance the fringe contrast by a factor of 3. This procedure improves the shot-to-shot sensitivity of the interferometer below 5 um/s^2. In addition, the atom interferometer is compared with a classical state-of-the-art accelerometer.
Matter-wave interferometry has enabled high precision measurements of inertial forces such as gravity and the Coriolis force. This is facilitated by the long-term stability of the physical properties of atoms and lasers. Recent experiments have demonstrated the operation of portable, robust sensors using atom interferometry. This has potential uses in the context of inertial navigation, where conventional devices suffer from long-term drifts due to bias instability. Furthermore, determining position via dead reckoning requires minimisation of dead time between measurements. This thesis presents the development of an atom interferometer for measuring horizontal accelerations. In this configuration, gravity induces motion across the laser wavefront, which constrains the tolerable level of wavefront distortions. Effective control of the experiment allows the interferometer to be operated at a rate of 4 Hz. A cold ensemble of 10^6 atoms in the same internal state is prepared in 150 ms. The interferometer operates using a sequence of three laser pulses separated by T= 25 ms to achieve sensitivity to horizontal accelerations. Combining this with a classical accelerometer provides a method of correcting for vibration-induced noise, as well as determining the interferometer fringe order. After an integration time of 40 s, the sensitivity to horizontal accelerations is better than 3^-6 m s^-2. Effects which limit this sensitivity are discussed.
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.
Light pulse atom interferometers have demonstrated remarkable sensitivity and stability for acceleration and rotation rate measurement. However, typical manifestations are designed for laboratory environments and thus rely on a fixed magnitude and direction of gravity, and limited ambient rotation rate. We have enhanced the application space of atom interferometers towards more dynamic environments, with special attention for inertial navigation. I present our work in the domain of short time-of-flight atom interferometry, whereby the magnitude of ensemble excursion is constrained. The limited interrogation time results in a significant loss of sensitivity. We recover a fraction of the lost sensitivity by operating with an enhanced duty-cycle and data-rate. To demonstrate this concept, we construct an atom interferometer accelerometer capable of operating at data-rates as high as 300 Hz with sensitivities at μg/rtHz levels, which represents a competitive figure for inertial navigation application. For the bulk of this work, we demonstrate a dual-axis sensor capable of simultaneous acceleration and rotation-rate measurements. The sensor relies on a technique we refer to as "ensemble exchange" which provides a high flux source of ultracold atoms by swapping atomic ensembles between two MOTs. We achieve a steady-state atom number of 7e6 atoms/shot using a minimal loading time of a few milliseconds each shot. Furthermore, we find this technique to be robust under dynamic conditions as large as 10 g of acceleration and 20 rad/s of rotation rate, representing a significant enhancement in ultra-cold atom sample preparation. The sensor achieves μg/rtHz and μrad/s/rtHz sensitivities, making this technique a compelling prospect for inertial navigation applications. Through the use of auxiliary cosensors and a real-time combinatorial loop with feedforward and feedback mechanisms, we demonstrate an unprecedented enhancement of the sensor dynamic range up to 20 mg. Finally, I will discuss a novel manifestation of short time-of-flight atom interferometry in a warm atomic vapor, which avoids the complication of cold sample preparation and has the potential for significantly simplified laser systems.
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 interferometers have been used to measure accelerations and rotations in many laboratories around the world. They have demonstrated very high long-term stability, making them well suited for development into an inertial navigation system. The next stage of development towards a full navigation system is to move these laboratory demonstrators into the field, and improve their resilience to the effects of a more turbulent environment. This thesis presents the development of the first transportable quantum inertial sensor at the Centre for Cold Matter at Imperial College London, developing on the laboratory-based systems which have already been demonstrated. Rubidium-87 atoms are delivered into an ultra-high vacuum chamber with a 2D+ magneto optical trap (MOT), before being trapped in a 3D MOT and cooled to below 5 µK using polarisation gradient cooling. Atoms are efficiently prepared in a magnetically insensitive energy level, before a narrow velocity class of atoms is selected, to reduce the effect of thermal dephasing mechanisms. A three pulse, π/2−π−π/2, interferometer sequence coherently splits, reverses and recombines the atom cloud, and the atomic energy level populations are recorded after the third pulse using fluorescence imaging. Interference fringes have been demonstrated for a number of different interferometer times in the laboratory, and initial characterisation of the fringe contrast has been performed. In addition to in-laboratory characterisation, data from three field trials, alongside the Royal Navy, are presented. The trials have enabled us to benchmark the system’s performance outside of the laboratory, and highlight the areas of development required for improved system performance in future field trials.