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Developing a transportable quantum inertial sensor

Henry Sewell

Published Feb 14, 2024• 0 citations
Publisher

Abstract

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.

DOI
10.25560/131046

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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.
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.
Developments in the precise control of quantum systems have led to advances in both fundamental research and emerging technologies in areas including quantum sensing and computation. One particularly useful application is atom interferometry, which uses laser pulses to split, redirect, and recombine atomic wavepackets, enabling precision measurements of inertial effects. This thesis presents the construction and characterisation of a compact cold atom interferometer, to meet the longer-term aim of developing an atomic gyroscope. We detail the setup of a rubidium-85 Magneto-Optical Trap (MOT) and the development of a laser to perform Raman transitions across the hyperfine Rb-85 levels. We then demonstrate and characterise the coherent manipulation of the atomic state of the rubidium as well as atom-interferometric fringes, in a Doppler-insensitive configuration. Systematic errors can degrade the performance of laser pulse interactions with cold atoms by reducing the fidelity of the atom–light interactions. In this thesis, we investigate the implementation of composite pulses and optimal control pulses, which use pulses with varying phase sequences and durations to mitigate against experimental inhomogeneities. We also show that phase modulators can introduce distortions in both the applied laser phase and the Raman modulation amplitude, which can in turn degrade pulse performance. To address this, we develop a compensation protocol that corrects for these non-linearities and demonstrate that it leads to a further improvement in pulse fidelity, for the Waltz and Knill composite pulses. Applying this protocol to the rotary echo sequence led to Rabi oscillations that could last 4 times longer, compared to the uncalibrated IQ modulator. Although an optimal control inversion pulse showed more resilience to the IQ modulator distortions, the Mach-Zehnder interferometer sequence was more susceptible to them. It was demonstrated that compensating for IQ modulator non-linearities improved the optimised fringe signal-to-noise ratio, and reduced the phase-offset error by a factor of 2.4.
We are developing an atom interferometer that measures horizontal accelerations to form a component of a quantum inertial measurement unit. Benefitting from the inherent stability of atom interferometry, this points to a navigation system that offers long-term accuracy without recourse to GPS. To aid compactness and transportability, the sub-components of the quantum sensor are mounted in a 19-inch rack. This includes an optical system of two titanium-sapphire lasers that delivers all of the optical pulses required to operate a highly sensitive atom interferometer using 87Rb atoms. Active compensation systems that allow operation of the quantum sensor in the noisy environment of a moving vehicle are described. Finally, we give an account of recent field trials on a variety of test platforms.
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.
In the past century, the development of gravimeters with low uncertainty and long-term stability has led to new fields of research in geodesy and geoscience. Decreasing the instrumental measurement uncertainty further will enable observations of previously inaccessible phenomena, for instance, mass transport in hydrology and volcanology. During the last decades, quantum sensors based on the interference of cold atoms have been developed. Using a cold atomic gas as test mass, the accuracy of these sensors is not limited by mechanical properties but by effects caused by the thermal expansion of the atomic ensemble. The application of ultra-cold atomic ensembles with lower expansion rates in atom interferometer gravimeters is projected to reduce the leading order uncertainties by more than an order of magnitude. At the same time, atom chip technology makes it possible to prepare ultra-cold atomic ensembles at a high repetition rate and to miniaturise the sensor size. These advancements promise the realisation of an absolute gravimeter with unprecedented accuracy. This thesis describes the design considerations and the assembly of the transportable Quantum Gravimeter (QG-1) based on light-pulse atom interferometry of Bose-Einstein condensates (BEC) prepared on an atom chip. It is estimated, that the two leading order uncertainties of systematic biases governing the instrumental measurement uncertainty of current generation cold atom gravimeters are reduced to less than 1 nm/s² in the QG-1 apparatus. The established design of an atom-chip-based BEC source pioneered in the Quantus collaboration is modified to meet the requirements of QG-1. A free optical aperture of 18 mm for the interferometry laser beam is realised by changing the orientation of the atom-chip-based BEC source. Therefore, a new layout of the mesoscopic wire structure of the atom chip is required. The design described in this thesis enables atom interferometry with a free falling test mass with a baseline of 330 mm. The retro-reflection mirror is placed inside the vacuum chamber to eliminate optical elements in the atom interferometer beam path. It is mounted on a custom designed tip/tilt-stage with compact size and a large dynamic range of up to a hundredfold of the Earth's rotation rate for characterisation. Furthermore, a compact, robust and transportable fibre based laser system with modular electronics and a computer control system are set up. The key result of this thesis is the reliable operation of the ultra-cold atomic source on the atom chip. After optimisation of the trap loading procedure for a high atom number and low excitation of oscillations, it was shown that the necessary design change of the atom chip allows for efficient operation. The compressed magnetic trap has a geometrically averaged trap frequency of 2π · 256 Hz and the trapped ensemble has a lifetime of 3.2 s. The evaporative cooling procedure starts with 3.3 · 10⁷ atoms at a temperature of 166 μK. Within 1.3 s, or 2.3 s for the complete sequence, 3000 atoms are prepared at a temperature of 160 nK close to the critical temperature for Bose-Einstein condensation.
Inertial sensors based on cold atom interferometry exhibit many interesting features for applications related to inertial navigation, particularly in terms of sensitivity and long-term stability. However, at present the typical atom interferometer is still very much an experiment—consisting of a bulky, static apparatus with a limited dynamic range and high sensitivity to environmental effects. To be compliant with mobile applications further development is needed. In this work, we present a compact and mobile experiment, which we recently used to achieve the first inertial measurements with an atomic accelerometer onboard an aircraft. By integrating classical inertial sensors into our apparatus, we are able to operate the atomic sensor well beyond its standard operating range, corresponding to half of an interference fringe. We report atom-based acceleration measurements along both the horizontal and vertical axes of the aircraft with one-shot sensitivities of 2.3 × 10−4 g over a range of ∼ 0.1 g. The same technology can be used to develop cold-atom gyroscopes, which could surpass the best optical gyroscopes in terms of long-term sensitivity. Our apparatus was also designed to study multi-axis atom interferometry with the goal of realizing a full inertial measurement unit comprised of the three axes of acceleration and rotation. Finally, we present a compact and tunable laser system, which constitutes an essential part of any cold-atom-based sensor. The architecture of the laser is based on phase modulating a single fiber-optic laser diode, and can be tuned over a range of 1 GHz in less than 200 μs.