In the context of the second quantum revolution, the field of quantum sensors is in full expansion. One of the corner stones of this field is inertial sensing through cold-atom interferometry which provides sensors of high precision and stability. Size reduction is one of the main tasks in order to develop atomic inertial sensors that could apply to inertial navigation: this leads to the development of atom-chips. It is with this context in mind that this thesis investigates an atom-chip gyro in a collaboration between the University of Sussex and l'Observatoire de Paris (France). The experimental work is hosted at l'Observatoire de Paris where we worked on atom-chip fabrication, comparing metal deposition by evaporation and by electroplating, and paying attention on the wire's roughness as it impacts on the magnetic waveguide and therefore the atoms. But the wire's roughness is only one source of noise to the magnetic waveguide. Low noise current supplies are also studied and characterised. Furthermore, using the density operator and coherent state basis we develop a simple and elegant formalism under a 1D approximation. We use a new approach to show how temperature inuences the propagation of a wavepacket by increasing the spatial spreading while decreasing the wavepacket coherence. We also built a model for the pumping dynamics of a cold-atom experiment in a single vacuum chamber which matches experimental data from l'Observatoire de Paris and is an important step to work on size and the dead time reduction of sensors. All these different steps present in this thesis take us towards a working inertial sensor. While working on noise characterisation which will always remain a fundamental issue in building an inertial sensor, this thesis presents new models and approaches that may benefit the development of quantum technologies
In the context of the second quantum revolution, the field of quantum sensors is in full expansion. One of the corner stones of this field is inertial sensing through cold-atom interferometry which provides sensors of high precision and stability. Size reduction is one of the main tasks in order to develop atomic inertial sensors that could apply to inertial navigation: this leads to the development of atom-chips. It is with this context in mind that this thesis investigates an atom-chip gyro in a collaboration between the University of Sussex and l'Observatoire de Paris (France). The experimental work is hosted at l'Observatoire de Paris where we worked on atom-chip fabrication, comparing metal deposition by evaporation and by electroplating, and paying attention on the wire's roughness as it impacts on the magnetic waveguide and therefore the atoms. But the wire's roughness is only one source of noise to the magnetic waveguide. Low noise current supplies are also studied and characterised. Furthermore, using the density operator and coherent state basis we develop a simple and elegant formalism under a 1D approximation. We use a new approach to show how temperature inuences the propagation of a wavepacket by increasing the spatial spreading while decreasing the wavepacket coherence. We also built a model for the pumping dynamics of a cold-atom experiment in a single vacuum chamber which matches experimental data from l'Observatoire de Paris and is an important step to work on size and the dead time reduction of sensors. All these different steps present in this thesis take us towards a working inertial sensor. While working on noise characterisation which will always remain a fundamental issue in building an inertial sensor, this thesis presents new models and approaches that may benefit the development of quantum technologies
This work reviews the topic of rotation sensing with compact cold atom interferometers. A representative set of compact free-falling cold atom gyroscopes is considered because, in different respects, they establish a rotation-measurement reference for cold guided-atom technologies. This review first discusses enabling technologies relevant to a set of key functional building blocks of an atom chip-based compact inertial sensor with cold guided atoms. These functionalities concern the accurate and reproducible positioning of atoms to initiate a measurement cycle, the coherent momentum transfer to the atom wave packets, the suppression of propagation-induced decoherence due to potential roughness, on-chip detection, and vacuum dynamics because of its impact on sensor stability, which is due to the measurement dead time. Based on the existing enabling technologies, the design of an atom chip gyroscope with guided atoms is formalized using a design case that treats design elements such as guiding, fabrication, scale factor, rotation-rate sensitivity, spectral response, important noise sources, and sensor stability.
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.
Atom chips are robust and extremely powerful toolboxes for quantum optical experiments, since they make it possible to create exceedingly precise magnetic traps for neutral atoms with minimal field modulations. Accurate manipulation of trapped atoms is feasible with magnetic and electric fields created on the atom chip. Therefore atom chips with high quality surfaces and extremely well defined wires were build (roughness < 20nm). Furthermore new generations of atom chips were developed, like the multi-layer atom chip with contact-free wire crossings, the sub-micron structured atom chip and the semiconducting atom chip. Extensive characterisation measurements of atom chip wires demonstrated that the wires stand current densities of up to 10^7A/cm^2 for seconds and voltage differences of more than 500V over 10µm. From the different materials tested doped silicon with a thin silicon dioxide layer is the best qualified substrate for atom chip fabrication. The atom chips fabricated during this thesis have been used in many successful experiments, yielding numerous results. Moreover this thesis established the basics for many further experiments in atom physics and quantum optics and delivers complete instructions for the fabrication of all developed atom chips as well as an introduction to the experiments.
This thesis details the work carried out in the development towards a fully miniaturised integrated atom chip. The main focus of the project so far has been on the construction of miniaturised vacuum chambers, the study of the eutectic bond and the manufacturing of integrated electric feedthroughs, together with the development of new cold atom trap geometries. Current cold atom technologies rely on the use of bulky optical systems and large vacuum chambers. We propose the miniaturisation of the entire system into a device comparable in size with a match box. Afirst step towards this goal is the miniaturisation of the vacuum system and atomic source. In the first part of this thesis we present planar microfabrication techniques, such as anodic bonding and eutectic bonding, as a solution for providing hermetic seals. Several partially working devices i.e miniaturised vacuum chambers with rubidium atom sources are presented and compared to commercial rubidium cells. As ultimately our integrated atom chips will be used in metrology and sensing devices, they require current-carrying wires within the vacuum. Towards this purpose we propose a method of building hermetically sealed electrical feedthroughs through the process of glass reflow in thick silicon substrates. The second part of this thesis outlines a brief theory towards cooling atoms, together with new geometries designed for use with miniaturised devices and the attempts of achieving a functional integrated atom chip. Finally, we present the development of a new method for achieving a cold atom source without the use of magnetic fields.
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.
Atom optics has been a highly active field of research with many scientific breakthroughs over the past two decades, largely due to successful advances in laser technology, microfabrication techniques, and the development of laser cooling and trapping of neutral atoms. This dissertation details several atom optics experiments with the motivation to develop tools and techniques for precision atom wave interferometry. It provides background information about atom optics and the fundamentals behind laser cooling and trapping, including basic techniques for cold gas thermometry and absorptive detection of atoms. A brief overview of magnetic trapping and guiding in tight wire-based traps is also provided before the experimental details are presented. ❧ We developed a novel laser source of 780 nm light using frequency-doubled 1560 nm fiber amplifier. This laser system provided up to a Watt of tunable frequency stabilized light for two Rb laser cooling and trapping experiments. One system generates Bose-Einstein condensates in an optical trap while the second is based on atom chip magnetic traps. The atom chip system, detailed in this thesis, was designed and built to develop the tools necessary for transport and loading large numbers of cold atoms and explore the potential for guided atom interferometry. Techniques and results from this experiment are presented, including an efficient magnetic transport and loading method to deliver cold atom to atom chip traps. We also developed a modeling tool for the magnetic fields formed by coiled wire geometries, as well as planar wire patterns. These models helped us design traps and determine adiabatic transportation of cold atoms between macro-scale traps and micro-traps formed on atom chips. Having achieved near unity transfer efficiency, we demonstrated that this approach promises to be a consistent method for loading large numbers of atoms into micro-traps. Furthermore, we discuss an in situ imaging technique to investigate magnetic field contours of the traps and the dynamics of atoms within those confining potentials. We also controlled the propagation along the atom chip guides by accelerating atoms with longitudinal magnetic gradients, and investigated an atom focusing scheme. ❧ While the atom chip wire guides perform a role analogous to optical fibers guiding light waves, ""free space"" cold atoms offer great opportunity for precision interferometry. We describe a second on-going atom optics experiment that measures gravity gradients using a pair of atom fountain interferometers separated by one meter. We have demonstrated Gravity Gradiometer resolution down to 4x10⁻⁹ g/m using a 40 kg test mass. The atomic physics subsystem is described in detail, including the vacuum, cold atom source, optics, magnetic coils and shields, and vibration isolation and compensation. The system is designed to be a compact, robust, transportable instrument, taking strides towards future gravity gradient measurements in the field. In the realm of space applications, there has been interest for micro-gravity science experiments aboard the International Space Station, along with instrument development for gravity mapping of Earth and planetary bodies with satellite-based instruments. Furthermore, there are ground-based applications for gravity imaging of local density distributions, precision measurement of gravity, as well as proposals for redefining the kilogram, detecting gravitational waves and determining the Gravitational constant.