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.
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.
The quantum gravimeter QG-1 is designed to perform local gravity measurements in field with residual uncertainties surpassing state-of-the-art absolute gravimeters. It will use delta-kick collimated Bose-Einstein condensates (BEC) of 87 Rb atoms as the source for matter-wave interferometry and aims for an uncertainty level below 3 nm/s 2 with a repetition rate of 0.5 Hz. In the frame of this thesis, the atom-chip source has been set up and characterized. In order to meet the requirements for transportability, the apparatus needs to be compact and robust. Additionally, a high BEC flux is desired for the targeted statistical uncertainty. To meet these requirements a double magneto-optical trap (MOT) configuration based on a 3-layer atom chip has been implemented. Furthermore, a compact high power fiber-based laser system has been developed to provide the light for atomic manipulation. To power and control this laser system and generate the magnetic fields necessary to trap atoms, a miniaturized electronic system with low power consumption has been utilized and characterized. The steps towards trapping the atoms inside a magnetic trap on which the evaporation will be implemented have been realized and optimized. A two-dimensional MOT has been employed and an optimized atomic beam flux of 2.2 × 10 9 atoms/s with a mean longitudinal velocity of 17.2 m/s and a narrow longitudinal velocity distribution of 2.2 m/s has been used to load a 3D-chip MOT. The resulting 3D-MOT has been characterized and a loading rate of 8.1×10 8 atoms/s has been attained. After loading for 300 ms, 2.4×10 8 atoms are trapped in the 3D-MOT. Applying a subsequent compressed MOT stage, the cloud density has been increased by a factor of 10. This denser atomic ensemble with 2.9 × 10 10 atoms/cm 3 has been further cooled in a polarization gradient cooling (PGC) stage to 5.6 μK in order to increase the mode match with the subsequent magnetic trap. In the initial large volume magnetic trap generated by the wire structures on the mesoscopic and the base layers of the atom-chip assembly, 3.6 × 10 7 atoms have been captured. This trap features a temperature of 44.3 μK, a trapping frequency of 9.1 Hz in its weak confining axis and a phase-space density of 4.5 × 10 −7 . The process of transferring the atoms into the final magnetic trap with higher trapping frequency for an efficient BEC evaporation, which is formed by the wire structures on the base and science chip has been tested. With this, all the necessary functionalities of a high-flux BEC source setup have been demonstrated.
Pioneering work on cold atom based quantum sensors introduced new measurement concepts to geosciences, promising independent tests of established methods as well as showing unprecedented stability, in the case of gravimetry. This naturally raises the question, whether unprecedented stability could be paired with unprecedented accuracy. Unfortunately, systematic effects originating from the thermal expansion and horizontal motion of the cold atom source prohibit this intrinsically. However, employing delta-kick collimated Bose-Einstein Condensates (BECs) instead, these leading order uncertainty contributions, i.e. wavefront aberrations and the Coriolis contribution, can be overcome. One of the major challenges here is to minimise the sensor size and power consumption to allow for mobile operation. This thesis elaborates on the development of a transportable BEC gravimeter referred to as the Quantum Gravimeter QG-1. A compact sensor size is realized by using an atom-chip based BEC source. The source performance is optimised at two operation locations producing samples of > 10^5 87Rb atoms after less than 1 s of evaporation time. The evaporation procedure is optimised for minimal residual centre-of-mass oscillation. Subsequently the atoms are released from the magnet trap, delta-kick collimated by a magnetic potential and prepared for interferometry by state transfer and purification. After this procedure up to 10^5 atoms remain in the desired, to first order, magnetically insensitive mF = 0 state, exhibiting an effective temperature of 19.8 nK in the z and 400 pK in the y direction. With this source atom interferometers of high fringe contrast of > 90 % could be operated for pulse separation times of up to 60 ms. The main focus of this work lies on the analysis of the first test of long-term operation. For this purpose the instrument was operated for eight consecutive days. This comprises a new milestone for the interruption-free operability of BEC gravimeters. The acceleration signal’s instability reveals a pressure-dependent long-term drift, which originated from the reference oscillator. Environmental and experimental data is used to correct the acceleration signal. This method reduces the long-term instability to the expected 1/√𝜏 behaviour and reveals correlation of the remaining instability with the sample’s position stability. The acceleration spectral density indicates a limitation of the short-term instability by residual vibrations of the passive isolation platform. The analysis procedure itself is evaluated by comparison of three different approaches of image evaluation. The important role of spatially resolving detection in the analysis of sources of instability, as well as in the characterisation of systematic effects is highlighted by application to setting an upper bound on the uncompensated Coriolis contribution and second order Zeeman shift. Future benefits of image-based detection for characterising systematics, such as wavefront aberrations, as well as its role in the alignment of the interferometer with respect to gravity are pointed out. Strategies for suppressing a phase shift originating from the BEC’s mean-field energy are introduced. These findings are a profound basis for entering the commissioning phase of the device, which includes the establishment of an initial uncertainty budget, the development of standardized measurement and evaluation procedures, the participation in comparisons and the preparation of measurement campaigns.
Abstract Gravimetry with low uncertainty and long-term stability opens up new fields of research in geodesy, especially in hydrology and volcanology. The main limitations in the accuracy of current generation cold atom gravimeters stem from the expansion rate and the residual centre-of-mass motion of their atomic test masses. Our transportable quantum gravimeter QG-1 aims at overcoming these limitations by performing atom interferometry with delta-kick collimated Bose–Einstein condensates generated by an atom chip. With our approach we anticipate to measure the local gravitational acceleration at geodetic campaigns with an uncertainty less than 1 nm/s2 surpassing the state-of-the-art classic and quantum based systems. In this paper, we discuss the design and performance assessment of QG-1. Graphical abstract
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.
Atom Interferometry methods open new prospectives for absolute acceleration measurements; the quantum features of matter and the employment of atomic test masses with manipulation technique of internal and external degrees of freedom lead to a new generation of quantum sensors. In this thesis I present the development of a prototype for a transportable sensor at system and subsystem level. An instrument with such characteristics can find applications in many felds. The work has been developed in the framework of two European projects respectively oriented to the realization of an atom interferometry based absolute gravimeter for geophysical and metrological applications and to the realization of a Space Atom Interferometer for tests in micro-gravity environment. In order to focus on the possible scientific landscape I briefy illustrate in the introduction the existing devices and their characteristic performances, pointing out how the atom interferometry may represent a valid alternative leading to a new generation of quantum devices. The study, the development and the realization stages for our instrument are presented either for the laser system realization and for the the implementation of the whole apparatus. This work leads to the development of a very compact atomic physics interferometric system responding to the scientific requirements that represent our target. From the laser system side it has been realized also a complete and compact apparatus organized in a modular framework with stand alone components characterized by high working performances. This laser subsystem also includes laser sources in an external cavity configuration specially oriented to transportable apparatuses; they are characterized by an intrinsical passive stability, robustness and very narrow emission linewidth. The experience with neutral atom manipulation and with atom cooling and trapping technique has been integrated with a direct implementation, characterization and optimization of a Two Dimensional Magneto Optical Trap as a cold atoms source for an operating Atom Interferometer. Such kind of implementation represents a fundamental topic for a transportable sensor realization.
Quantum sensors based on atomic interferometry are becoming valued tools for precision measurements in various research areas such as metrology, Earth sciences and inertial sensors. However, the verification of their accuracy is linked to the resolution and thus the free-fall time of the interferometer, which can only be extended on Earth by levitation or other methods, which themselves usually result in a loss of accuracy. Especially in the field of basic research, space-based measurement apparatuses are therefore fascinating prospects to set new standards for fundamental tests and measurements in Earth observation. In space, it is possible to drop the measuring devices alongside their test objects as long as desired, which allows a considerable extension of interferometry time. An important prerequisite for interferometry on such long time scales is the minimization of propagation velocity and initial expansion, which can only be achieved with ultra-cold atomic ensembles such as a bose-einstein condensate (BEC). Therefore, the creation of such a BEC on a compact, space-based platform represents a central challenge for the feasibility of atom interferometers in space. An important step for the necessary adaptation of existing laboratory equipment to space platforms is the sounding rocket mission maius-1, which is to develop and test both technology and methodology for the use of quantum sensors in space. The core of the scientific payload of maius-1 is an ultra-compact cold atomic source, based on the atom chip technology, employed to generate BEC in magnetic traps. The precise magnetic field control with atomic chips, in an environment like that of a sounding rocket, requires specialized current drivers that combine ruggedness, compactness and excellent noise performance that is yet unattainable with commercial technology. This thesis presents design and characterization of a new generation of compact current drivers for this purpose. Based on a typical preparation of a BEC, core specifications for current drivers are derived and their effects on the experiment are discussed. These specifications are then supplemented by the technical requirements of a sounding rocket. The evaluation of the requirements catalogue results in three current driver designs. First of all, an analog, flexible prototype is presented, which, in addition to the demonstration of miniaturization and high power densities, allows many possible settings of control parameters and has been used in the evaluation phase of the maius-1 atomic chip section. The experiences from this phase flowed into the designs of the maius flight hardware, which offer two architectures, adapted to the given technical conditions: one model for the employment at atom chips and one model to drive the external coils around the atom chip. The design process was supported by detailed circuit simulation. They enable fast optimization of control architecture and parameters for any load. Particularly in the case of coils, which are of crucial importance for magnetic traps, the prediction quality obtained represents a major technological advantage in terms of optimizing switching behavior, since predecessor experiments previously depended on heuristic methods for this task. With a volume reduction of more than one order of magnitude compared to laboratory electronics, and output currents of up to 10 A, the current driver modules of maius can be used under harsh temperature conditions from 10 to 70 ℃. They show a temperature drift of 100 ppm/K (chip) or 32 ppm/K (coils). In thermal equilibrium, the drivers work with a relative current stability of 3⋅10−5 (chip) and 5⋅10−6 (coils). Technical current noise is a decisive limitation of the lifetimes in magnetic atom traps. Thus, the noise characteristics of the designs are evaluated in a spectral analysis and compared with commercial laboratory equipment from previous experiments. The thereby obtained values for the integrated current noise of 108 μARMS (Chip) and 64 μARMS (Coils) (1Hz-99.8 kHz), represent an improvement by a factor of 3.8, or 6.4 in comparison to the commercial reference. To conclude the first characterization of the developed designs before the launch of maius the first operational measurements of the integrated source of cold atoms are presented and evaluated in view of temperature drift of the current driver modules in the experiment. The achieved performance values set a new benchmark in the field of compactification and noise performance and thus provide a key technology for compact, atomic chip-based quantum sensors. The presented designs operated successfully on the maius-1 mission which on January 23rd, 2017, among other things, could demonstrate the first creation of a BEC in space.