Compact, low-noise current drivers for quantum sensors with atom chips

Abstract

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.