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

Manuel Popp

Published Jan 1, 2018• 0 citations
Publisher

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.

DOI
10.15488/3688

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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.
Bose-Einstein-Condensates (BECs) can be used as a very sensitive tool for experiments on fundamental questions in physics like testing the equivalence principle using matter wave interferometry. Since the sensitivity of these experiments in ground-based environments is limited by the available free fall time, the QUANTUS project started to perform BEC interferometry experiments in micro-gravity. After successful campaigns in the drop tower, the next step is a space-borne experiment. The MAIUS-mission will be an atom-optical experiment that will show the feasibility of experiments with ultra-cold quantum gases in microgravity in a sounding rocket. The experiment will create a BEC of 10(5) (87)Rb-atoms in less than 5 s and will demonstrate application of basic atom interferometer techniques over a flight time of 6 min. The hardware is specifically designed to match the requirements of a sounding rocket mission. Special attention is thereby spent on the appropriate magnetic shielding from varying magnetic fields during the rocket flight, since the experiment procedures are very sensitive to external magnetic fields. A three-layer magnetic shielding provides a high shielding effectiveness factor of at least 1000 for an undisturbed operation of the experiment. The design of this magnetic shielding, the magnetic properties, simulations, and tests of its suitability for a sounding rocket flight are presented in this article.
The QUANTUS consortium started activities to perform atom interferometry on a sounding rocket in the year 2011. Three sounding rocket missions (MAIUS-1 to 3) have been planned to demonstrate Bose-Einstein condensation and atom interferometry with rubidium and potassium atoms. Two different payloads MAIUS-A and MAIUS-B are being designed, qualified and integrated in the time frame from 2011 until 2020. The MAIUS-A scientific payload demonstrated the first Bose-Einstein Condensates in Space in the challenging environment aboard a two-staged VSB-30 sounding rocket on the 23rd of January 2017. In order to achieve this ambitious scientific goal the experiment used various sensitive instruments imposing strong requirements on the thermal and mechanical design of the scientific payload. This paper gives a summary of the MAIUS-A and MAIUS-B scientific payload design. Furthermore, the lessons learned from the MAIUS-1 maiden flight as well as the integration and testing activities and their impact on the MAIUS-B scientific payload design are presented. The paper closes with an outlook on future atom interferometry payloads on sounding rockets and beyond.
Quantum sensors based on the interference of cold atoms have advanced to the forefront of precision measurements in geodesy, metrology and tests of fundamental physics. The ultimate potential of these devices is realized using quantum degenerate atoms in extended free fall. This can be achieved on microgravity platforms such as drop towers, parabolic flights, ballistic rockets, satellites and space stations. The transition to mobile and robust devices that can withstand the demands of these environments comes with many challenges. Quantum sensors need to be scaled down and integrated without compromising their performance. In fact, they need to significantly outpace conventional instruments, since microgravity time is an expensive resource and limited to a few seconds at a time on the most accessible platforms. This thesis describes the construction, qualification and operation of a miniaturized ultracold atom experiment that meets these challenges. The QUANTUS-2 apparatus features a payload weight of 147 kg and a payload volume of 0.3 m^3. It generates Bose-Einstein condensates of 4×10^5 ^87Rb atoms every 1.6 seconds, a flux of ultra-cold atoms that is on par with the best lab-sized devices. Ensembles of 1×10^5 atoms can be created at a 1 Hz rate. It is currently the fastest machine of its kind and achieves the highest atom number of any atom chip setup. The apparatus continuously withstands peak accelerations of up to 45 g during microgravity campaigns at the drop tower facility in Bremen, Germany. Here, the payload has accrued 208 drops and 9 catapult launches over 24 month. The setup is the first atom optics experiment to stand up to the technical demands of catapult operation. Four condensates can be created and observed consecutively during nine seconds of free fall in a single catapult launch. In total, the experiment has been suspended in microgravity for over 17 minutes. With the record source performance, the repetition rate for microgravity experiments with ultra-cold atoms was increased by a factor of four compared to previous devices. The total atom number was increased by a factor of 40, vastly improving the signal to noise ratio for absorption images of spatially extended clouds. The ensembles can be prepared consistently over many weeks of drop tower operation. The variance of the mean center of mass velocity in two observable directions is 7.3 μm/s and 6.9 μm/s. Magnetic lensing techniques were employed to manipulate the expansion of the ensembles. First results yield a residual expansion rate in three dimensions of σ_v = 116.9 ± 13.9 μm/s, which implies a three-dimensional effective temperature of T = 47.6 ± 11.3 pK at an average condensate atom number of N = 93000. These values constitute the best collimation of any atomic ensemble and the most promising source for atom interferometry reported to date. Optimizing the current lensing sequence will reduce the expansion rate further to effective temperatures in the femtokelvin regime. The level of control demonstrated over the condensates is highly relevant for the advancement of matter-wave optics and quantum sensors. Controlling the motion and size of atomic clouds is intrinsically tied to many systematic effects in high precision measurements. QUANTUS-2 will provide a platform to explore and mitigate these limitations on unprecedented time scales of up to seven seconds of free evolution.
The QUANTUS consortium started activities to perform atom interferometry on a sounding rocket in the year 2011. Three sounding rocket missions (MAIUS-1 to MAIUS-3) have been planned to demonstrate BoseEinstein condensation and atom interferometry with rubidium and potassium atoms. Two different payloads MAIUS-A and MAIUS-B have been designed qualified and integrated in the time frame from 2011 until 2020. The MAIUS-A scientific payload is capable of performing single species experiment with rubidium only and was used as a technology demonstrator in the MAIUS-1 mission. The MAIUS-1 mission was conducted in fall 2016. On the 23rd of January 2017 the payload MAIUS-A was launched on a VSB-30 sounding rocket. The payload created the first Bose-Einstein condensate (BEC) and performed atom interferometry aboard a sounding rocket with rubidium 87 atoms [Becker 2018]. Its successor MAIUS-B will be able to perform simultaneous experiments with Bose-Einstein Condensates of Rubidium-87 and Potassium-41 atoms. In order to achieve this ambitious scientific goal the experiments use various sensitive instruments imposing strong requirements on the thermal and mechanical design of the scientific payload. This paper gives a summary of the MAIUS-A and MAIUS-B scientific payload design with a focus on the thermal control system. Furthermore, the design of the GSE and the water cooling umbilicals is presented. The paper closes with an outlook on future atom interferometry payloads on sounding rockets and beyond.
Recent developments in quantum technology have resulted in a new generation of sensors for measuring inertial quantities, such as acceleration and rotation. These sensors can exhibit unprecedented sensitivity and accuracy when operated in space, where the free-fall interrogation time can be extended at will and where the environment noise is minimal. European laboratories have played a leading role in this field by developing concepts and tools to operate these quantum sensors in relevant environment, such as parabolic flights, free-fall towers, or sounding rockets. With the recent achievement of Bose–Einstein condensation on the International Space Station, the challenge is now to reach a technology readiness level sufficiently high at both component and system levels to provide “off the shelf” payload for future generations of space missions in geodesy or fundamental physics. In this roadmap, we provide an extensive review on the status of all common parts, needs, and subsystems for the application of atom-based interferometers in space, in order to push for the development of generic technology components.