Since the possibility of trapping and cooling neutral atoms, ultracold quantum gases have shifted boundaries in a growing field of modern physics based on the first observation of Bose-Einstein condensates in 1995 and appreciated by the Nobel Prizes in 1997 and 2001. The current developments in the domain of atom optics lead to an utilization of ultracold quantum matter techniques in unique practical applications as high-precision atomic clocks, atom interferometer technologies and inertial sensing instruments for gravity field mapping, underground structure detection, autonomous navigation, as well as precision measurements in fundamental physics. The expectations of even higher precision measurements can be performed by arbitrarily extending the time of unperturbed evolution of those quantum systems. In respect thereof weightlessness provides an outstanding basis for such applications and measurements. Motivated by these prospects, many national and international groups have initialized research programs aiming for compact, transportable and ruggedly designed atom-optical experiments, which might be launched in parabolic flights and space applications. Thanks to an easy access to low gravity on earth, realization of quantum degenerated gases in excellent microgravity conditions at the Drop Tower Bremen opened a new kind of perspectives on atom-optical experiments, e.g., to currently achieve longest expansion times of Bose-Einstein condensates within the QUANTUS pilot project (up to one second). Thus, ultracold quantum matter in an environment of weightlessness represents an emerging area of science in quantum engineering with an impressive potential for a future technology and multidisciplinary applications.
Since the possibility of trapping and cooling neutral atoms, ultracold quantum gases have shifted boundaries in a growing field of modern physics based on the first observation of Bose-Einstein condensates in 1995 and appreciated by the Nobel Prizes in 1997 and 2001. The current developments in the domain of atom optics lead to an utilization of ultracold quantum matter techniques in unique practical applications as high-precision atomic clocks, atom interferometer technologies and inertial sensing instruments for gravity field mapping, underground structure detection, autonomous navigation, as well as precision measurements in fundamental physics. The expectations of even higher precision measurements can be performed by arbitrarily extending the time of unperturbed evolution of those quantum systems. In respect thereof weightlessness provides an outstanding basis for such applications and measurements. Motivated by these prospects, many national and international groups have initialized research programs aiming for compact, transportable and ruggedly designed atom-optical experiments, which might be launched in parabolic flights and space applications. Thanks to an easy access to low gravity on earth, realization of quantum degenerated gases in excellent microgravity conditions at the Drop Tower Bremen opened a new kind of perspectives on atom-optical experiments, e.g., to currently achieve longest expansion times of Bose-Einstein condensates within the QUANTUS pilot project (up to one second). Thus, ultracold quantum matter in an environment of weightlessness represents an emerging area of science in quantum engineering with an impressive potential for a future technology and multidisciplinary applications.
Low gravity provides an outstanding basis for precision measurements in atom optics pursuing multi-disciplines in fundamental physics. On the other hand it leads to an utilization of ultracold quantum matter techniques in unique practical applications. We report on the first establishments of atom optical experiments in the gravitation-free conditions at the Drop Tower Bremen in Germany, a facility of the Center of Applied Space Technology and Microgravity (ZARM). The chosen drop tower has an easy access to low gravity on earth with a daily usage, a more than sufficient time of free fall for experiments in quantum regimes (about 4s in drop mode and about 9s in catapult mode) and a well quality of weightlessness of 10-6g. Our report demonstrates the results of the precursor pilot project ATKAT ("atom catapult") and the current pilot project QUANTUS ("quantum systems under weightlesness") . The pure technological experiment ATKAT could successfully realize a compact and robust setup for trapping and cooling neutral rubidium atoms ( 87Rb) in microgravity conditions. After ATKAT's test accomplishment we have started the parallel developed QUANTUS main pilot projekt to investigate quantum degenerated gases in free fall. The first Bose-Einstein condensate (BEC) in a weightless environment world wide and longest evolution times of the 87Rb BEC up to 1s could be achieved within the science experiment QUANTUS. In respect thereof our collaboration opens further opportunities in atom optics. At least we will give an outlook of the new QUANTUS II project and its supporting project PRIMUS ("precision interferometry with matter waves under weightlessness") aiming on a precise inertial sensor by using an atom interferometer under weightlessness.
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.
Ultra-cold quantum gases in space promise to boost the sensitivity of matter-wave interferometers. Applications of the latter extend from fundamental physics over the use in navigation to interdisciplinary applications such as geodesy, e.g. satellite gravimetry [1, 2]. Exploiting quantum gases for high-precision interferometry places high demands on their control and manipulation. We take benefit of various microgravity platforms such as the Bremen drop tower [3], the Einstein elevator in Hannover [4], sounding rockets [5,6] and the international space station [7] to advance the necessary methods. The DLR-mission MAIUS-1 demonstrated Bose-Einstein condensation and performed first interferometry experiments [4]. NASA’s Cold Atom Laboratory continues this research in orbit on the ISS [7]. In addition, atom interferometry is pursued in highly dynamic environments such as parabolic flights [8]. Starting from a rubidium Bose-Einstein condensate, recently lowest expansion energies have been achieved by us in the Bremen drop tower as required for extending atom interferometry over several seconds [9]. Extending these methods to quantum mixtures not only opens up new physics in the absence of buoyancy, but also faces challenges regarding their use for interferometry. Interferometers based on two chemical elements have been proposed for quantum tests of the equivalence principle on the ISS as well as on satellites. Currently we prepare a sounding rocket mission to investigate the simultaneous generation and manipulation of potassium and rubidium condensates [10]. Together with CAL [7], these experiments will prepare the DLR-NASA multi-user facility BECCAL for research on quantum gas mixtures and interferometry [11] as well as enhance the readiness level of methods required for STE-QUEST [12], a proposal for a satellite mission currently studied in an modified version within ESA’s VOYAGE 2050 program [13].
Ultra-cold atoms have become indispensable in quantum sensing, quantum computing, and quantum communication applications, as well as in fundamental sciences. In quantum sensing, they enable precision measurements with outstanding sensitivities that can be of great benefit in navigation, geodesy, and fundamental physics applications. The sensitivity of the atom interferometers used herefore typically scales with the square of the interrogation time. Therefore, in atom interferometric measurements, one seeks ensembles that feature a particularly low internal kinetic energy and can thus be detected after a long time-of-flight. The lowest values are achieved using Bose-Einstein condensates in conjunction with delta-kick collimation, a technique for the collimation of the atomic wave function. Bose-Einstein condensation can be achieved using a multi-stage laser cooling scheme, followed by evaporative cooling, which finally brings the temperature down to the critical temperature, typically in the nanokelvin regime. In addition to the low thermal spread, microgravity enables particularly long observation times, as it avoids limitations of the interrogation time due to vacuum chamber constraints, which are present when falling freely under the influence of gravity. Additionally, microgravity enables measurements that are not possible under normal gravity conditions, such as buoyancy-free miscibility studies, truly uniform potentials, and advanced cold atom quantum memories, which are projected to feature unprecedented time-bandwidth products. For the preparation of a Bose-Einstein condensate, two techniques are widely used in science, namely magnetic traps and optical dipole traps. For microgravity missions, magnetic traps on atom chips have been used in nearly all setups so far due to their particularly low SWAP budget and high cold atom flux. In contrast, ground-based experiments often prefer optical traps due to their higher versatility, full optical access, and the absence of surfaces in proximity to the atoms, which is fundamental for precision measurements. Additionally, optical traps enable the manipulation of intra- and inter-species scattering lengths using Feshbach resonances. Compared to atom chip traps, they also feature an intrinsically better symmetry of the trapping potential, in theory leading to fewer aberrations in matter-wave optics. The latter could enable the generation of ensembles in the yet unreached femtokelvin regime using delta-kick collimation with fewer perturbations. This thesis is about the realization of a 1064nm optical dipole trap set up for the microgravity environment of the Bremen Drop Tower. It aims to highlight the advantages of optical traps in conjunction with microgravity and prepare optical trapping technology for use in future microgravity and space missions. The thesis describes the experimental setup, the direct loading process, the characterization of the optical potential, and the optimization and investigation of the evaporative cooling in the optical dipole trap. For the evaporation process, the influence of gravity is comprehensively investigated through analytical calculations and microgravity tests. Bose-Einstein condensation is demonstrated, proving the achievement of condensation through absorption imaging of the spatial distribution, tests on the free evolution of the ensemble, and investigation of the phase-space densities achieved. Up to (74 ± 4)% condensation of an ensemble featuring (30 000 ± 13 000) atoms at a kinetic temperature of (37 ± 3) nK is demonstrated. The evaporation shows high efficiency down to a 3 s evaporation time, while a critical phase-space density can be achieved within (1.29 ± 0.07) s using an even faster evaporation ramp. This opens the door to fundamental physics experiments on Bose-Einstein condensates in microgravity, as well as applications pursuing atomic ensembles with exceptionally low expansion velocities.
Cold and ultracold ensembles of atoms are increasingly used in different applications in science and technology. In recent years they also have been used as the source of optical atomic clocks or atom interferometers. With the latter it is possible to perform various fundamental physical measurements with very high accuracy, such as measurements of the fine structure constant or the gravitational constant. In addition, tests of fundamental theories, like general relativity, can be realized. This includes tests of the weak equivalence principle. Under weightlessness the sensitivity of a measurement of this kind can be signifcantly increased due to the longer interferometer time. This thesis presents the realization of a miniaturized laboratory system to analyze cold atomic ensembles in microgravity. The system provides, as the first of its kind, a cold 2 species mixture of rubidium and potassium atoms in the drop tower, which forms the starting point for a non classical measurement of the universality of free fall. Adding a second species implies a doubling of the complexity of the experiment compared to existing 1 species experiments in microgravity. In a system, comprising a source chamber and an experiment chamber, 87Rb and 39K atoms are cooled by magneto optical traps. A newly designed 2 species oven for use in a zero gravity environment provides the required steam pressure in the source chamber. A FE simulation confirms the experimentally measured parameters of the source. A combined and miniaturized state of the art laser system allows the preparation and detection of the atoms and also contains additional laser modules to implement the needed laser pulse beam splitter for an atom interferometer in the future. The 2 species laser system is designed in such a way that it takes place on a capsule platform with a diameter of 70 cm. It was tested in a drop tower campaign in terms of stability. In addition, the drop capsule includes an optical dipole trap with a wavelength of 1950 nm to trap atoms with this technology and cool them evaporatively in weightlessness for the first time. The light of a thulium fiber laser with an optical output power of 26 W is guided to the experiment chamber by a free beam setup and generates the dipole trap. An optical dipole trap is a promising alternative to atom chip experiments with magnetic traps. One of the advantages are better starting conditions for an atom interferometer. All in all, the realized laboratory system fulfills pathfinder characteristics and is a technology demonstrator for space projects with atom optical focus. The realization of a cold 2 species ensemble and an optical dipole trap for use in microgravity offer further possibilities for future satellite based atom interferometers.
Quantum technology based on cold-atom interferometers is showing great promise for fields such as inertial sensing and fundamental physics. However, the best precision achievable on Earth is limited by the free-fall time of the atoms, and their full potential can only be realized in Space where interrogation times of many seconds will lead to unprecedented sensitivity. Various mission scenarios are presently being pursued, which plan to implement matter-waves inertial sensors to detect gravitational waves, to measure the Earth gravity field or to test fundamental physics such as the Equivalence Principle.We developed a cold atom sensor experiment adapted to microgravity platforms, especially the ZERO-G plane. To achieve a test of the Universality of Free Fall, we realize the first onboard operation of simultaneous ^{87}Rb - ^{39}K interferometers in the weightlessness environment produced during parabolic flight. The large vibration levels (10 ^{-2} g/Hz ^{1/2}), acceleration range and rotation rates (5 deg/s) during flight present significant technical challenges for atom interferometers. By hybridizing each atomic sensor with a common mechanical accelerometer, we achieve a sensitivity of 3.4 10 ^{-5} g per shot-more than 1600 times below the ambient vibration level onboard the aircraft. We demonstrate the capability of our dual-quantum sensor by measuring the Eötvös parameter with systematic-limited uncertainties of 3.0 10 ^{-4} during micro-gravity. This constitutes the first quantum test of the equivalence principle in a free falling vehicle. Our results can be extended to the trajectory of a satellite for future Space missions. We also achieved a simultaneous ^{87}Rb- ^{39}K interferometer on ground with a long term stability on the differential acceleration measurement around 5 10 ^{-8}g, limited by the fall of the atom clouds. To increase the interrogation time and the sensitivity, a ZERO-G simulator has been installed in our laboratory and allows our experiment to be in free fall during 500 ms every 12 seconds, to achieve highly repeatable measurements in microgravity. We expect to increase our short-term sensitivity by two orders of magnitude, and consequently reach a long-term measurement of the Eötvös parameter at the level of 10 ^{-11}.To benefit from the free fall of the experiment completely, it is necessary to produce atoms clouds with a reduced velocity dispersion. To reach this goal, we put ^{87}Rb atoms in an optical dipole trap supplied by a compact fiber laser compliant with onboard applications and decrease the optical power on a very controlled way to obtain evaporative cooling. Preliminary results of ultra-fast evaporative cooling (less than a few seconds) onboard the ZERO-G plane allowed us to reach a regime of temperature compliant with an interrogation time of the order of one second. This method and technology will be tested on the ZERO-G simulator as well.