Optical interferometers are well known for their high precession in several measurement situations. They are based on a readout of phase differences between optical path length. In contrast, atom interferometers rely on the De Broglie wavelength of atoms and their corresponding phase. A cloud of atoms is split and recombined with light pulses, while in between, the atoms are moving freely. These precise quantum tools will enhance a broad variety of measurements, ranging from large scale phenomena like gravitational wave detection to short scale Casimir-Polder forces and everything in between. The sensitivity of atom interferometers scales with the time squared between light pulses, which is generally limited by gravity and temperature. One technical challenging sollution is the realization of atom interferometers in microgravity. While ultra cold atoms can be prepared in optical and magnetic traps, under weightlessness their preparation is limited to magnetic traps until now. These experiments are based on atom chips with highly asymmetric trapping potentials. The improved symmetry of optical traps can improve the most advanced cooling technique, called delta kick collimation, leading to atomic clouds in the yet unreached femtokelvin range. This thesis is about the first realization of an optical trapping potential in microgravity. It describes the experimental setup, identifies a molasseses technique as the optimal process for dipole trap loading and investigates the further cooling process under the influence and in the absence of gravity. In optical and magnetic traps, the preparation of ultracold atoms is based on evaporation. However, the underlying physics to drive this process are fundamentally different and for optical traps, evaporation is driven by gravity. It is demonstrated that evaporation from an optical potential performs approximately equal with and without gravity, due to a strong mixing of the trapping potentials in all three spatial dimensions. These findings are confirmed with computational simulations, based on the Direct Simulation Monte Carlo method. Atomic ensembles of rubidium-87 with temperatures as low as a 300 nK were generated in the microgravity environment of the drop tower in Bremen with an evaporation time as short as 0.5 s. The final confining trap was too shallow to be reproduced in a laboratory environment. The findings of this thesis will possibly guide the way to improved atom interferometers, with countless applications in precise sensing. Furthermore, optical potentials in microgravity on their own will open up a broad field of fundamental physical experiments. One example is the applicability of magnetic Feshbach resonances. Allowing almost arbitrary tuning of the interatomic scattering length offers new insights in scattering processes or miscibility scenarios.
Optical interferometers are well known for their high precession in several measurement situations. They are based on a readout of phase differences between optical path length. In contrast, atom interferometers rely on the De Broglie wavelength of atoms and their corresponding phase. A cloud of atoms is split and recombined with light pulses, while in between, the atoms are moving freely. These precise quantum tools will enhance a broad variety of measurements, ranging from large scale phenomena like gravitational wave detection to short scale Casimir-Polder forces and everything in between. The sensitivity of atom interferometers scales with the time squared between light pulses, which is generally limited by gravity and temperature. One technical challenging sollution is the realization of atom interferometers in microgravity. While ultra cold atoms can be prepared in optical and magnetic traps, under weightlessness their preparation is limited to magnetic traps until now. These experiments are based on atom chips with highly asymmetric trapping potentials. The improved symmetry of optical traps can improve the most advanced cooling technique, called delta kick collimation, leading to atomic clouds in the yet unreached femtokelvin range. This thesis is about the first realization of an optical trapping potential in microgravity. It describes the experimental setup, identifies a molasseses technique as the optimal process for dipole trap loading and investigates the further cooling process under the influence and in the absence of gravity. In optical and magnetic traps, the preparation of ultracold atoms is based on evaporation. However, the underlying physics to drive this process are fundamentally different and for optical traps, evaporation is driven by gravity. It is demonstrated that evaporation from an optical potential performs approximately equal with and without gravity, due to a strong mixing of the trapping potentials in all three spatial dimensions. These findings are confirmed with computational simulations, based on the Direct Simulation Monte Carlo method. Atomic ensembles of rubidium-87 with temperatures as low as a 300 nK were generated in the microgravity environment of the drop tower in Bremen with an evaporation time as short as 0.5 s. The final confining trap was too shallow to be reproduced in a laboratory environment. The findings of this thesis will possibly guide the way to improved atom interferometers, with countless applications in precise sensing. Furthermore, optical potentials in microgravity on their own will open up a broad field of fundamental physical experiments. One example is the applicability of magnetic Feshbach resonances. Allowing almost arbitrary tuning of the interatomic scattering length offers new insights in scattering processes or miscibility scenarios.
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.
Most cold atoms experiments in microgravity platforms or in Space are achieved using atom chips, leading to limitations in terms of optical access and inhomogeneous magnetic fields. Optical dipole traps do not have these drawbacks but have difficulties producing atomic samples with a large number of atoms at ultra low temperature in the absence of gravity. Here, we report on an efficient evaporative cooling in two-crossed laser beams during parabolic flights. Time-averaged potentials combine the advantages of large capture volume and trap compression, increasing the initial phase space density and collision rate to favor the evaporative process. With this technique we demonstrate the production of an ultra cold gas of 2.5×104 rubidium atoms at a temperature below 100 nK in less than 4 seconds. Our experiment paves the way for the development of quantum sensors applied to fundamental physics and geodesy as well as the study of ultracold atomic physics in Space.
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.
This thesis describes recent experiments conducted on a Bose-Einstein condensate in an optical waveguide. This optical potential confines the atoms against gravity in the vertical dimension, guiding them to freely propagate in the horizontal. Being supported against gravity enables long expansion times of hundreds of milliseconds, facilitating techniques such as delta-kick cooling. In an ideal atom interferometer, the beamsplitters would impart a large momentum splitting between the interfering states in order to increase the signal. However, their effective use requires both an even narrower momentum width source and good vibration isolation. Three such techniques were investigated in this thesis: reflection from a repulsive light potential barrier, Bragg transitions from an optical lattice (which are effectively bouncing atoms off a moving grating), and Bloch-acceleration by loading the atoms into such an optical lattice and then accelerating the combined system. It is found that a combination of both Bragg and Bloch provides the most promising route to truly large momentum transfer in a system which is sensitive only to acceleration. Lastly, large-momentum transfer techniques can be used to effectively increase the way in which the output signal scales with time, creating interferometers which generate the same sensitivity faster (increasing the bandwidth of a sensor), or generate a much better sensitivity in the same time. We form a dual condensate of both rubidium-85 an rubidium-87 via sympathetic cooling. The collisional properties of Rubidium-85 can be modified by applying an external magnetic field, at an easy-to-experimentally-reach value of between 150-170G. It turns out that this combination of atomic isotopes is ideal for an interferometric test of the weak equivalence principle, one of the underpinnings of General Relativity. This thesis also presents the results of the first Bose-condensed version of such a dual-species interferometer. By removing any 87Rb left after condensation, we have created a pure 85Rb BEC. Using this we can now explore how the inter-atomic interactions affect the phase shift of a condensed atom interferometer, as we have complete control over the interaction strength. A condensate with no inter-particle interactions should behave like an ideal particle. With a small attractive interaction between atoms, it is possible to create a self-trapped cloud of atoms which propagates dispersionlessly. This cloud is known as a soliton, and it is predicted to have even more interesting quantum mechanical properties. For example it is predicted that by colliding two such solitons, an entangled state can be generated. Our results indicate that the dispersionless character of the soliton out-performs all other interaction strengths in an atom interferometer, including even the non-interacting cloud.
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.
Inertial navigation systems are vital for modern society, be it on airplanes or in the context of autonomous mobility. On the other hand atom interferometers are exquisite measurement devices for inertia in fundamental physics and field applications, but are mostly limited to measuring one inertial observable. By utilizing time averaged optical potentials, the ultra-cold atomic ensemble input to the interferometer can be prepared in nearly arbitrary fashion. This results in a boost of preparation speed and number of atoms in the interferometer by increasing the size of the trap to match the size of the pre-cooled ensemble for optimal loading. Successive fast reduction of the extend of the trap leads to optimal evaporation efficiency and leads to increased sensitivity of the device, due to reduced cycle time of the experiment and increased atom number. Beyond this, the technique allows for new sensor geometries via the creation of multiple single wells, allowing for differential atom interferometry with numerous simultaneous interferometers. In this thesis, the first matter-wave analogue of a classical accelerometer array is implemented via the creation of a 3 × 3 array of Bose-Einstein condensates, consisting of 3 × 105 atoms and spanning a total area of 1.6 mm2 . Successive manipulation of the matter-waves with atom optics, coherently addressing the momentum states of the ensembles, allows for splitting, redirecting and readout pulses, realizing Mach-Zehnder type atom interferometers. Correlation of the nine 1D-acceleration measurements output by the interferometers yields measurements of linear acceleration, Euler- and centrifugal acceleration. The estimated sensitivity limits per shot are 6 × 10−5m s−2 for linear acceleration, 0.19 rad s − 2 for rotational acceleration and 203 mrad s−1 for rotation rate and are mainly limited due to the short free fall time of the ensembles, constrained by the atom optics light field. The correlation also allowed to determine the rotation center, being close to the retro reflection mirror of the atom optics light field. Crucial for the realization of this measurement was the implementation of time averaged potentials as stated above, facilitated through two 2D acousto-optical deflectors and a software defined radio, allowing for generating the required RF-signals. The techniques to simultaneously load and evaporate multiple ensembles into distinct traps from a single cloud of cold atoms and subsequently enlarge the spacing between the ensembles with adiabatic transport ramps are of highest interest for e.g. the Very Long Baseline Atom Interferometer facility in Hannover. Due to the correlated outputs systematic errors, such as wave front abberations, can be nullified. Furthermore the scheme can be extended to 3D matter-wave arrays, fulfilling all requirements necessary for an autonomous inertial measurement unit.