Rasel, Ernst M.
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Rasel, Ernst M.
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Rasel, Ernst M.
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Item-typ:Veröffentlichung, Matter-wave Interferometry for space-borne Inertial Sensors(2019-02-15); ; ; Inertial sensors based on matter-wave interferometry are currently approaching the precision and accuracy of state-of-the-art classical sensors. While these devices are often realised with ultracold but not Bose-condensed atoms as matter waves, employing Bose-Einstein Condensates (BEC) promises to overcome certain limitations, especially those related to the ensemble's expansion. The point-source like character of BECs also enables utilising spatial interference patterns to measure e.g. rotation rates in single-shot experiments. Matter wave-based inertial sensors are considered for experiments ranging from Gravitational Wave detection to tests of the Universality of Free Fall (UFF) to gain insight into the joint between Quantum Mechanics and General Relativity. In the scope of this thesis, matter-wave interferometry with BECs was demonstrated for the first time in a microgravity environment with the QUANTUS-1 apparatus. The same instrument was then employed as a quantum tiltmeter utilising a novel beam-splitting mechanism known as Bragg Double Diffraction. To this end, the QUANTUS-1 apparatus designed as a BEC instrument to be operated in the drop tower at ZARM at University of Bremen was equipped with optics and laser systems required for performing matter-wave interferometry based on Bragg Diffraction. The apparatus employs an atom chip to create BECs of around 10000 Rubidium 87 atoms within 15 s. With a Mach-Zehnder like interferometer scheme, spatial interference fringes were observed after a free evolution time in the interferometer of up to 677 ms. To achieve these long time scales, a method known as Delta-Kick Collimation (DKC) was adapted to slow the expansion of the BEC to a kinetic energy equivalent below 1 nK, and the atoms were transferred to a non-magnetic Zeeman state via an adiabatic rapid passage (ARP). A similar interferometer scheme with a newly developed beam-splitter mechanism known as Bragg Double Diffraction was used to measure the tilt of the instrument on ground with a precision of up to 4.4 AA rad. This thesis presents an overview of the apparatus including ground-based characterisations of all required experimental steps. Results from over 400 free fall experiments are evaluated for expansion studies of the BEC and matter-wave interferometry in microgravity. The time-evolution of first and second order Bragg Double Diffraction beam splitters is studied, and an interferometer sensitive to the tilt of the instrument is implemented. Based on this work, a gravimeter with a new launch mechanism comprising Bragg beam splitters and Bloch oscillations to enable atomic fountains in atom-chip based devices was developed. The microgravity experiments were adapted for the MAIUS-1 sounding-rocket instrument to create the first man-made BEC in outer space and study the feasibility of operating matter-wave interferometers on space-borne platforms. The results of this thesis lay the groundwork for future space-borne missions using matter-wave interferometry for precision measurements of inertial forces.Dissertation629 572 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Atom interferometry with picokelvin ensembles in microgravity(2022-02-25); ; ; Atom interferometry enables precision measurements with outstanding sensitivities in a broad field of applications, ranging from fundamental physics to applications in geodesy or navigation. The development of robust and mobile devices paves the way for future satellite missions, e.g. striving for improved spaceborne gravimetry or a precision test of the universality of free fall. The sensitivity of an atom interferometer scales quadratically with the interrogation time. Consequentially, exceptional sensitivities can be reached for interferometry with free evolving ensembles on time scales of several seconds, achievable by operating on a microgravity platform. Such long interrogation times necessarily require ensembles with ultra-low expansion rates, making collimated Bose-Einstein condensates (BEC) the ideal input states. Therefore a method called magnetic lensing is used to narrow the momentum distribution. Together with the very good coherence properties of BECs, this reduces uncertainties in the interferometric measurement and enables high-fidelity beam splitter processes like Bragg diffraction. Within the scope of this thesis, a novel matter-wave lens system is presented to lower the internal kinetic energy of a BEC to the picokelvin regime, which is then used to perform interferometric measurements in microgravity. This is achieved with the QUANTUS-2 apparatus, a high-flux rubidium BEC machine based on atom chip technology, which operates at the drop tower in Bremen. Exploiting the excitation of a quadrupole mode in combination with a magnetic lens attains three-dimensional collimation of the BEC. With this technique, an unprecedented residual kinetic energy of $\sfrac{3}{2}k_B\cdot38\,$pK is achieved, where the ensemble is observed after an interrogation time of 2$\,$s with a high signal-to-noise ratio. Upgrading the experiment to realize single and double Bragg diffraction enables the first demonstration of a double Bragg-based interferometer in microgravity with a retro-reflection setup. The symmetric splitting achieved with the double Bragg process doubles the enclosed interferometer area and reduces systematic effects compared to single diffraction techniques. A complete characterization is performed to optimize the beam splitting process and verify the feasibility of atom chip setups for interferometric measurements. The potential of magnetically lensed BECs for interferometric measurements is investigated by probing the spatial coherence. To this end, a novel application of shear interferometry is developed to investigate the divergence of the magnetically lensed ensemble in analogy to an optical shear plate. Based on the interferometry pattern, the imperfections of the magnetic lens potential are studied, and the lens strength is optimized. Shear interferometry even enables the spatially resolved determination of the BEC's velocity field based on the interferometry pattern. Consequentially, the internal kinetic energy can be deduced from a single absorption image. Especially compact, ground-based atom interferometers can profit from this characterization method since extended times of flight are not required. This shear interferometry represents a versatile tool to study BEC dynamics independently of the application in matter-wave optics. The first demonstration of interferometry with picokelvin atomic ensembles and the tools developed in this work provide the basis to realize atom interferometry on extended time scales of several seconds. This will ultimately enable future space missions to employ cold atom interferometry at unrivalled levels of precision.Dissertation402 370 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Atom interferometry in a twin lattice(2020-03-20); ; ; Atom interferometers represent a well-proven tool for precision measurements and are used for a variety of applications ranging from geodesy and inertial navigation to fundamental questions in physics. State-of-the-art sensors typically operate with laser-cooled atoms, which exhibit a relatively large spatial and momentum width. The systematic uncertainties associated with the latter can be overcome by employing Bose-Einstein condensates (BECs). The sensitivity of an atom interferometer can be enhanced by the interrogation time as well as by a large momentum separation of the atomic wave packets. Hereby, a small atomic velocity width is crucial to achieve high-fidelity manipulation. Within this thesis, the realization of a novel beam splitter for the transfer of large momentum on an ultra-cold atom cloud is presented. The condensate is generated in a miniaturized atom-chip based setup and its expansion rate is further reduced by delta-kick collimation. The beam splitting light field consists of two counterpropagating lattices with orthogonal polarizations. In such a twin lattice, the efficient combination of double Bragg diffraction with Bloch oscillations promises to overcome current limitations of large momentum transfer. Bloch transfer efficiencies of more than 99.9% per photon recoil (ħk) can be achieved and, thus, excellent scalability is provided. The symmetry of the twin lattice enables the suppression of systematic errors. These features allow for the realization of a symmetric Mach-Zehnder-type geometry, where contrast can be observed up to a maximum splitting of 408ħk corresponding to a total transfer of 1632ħk. To our knowledge, this represents the largest momentum separation in an atom interferometer reported so far. A detailed experimental and theoretical study reveals that the current limitations are solely caused by technical properties of the experiment. In particular, light field distortions arising due to the diffraction of the laser beam at different apertures cause a dephasing leading to a contrast decay. The results open up new routes for the miniaturization of inertial quantum sensors as well as for gravitational wave detectors. In addition, the combination of an atom-chip trap with an optical dipole trap is investigated, which serves as a pathfinder experiment for atom interferometry in optical waveguides. Waveguides allow extending the interrogation time without an increase of the interferometer region. Optimizing the spatial overlap of both traps as well as the temporal sequence, the BEC can be transferred into the dipole trap with an efficiency of over 99%. Such a setup also has applications in future atom-chip experiments featuring two atomic species, whose interactions can only be controlled in a purely optical trap.Dissertation629 810 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, An Optical DipoleTrap in Microgravity(2019-04-26); ; ; 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.Dissertation480 235