This thesis describes theoretical work carried out in connection with the high-mass matter-wave interference experiments at the University of Vienna. Its scope ranges from the simulation and analysis of experimental data to the proposal of a trapped interference scheme applicable to a variety of matter-wave platforms. The first main result of the thesis is the phase-space description of a near-field interferometer with any number of gratings. This theory is used to analyze a four-grating Talbot-Lau interferometer, in which passive compensation of low-frequency vibrations and of the Coriolis force are expected. The compensation is numerically confirmed and its effectiveness is compared to existing techniques. The second main result is the demonstration of Bragg diffraction of complex molecules on optical gratings, to which the author contributed the basic experimental design, simulations, and the interpretation of the data. This demonstration is a first step towards applying large momentum transfer coherent manipulation techniques to the interference of heavy molecules and clusters. The third main result is a new interference scheme for matter waves in toroidal confinement. The beam splitting and recombination in this scheme rely on the free evolution in the waveguide and require no interaction with the matter waves. The scheme is within reach of state-of-the-art experiments with Bose-Einstein condensates, where it could be used to measure weak atom-atom interactions or magnetic field gradients.
This thesis describes theoretical work carried out in connection with the high-mass matter-wave interference experiments at the University of Vienna. Its scope ranges from the simulation and analysis of experimental data to the proposal of a trapped interference scheme applicable to a variety of matter-wave platforms. The first main result of the thesis is the phase-space description of a near-field interferometer with any number of gratings. This theory is used to analyze a four-grating Talbot-Lau interferometer, in which passive compensation of low-frequency vibrations and of the Coriolis force are expected. The compensation is numerically confirmed and its effectiveness is compared to existing techniques. The second main result is the demonstration of Bragg diffraction of complex molecules on optical gratings, to which the author contributed the basic experimental design, simulations, and the interpretation of the data. This demonstration is a first step towards applying large momentum transfer coherent manipulation techniques to the interference of heavy molecules and clusters. The third main result is a new interference scheme for matter waves in toroidal confinement. The beam splitting and recombination in this scheme rely on the free evolution in the waveguide and require no interaction with the matter waves. The scheme is within reach of state-of-the-art experiments with Bose-Einstein condensates, where it could be used to measure weak atom-atom interactions or magnetic field gradients.
We show our progress in setting up a three grating near-field Talbot-Lau interferometer (TLI) for large molecules such as C60. The TLI is the preferred tool for showing the wave nature of large particles, as it accepts beams of poor spatial coherence and because of its scaling properties with increasing mass of the particles. The spatial coherence is prepared by the first grating, it can be seen as an array of many collimation slits. Our setup is highly flexible in terms of the possibility of changing most of the parameters (such as distances between the gratings and grating angles) from the outside of the UHV-chamber. As the interferometer and the molecular beam source are place in separate vacuum chambers, refilling the oven and adjusting the setup parameters to different molecules is possible without opening the interferometer chamber. This is necessary, as the pressure in the interferometer chamber has to be lower than 5.10-7 mbar in order to observe high interference contrast. The TLI is highly sensitive to vibrations, especially in the range of 1 kHz (the inverse time-of-flight of the molecules through the interferometer), which makes vibration damping crucial for the success of observing interference. We observed vibration amplitudes of up to 75 nm at low frequencies (158 Hz). Compared to the inverse time-of-flight of the molecules, these vibrations are not 'seen' by the molecules and are non-critical. We observed interference of C60 with a contrast of 27 %, by scanning the self-image of the diffraction grating. In the present setup, an interference contrast of 60 % should be observed with perfect alignment of the gratings. Time-of-flight measurements showed small modulations of the velocity distribution at for interference expected distances of 2.5 ms. We also show the possibility of reconstructing the Wigner distribution function out of measurements for the case of Talbot interference under experimental conditions.
We discuss modern developments in quantum optics with organic molecules, clusters and nanoparticles -- in particular recent realizations of near-field matter-wave interferometry. A unified theoretical description in phase space allows us to describe quantum interferometry in position space and in the time domain on an equal footing. In order to establish matter-wave interferometers as a universal tool, which can accept and address a variety of nanoparticles, we elaborate on new quantum optical elements, such as diffraction gratings made of matter and light, as well as their absorptive and dispersive interaction with complex materials. We present Talbot-Lau interferometry (TLI), the Kapitza-Dirac-Talbot-Lau interferometer (KDTLI) and interferometry with pulsed optical gratings (OTIMA) as the most advanced devices to study the quantum wave nature of composite matter. These experiments define the current mass and complexity record in interferometric explorations of quantum macroscopicity and they open new avenues to quantum assisted metrology with applications in physical chemistry and biomolecular physics.
In order to explore the quantum-to-classical transition, the ambition of this thesis was the design and synthesis of tailor-made molecules for matter-wave interference experiments. Firstly, the focus was set on a deeper understanding of the influence of a molecule’s internal structure, especially its polarizability, on quantum interference. Furthermore, the limits of quantum mechanics were addressed by increasing the size of the interfering objects. The metrological aspect of molecule interferometry - to measure influences of molecule properties on the wave nature - is described in the first part of this thesis. Two fourfold perfluorohexyl functionalized constitutional isomers were synthesized in order to demonstrate that different molecular conformations can be distinguished in quantum interference experiments. A tetrahedral isomer and a rod-like oligo(phenylene ethynylene) derivative were applied in a near-field Kapitza-Dirac-Talbot-Lau interferometer. Although de Broglie quantum interference describes the center of mass motion of a massive body, it was shown to be sensitive to the internal molecular structure of the constitutional isomers. The different total susceptibilities of both isomers lead to different de Broglie interference shifts in the presence of external electric fields. The isomers thus become distinguishable in spite of their identical mass and chemical sum formula. Besides metrological applications, a main goal of this thesis was to synthesize model compounds for molecule interferometry to increase the preexisting mass record for quantum interferometry, which was held by the fluorofullerene C60F48 with a molecular weight of 1 633 g/mol. Near-field interferometry is a promising concept to set new records and thus to approach the quantum-to-classical transition. To meet the requirements of quantum studies with complex organic molecules in a Kapitza-Dirac-Talbot-Lau near-field interferometer – high volatility, high stability, and high molecular mass – we chose the functionalization of tetraarylporphyrins with perfluoroalkyl moieties. A modular synthesis of seven perfluoroalkyl-substituted porphyrin derivatives with up to 16 fluorous alkyl chains was presented. With this series in hand, it was possible to observe interference patterns for three of the seven derivatives. The heaviest molecule of this series for which the wave nature has been observed has a 3.3 fold higher molecular weight than the preexisting mass record for quantum interference. These experiments open a new window for quantum experiments with nanoobjects in a complexity class comparable to that of small proteins. The experimental setup of the near-field Kapitza-Dirac-Talbot-Lau interferometer is not restricted to monodisperse and pure starting compounds. It also allows for experiments with compound mixtures because individual components of compound libraries can be detected in the mass spectrometric detection unit. The preparation of porphyrin libraries gave access to compounds in a new mass region (> 10 000 g/mol). Preliminary investigations in near-field interferometers with these structures revealed suitable molecule beam properties. With these porphyrin libraries in hand, it is expected to exceed for the first time the ceiling of 10 000 g/mol for an interfering object in the future. Large, stable, highly volatile and fluorescent molecules were the goal of synthetic work that was performed for novel far-field interference experiments in a setup based on fluorescence detection. For this purpose three fluorescent fluorous phthalocyanine derivatives were prepared. With the help of a new laser-controlled micro-evaporation source it was possible to produce a beam of molecules with the required intensity and coherence. The observation of the wave nature of a free base phthalocyanine having a molecular weight of 1 299 g/mol set a new mass record for far-field molecule interferometry. The preexisting record for far-field interferometry was held by the fullerene C70 with a mass of 841 g/mol. To conclude, major contributions to the exploration of the frontiers of quantum mechanics were achieved by matter-wave interferometry using the perfluoroalkyl-functionalized molecules described in this thesis. \n
We present a theoretical framework to describe the effects of decoherence on matter waves in Talbot-Lau interferometry. Using a Wigner description of the stationary beam the loss of interference contrast can be calculated in closed form. The formulation includes both the decohering coupling to the environment and the coherent interaction with the grating walls. It facilitates the quantitative distinction of genuine quantum interference from the expectations of classical mechanics. We provide realistic microscopic descriptions of the experimentally relevant interactions in terms of the bulk properties of the particles and show that the treatment is equivalent to solving the corresponding master equation in paraxial approximation.
This cumulative thesis consists of four publications [1-4], presented as three contributions (I) - (III), where the second one (II) consists of the joint publications [2,3]. All publications were written during the author's research activities at the Institute of Quantum Physics under the supervision of Prof. Dr. W. P. Schleich. All of the presented publications consider light-matter interactions and their potential application in atom interferometry, a comparatively young and rapidly evolving technique. Atom interferometers find application in high-precision measurements, the determination of physical constants, the testing of fundamental physics, and the provision of gravimeters and gyroscopes. The advancement of technology and science has led to significant improvements in performance and the development of ambitious mobile and space-based devices. The recent success of several experiments has led to the proposal of further experiments with the aim of opening up new frontiers. However, the construction of future devices necessitates a decision regarding the employed diffraction mechanism and geometry, depending on the specific technical and scientific requirements. Furthermore, it is necessary to enhance the sensitivity of future devices. The contributions in this dissertation address these issues. The first contribution (I) focuses on the diffraction of atoms by laser pulses which can be employed to generate atom-optical diffraction elements like beam splitters and mirrors. These diffraction elements can be combined to construct atom interferometers, which have been successfully employed for precision measurements regarding fundamental physics as well as practical applications. The article presents a numerical comparison of two distinct diffraction mechanisms (Bragg and Raman) and diffraction geometries (single and double diffraction) in microgravity. It discusses the fundamental differences between the mechanisms and geometries with respect to the diffraction efficiency, taking into account the width of the initial wave function and the duration of the light pulse. Additionally, it explores the transition between the two diffraction geometries when starting from an atom initially at rest and increasing its momentum. The results may serve as a reference for experimentalists in selecting the most suitable configuration, based on their aims and requirements in the realization of more advanced atom interferometers. The second contribution (II) is a follow-up to the previous one, but focuses exclusively on a single diffraction type in microgravity, namely the so-called double Raman diffraction. In contrast to the previous article, where we only discussed single two-photon processes, we consider in this article a six-photon process, also known as third-order diffraction, as well as three subsequent two-photon processes, which both lead to the same amount of transferred momentum and can both be used to realize large-momentum transfer (LMT). We demonstrate that third-order double Raman diffraction is a viable and advantageous method for the diffraction of small momentum distributions like Bose-Einstein condensates (BECs) compared to a sequence of three two-photon processes. Furthermore, we show that third-order Raman pulses can be successfully applied to create higher-order Raman atom interferometers. Third-order diffraction can be perfectly combined with other LMT techniques and, moreover, allows for a reduction in the overall duration of the diffraction process and the complexity of the experimental set-up, as only two distinct laser frequencies are required. Moreover, we present a third-order Mach-Zehnder interferometer with spurious paths being disregarded. As demonstrated later on in [3], this interferometer generates optimal contrast and a substantial amplitude. The third contribution (III) extends the semi-classical λ-scheme of single Raman diffraction, as discussed in detail in contribution (I), into a fully quantized multi-atom multi-λ configuration with center-of-mass motion inside a cavity. Therefore, the incorporation of an internal structure allows for a more realistic modeling of the atoms, which is relevant for multi-photon interaction or cooling processes. We use for our analysis the framework of second quantization, i.e., we take into account the field-theoretical nature of the multi-particle atomic field as well as of the diffracting light field. We reduce this model with a Hamiltonian averaging theory applied to the atomic field operators and obtain an effective Hamiltonian, which determines the time evolution of the system. We demonstrate the efficiency of our model by the application of a typical initial state. Moreover, for a specific input state, we obtain the Hong-Ou-Mandel effect simultaneously in the atomic and the optical system. Our many-body formalism is perfectly suited to theoretically describe contemporary high-precision atom interferometric experiments based on entanglement between the atoms, that aim to surpass the shot-noise limit.
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.