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Quantum technologies

Dieter Meschede's research group
Home PhD theses
PhD theses


  • M. Sajid
    Magnetic Quantum Walks of Neutral Atoms in Optical Lattices, (2018), PhD thesisBibTeXPDF

    This thesis focuses on the simulation of the physics of a charged particle under an external magnetic field by using discrete-time quantum walks of a spin-1/2 particle in a two-dimensional lattice. By Floquet-engineering the quantum-walk protocol, an Aharonov–Bohm geometric phase is imprinted onto closed-loop paths in the lattice, thus realizing an abelian gauge field—the analog of a magnetic flux threading a two-dimensional electron gas. I show that in the strong-field regime, i.e. when the flux per plaquette of the lattice is a sizable fraction of the flux quantum, magnetic quantum walks give rise to nearly flat energy bands. I demonstrate that the system behaves like a Chern insulator by computing the Chern numbers of the energy bands and studying the excitation of the midgap topologically protected edge modes. These modes are extended all along the boundaries of the magnetic domains and remain robust against perturbations that respect the gap closing conditions. Furthermore, I discuss a possible experimental implementation of this scheme using neutral atoms trapped in two dimensional spin-dependent optical lattices. The proposed scheme has a number of unique features, e.g. it allows one to generate arbitrary magnetic-field landscapes, including those with sharp boundaries along which topologically protected edge states can be localized and probed. Additionally, I introduce the scattering matrix approach in discrete-time quantum walks to probe the Hofstadter spectrum and compute its topological invariants. By opening up a discrete-time quantum walk system and connecting it to metallic leads, I demonstrate that the reflection/transmission probabilities of a particle from the scattering region give information on the energy spectrum and topological invariants of the system. Although the work presented here focuses on the physics of a single particle in a clean system, it sets the stage for studies of many-body topological states in the presence of interactions and disorder.

  • T. Macha
    Storage of Short Light Pulses in a Fiber–Based Atom–Cavity System, (2018), PhD thesisBibTeXPDF

    In this work I theoretically investigate and experimentally realize the storage of short light-pulses in a fiber-based atom-cavity system. Our miniaturized optical resonator – with seven times the natural atomic linewidth and a small mode volume – simultaneously ensures a high bandwidth and operation in the strong-coupling regime. In particular, it enables the storage of light pulses with on average one photon and a temporal extent of less than 10 ns, which is more than a factor of two shorter than the atomic excited state lifetime of rubidium. We obtain a storage efficiency of 8%, consistent with both cavity losses and the employed level scheme.
     In order to improve the coupling and number of measurements for which a single atom can be recycled, we use dipole-trap assisted, degenerate Raman sideband cooling and a further development of our carrier-free Raman sideband cooling scheme, which permits a three-dimensional ground state population of 70%. The new techniques increase the measurement repetition rate by two orders of magnitude to ∼ 2 kHz. Moreover, for the first time we achieve a Zeeman state preparation fidelity above 95% in our experiment.
     On this basis, I present the deterministic generation of single photons in the near-adiabatic limit. By shaping the control laser pulse, we do not only show that we can control the temporal waveform of retrieved photons, but also reach a faster extraction from the cavity-coupled atom than possible in free-space. The quantum nature of the retrieved light is verified by measuring a second-order correlation function, which yields the expected antibunching. Moreover, the generation of photons in the cavity mode with an efficiency exceeding 66% is used as a fast hyperfine-state detection method, since our traditional, non-destructive state detection via a probe laser is no longer applicable in a Raman configuration due to the absence of a cycling transition. In order to realize Raman coupling between the two hyperfine ground states, we develop a scheme for shifting the cavity resonance frequency between two hyperfine transitions. During the scan, we are furthermore able to determine the atom-cavity coupling strength via the vacuum Rabi splitting in each individual measurement – a useful tool for post-selection of acquired data sets.
     By employing a numerical simulation based on a full quantum-mechanical master equation, I find the strategy to store a coherent laser pulse with the maximum possible efficiency for a given system. Although the cavity input field is treated classically, our simulation model is able to calculate efficiencies for a pure single-photon Fock-state input. Moreover, numerical optimal control methods enable us to find control pulses with storage efficiencies slightly above those achieved for temporally-scaled adiabatic control pulses. For our specific system, we finally demonstrate the non-adiabatic storage of a short, coherent light pulse.
     The ability to interact with pulses of high bandwidths encourages quantum hybrid experiments with quantum dots as single-photon sources. In this context, the stabilization of their emission frequency to an atomic transition is required. In collaboration with the IFW Dresden, I present a technique to counteract long-term frequency drifts by applying rate-based feedback to a strain-tunable quantum dot, which results in frequency deviations smaller than 1.5% of its emission linewidth. By simultaneously stabilizing the emission frequency of two quantum dots in separate cryostats, we enhance their two-photon interference visibility in a Hong-Ou-Mandel measurement from 31% to 41%, which corresponds to the maximum reachable visibility for the given emitters. Frequency-stable, efficient photon sources together with atom-cavity based quantum memories may facilitate the realization of quantum networks.


  • J. C. Gallego
    Strong Coupling between Small Atomic Ensembles and an Open Fiber Cavity, (2017), PhD thesisBibTeXPDF

    In this work I present the experimental realization of a versatile platform for the interplay between light and matter at the single-quanta level. In particular, I demonstrate the high cooperativity of small ensembles of rubidium atoms strongly coupled to a state-of-the-art, open fiber-based microcavity, emphasizing the capabilities of the system as an efficient source and storage device for single photons.

    The first part of this thesis focusses on the construction and characterization of the microresonator, which is composed of two dielectric mirrors machined on the end-facets of optical glass fibers. Through the implementation of an in-house facility, a large number of fiber-based mirrors are manufactured and precisely characterized. The intrinsic properties of this particular type of resonator are then analyzed and discussed. I present a theoretical model that explains, for the first time, the asymmetry in their reflective line shape and that has important implications for the optimal alignment of fiber-based cavities.

    In the following chapter, I introduce the main experimental apparatus, which contains a miniaturized fiber cavity — with small mode volume and a linewidth of ϰ=2π×25 MHz — that is actively stabilized and integrated in a compact assembly. The monolithic structure features several high–numerical aperture (NA) lenses that provide the necessary tools for the trapping, manipulation and high-resolution imaging of atoms inside the resonator. Neutral rubidium atoms are delivered by an optical conveyor belt from the cooling region into the cavity mode, where their deterministic coupling to the resonator is ensured by the tight confinement of a 3D optical lattice. The high linewidth of our open cavity also prevents the manifestation of cavity-heating mechanisms, enabling a constant monitoring of the atom’s presence by probing the cavity field without increased trap losses. This atom detection method allows us to perform real-time optical feedback in the transport scheme and to observe the characteristic vacuum Rabi splitting for individual atoms in a non-destructive manner.

    The rest of this work focusses on the interplay between atomic and photonic excitations inside the resonator. Due to the small mode volume of the microcavity, coupling strengths up to g=2π×100 MHz are observed for single atoms, corresponding to light–matter interaction in the strong coupling regime. The system’s cooperativity is collectively enhanced more than five times when placing a small atomic ensemble inside the resonator. Such a fast interaction rate — along with the relatively high transmission of the input cavity mirror — provides a rapid, non-destructive readout of the internal hyperfine state of a coupled atom when probing the cavity field. The state-detection method yields fidelities of 99.8% in 5 ms with less than 1% population transfer and negligible atom losses.

    The last part of the thesis is dedicated to the study of the influence of the cavity on the emission properties of an atom. I show how, despite the small solid angle covered by the cavity mode, the resonator alters the radiation pattern of an externally pumped single atom and increases its emission rate by a factor of 20 due to the process known as cavity back-action. More than 85% of the emitted photons are collected by the single cavity-mode — as a result of the strong Purcell enhancement — and subsequently channeled out by one of the fiber mirrors. A characterization of the photon statistics of the cavity output shows a clear antibunching dip, confirming that the emission corresponds to a single quantum emitter and that our system can be used as a readily fiber-coupled, efficient single-photon source. The geometry of our fiber-based resonator provides wide optical access that — in combination with the high-NA lenses — allows us to study the free-space emission rate of an atom coupled to the cavity. The various coupling strengths associated to different positions of the atom in the cavity mode lead to a clear visualization of the cavity back-action for all cooperativity regimes.

    The high cooperativity, intrinsic fiber coupling and scalability properties of our system make it suitable for the realization of an efficient, high-bandwidth quantum memory and its implementation in quantum networks. Additionally, the ability to couple an ensemble of indistinguishable atoms to the same cavity mode provides a versatile platform for the study of multipartite entangled states.

  • C. Robens
    Testing the Quantumness of Atom Trajectories, (2017), PhD thesisBibTeXPDF

    This thesis reports on a novel concept of state-dependent transport, which achieves an unprecedented control over the position of individual atoms in optical lattices. Utilizing this control I demonstrate an experimental violation of the Leggett Garg inequality, which rigorously excludes (i.e. falsifies) any explanation of quantum transport based on classical, well-defined trajectories. Furthermore, I demonstrate the generation of arbitrary low-entropy states of neutral atoms following a bottom-up approach by rearranging a dilute thermal ensemble into a predefined, ordered distribution in a one-dimensional optical lattice. Additionally, I probe two-particle quantum interference effects of two atom trajectories by realizing a microwave Hong-Ou-Mandel interferometer with massive particles, which are cooled into the vibrational ground state.

    The first part of this thesis reports on several new experimental tools and techniques: three-dimensional ground state cooling of single atoms, which are trapped in the combined potential of a polarization-synthesized optical lattice and a blue-detuned hollow dipole potential; A high-NA (0.92) objective lens achieving a diffraction limited resolution of 460 nm; and an improved super-resolution algorithm, which resolves the position of individual atoms in small clusters at high filling factors, even when each lattice site is occupied.

    The next part is devoted to the conceptually new optical-lattice technique that relies on a high-precision, high-bandwidth synthesis of light polarization. Polarization-synthesized optical lattices provide two fully controllable optical lattice potentials, each of them confining only atoms in either one of the two long-lived hyperfine states. By employing one lattice as the storage register and the other one as the shift register, I provide a proof of concept that selected regions of the periodic potential can be filled with one particle per site.

    In the following part I report on a stringent test of the non-classicality of the motion of a massive quantum particle, which propagates on a discrete lattice. Measuring temporal correlations of the position of single atoms performing a quantum walk, we observe a 6 σ (standard deviation) violation of the Leggett-Garg inequality. The experiment is carried out using so-called ideal negative measurements – an essential requisite for any genuine Leggett-Garg test – which acquire information about the atom’s position while avoiding any direct interaction with it. This interaction-free measurement is based on our polarization-synthesized optical lattice, which allows us to directly probe the absence rather than the presence of atoms at a chosen lattice site. Beyond its fundamental aspect, I demonstrate the application of the Leggett-Garg correlation function as a witness of quantum superposition. The witness allows us to discriminate the quantumness of different types of walks spanning from merely classical to quantum dynamics and further to witness the decoherence of a quantum state.

    In the last experimental part I will discuss recent results on collisional losses due to inelastic collisions occurring at high two-atom densities and demonstrate a Hong-Ou-Mandel interference with massive particles. Our precise control over individual indistinguishable particles embodies a direct analogue of the original Hong-Ou-Mandel experiment. By carrying out a Monte Carlo analysis of our experimental data, I demonstrate a signature of the two-particle interference of two-atom trajectories with a statistical significance of 4 σ.

    In the final part I will introduce several new experiments which can be realized with the tools and techniques developed in this thesis, spanning from the detection of topologically protected edge states to the prospect of building a one-million-operation quantum cellular automaton.


  • M. M. Dorantes
    Fast non-destructive internal state detection of neutral atoms in optical potentials, (2016), PhD thesisBibTeXPDF

    Neutral atoms trapped in optical lattices are promising candidates for quantum information processing and quantum simulation. Over the last decades, elegant tools for the manipulation of the internal and external states of optically trapped atoms have been developed. The crucial capability of scalable internal state readout in these systems, however, still relies on destructive methods. In spite of the important role of near-resonant illumination for the manipulation and detection of atoms in the lattice, there also exists a significant lack of studies on the heating and cooling dynamics of optically trapped atoms interacting with near-resonant light. An in-depth understanding of the heating and cooling processes is essential to finding the conditions of illumination that enable the non-destructive internal state readout of multiple atoms.

    This work presents an experimental system to cool, trap, manipulate, and detect the internal and external states of a small ensemble of 87Rb neutral atoms trapped in a one-dimensional optical lattice. A high photon detection efficiency in our experimental system allows for fast fluorescence imaging with acquisition times of 20 ms and fast position determination of atoms in the optical lattice with an accuracy of ∼40 nm.

    Using this experimental system, we investigate the heating dynamics of a neutral atom trapped in a standing wave dipole trap illuminated by a single near-resonant laser beam. A theoretical description to describe our measurements is provided in two experimentally relevant regimes. First, we consider the case of a weak near-resonant beam and later the case of off-resonant illumination. From this analysis, we find settings for the illumination light which allows an atom to scatter many photons before it is expelled from the trap.

    Building on these results we demonstrate simultaneous, non-destructive determination of the internal state of spatially resolved atoms trapped in a one-dimensional optical lattice with a fidelity of 98.6 ± 0.2% and a survival probability of 99.0 ± 0.2%. During the readout process, less than 2% of the atoms change their initial ground state.

    In order to determine the state of atoms that are not spatially resolved, a novel image analysis technique is presented. The technique uses Bayesian methods, which include the statistics of the detected photons as well as the response from the EMCCD camera. The Bayesian method is implemented on experimental data for atoms trapped in a one-dimensional optical lattice and its accuracy is tested by numerical simulations. In addition, an extension of this algorithm for atoms trapped in two-dimensional lattices is provided.

    Finally, the non-destructive state detection method is utilized as a tool for the state determination following the coherent control of the internal and external states of atoms in the optical trap. Here Raman sideband cooling is implemented and utilized in an atomic compression sequence for the creation of a small and dense atomic ensemble. These techniques will play an important role in experiments studying the collective light interaction of the atomic ensemble in a recently added optical fiber cavity.

  • S. Brakhane
    The Quantum Walk Microscope, (2016), PhD thesisBibTeXPDF

    In this thesis, I present single-site detection of neutral atoms stored in a three-dimensional optical lattice using a numerical aperture objective lens (NAdesign = 0.92). The combination of high-resolution imaging with state-dependent trapping along two-direction of the lattice opens up the path towards quantum simulations via quantum walks. Suppressing the interactions of a quantum system with the environment is essential for all quantum simulation experiments. It demands a precise control of both the external magnetic (stray) fields and the polarization properties of laser beams inside the vacuum chamber. I designed a metal shielding to reduce magnetic field fluctuations and designed, assembled and characterized a novel ultra-high vacuum glass cell. The glass cell consists of special glass material and exhibits an ultra-low birefringence Δn of a few times 10−8 to highly suppress polarization disturbances originating from stress birefringence in vacuum windows. Furthermore, anti-reflection coatings avoid reflections on all window surfaces. The cell hosts the assembled vacuum-compatible objective, that exhibits a diffraction limited resolution of up to 453 nm and allows to optically resolve the spacing of the optical lattice. Fluorescence images of single trapped atoms are used to characterize the imaging system. The filling, orientation and geometry of the optical lattice is precisely reconstructed using positions of atoms that can be determined from fluorescence images. Furthermore, I present a scheme to realize state-dependent transport and discuss its robustness against experimental imperfections in a technical implementation. This transport scheme enable the realization of discrete-time quantum walks with neutral atoms in two dimensions. These quantum walks pave the way towards the simulation of artificial magnetic fields and topologically protected edge states.


  • R. Reimann
    Cooling and Cooperative Coupling of Single Atoms in an Optical Cavity, (2014), PhD thesisBibTeXPDF
    In this work the motional state of single cesium atoms strongly coupled to an optical high-finesse cavity is controlled and manipulated by a novel Raman cooling scheme. Furthermore, cavity-modified super- and subradiant Rayleigh scattering of two atoms is observed and explained by collective coupling of the atoms to the cavity mode. We start with the description and comparison of different intra-cavity cooling schemes that allow us to control the motional states of atoms. Cavity cooling is experimentally and theoretically investigated for the two cases of pumping the cavity and driving the atom. In contrast to other cooling schemes, such as EIT- or Raman cooling, our analysis shows that we cannot use cavity cooling for efficient ground-state preparation, but it serves as a precooling scheme for the sideband-cooling methods. Comparing the more efficient sideband cooling techniques EIT and Raman cooling, we find that the experimental efficiency of EIT cooling could not be determined. Therefore we choose a novel, easily implemented Raman cooling technique that features an intrinsic suppression of the carrier transition. This is achieved by trapping the atom at the node of a blue detuned intra-cavity standing wave dipole trap that simultaneously acts as one field for the two-photon Raman coupling. We apply this method to perform carrier-free Raman cooling to the two-dimensional vibrational ground state and to coherently manipulate the atomic motion. The motional state of the atom after Raman cooling is extracted by Raman spectroscopy using a fast and non-destructive atomic state detection scheme, whereby high repetition rates and good signal-to-noise ratios of sideband spectra are achieved. In a last experiment we observe cooperative radiation of exactly two neutral atoms strongly coupled to our cavity. Driving both atoms with a common laser beam, we measure super- and subradiant Rayleigh scattering into the cavity mode depending on the relative distance between the two atoms. Surprisingly, due to cavity backaction onto the atoms, the cavity output power for superradiant scattering by two atoms is almost equal to the single atom case. We explain these effects quantitatively by a classical model as well as by a quantum mechanical one based on Dicke states. Furthermore, information on the relative phases of the light fields at the atom positions are extracted, and the carrier-free Raman cooling scheme is applied to reduce the jump rate between super- and subradiant configurations.


  • A. Steffen
    Single atom interferometers and Bloch oscillations in quantum walks, (2013), PhD thesisBibTeXPDF
    This thesis deals with the digital manipulation of the position and spin of neutral Caesium atoms in an optical lattice. I investigate coherent phenomena based on interferences between the trajectories of a single atom. Individual atoms are split by making use of our state-dependent lattice to shift different spin states in opposite directions, leading to coherent superpositions of spin and position state. This offers many possibilities; in this work, we chose to investigate atom interferometry and quantum walks in potential gradients. Chapter 1 is a brief introduction to the importance of phase in quantum mechanics. In chapter 2, I provide an introduction to our experimental apparatus with particular focus on state-dependent shifting and correct alignment procedures. Our model for decoherence in the lattice is also presented, with emphasis on the polarization state of the lattice lasers. Chapter 3 presents the first of two measurement campaigns, which employs a single atom interferometer with a flexible geometry. We investigate a laser intensity gradient present in the system and demonstrate how several interferometer geometries can be compared to glean extra information about the symmetries of a potential gradient, such as its spin state dependence. A deliberately applied inertial force serves as a proof-of-principle for accelerometry and is correctly measured. Chapter 4 contains the results of the second measurement campaign, which focussed on quantum walks. Quantum walks are a quantum analog to classical random walks and possess remarkable spreading properties. A theoretical model is presented, including a band structure picture of the walk. Unlike previous experiments, the walk can now be performed in a potential gradient, giving rise to new physics, in particular Bloch oscillations, which manifest as oscillations of the distribution width. Experimental results first confirm the predictions made by our model and show quantum walks of up to 100 steps with coherent behaviour. Walks in potential gradients are measured and indeed show clear signatures of Bloch oscillations. This is particularly remarkable because the quantum walk is effectively mimicking an electron in a solid, forming a basic quantum simulator. Chapter 5 is a conclusion and a preview on ongoing technical improvements that stand to significantly extend the experimental capabilities.


  • N. Spethmann
    Single impurity atoms immersed in an ultracold gas, (2012), PhD thesisBibTeXPDF
    In this thesis, experiments with an ultracold gas doped with few and single atoms of another species are presented. The techniques to adequately prepare and manipulate an ultracold Rb gas and to dope it with a precisely known number of few Cs atoms are introduced. These techniques allow the time-resolved observation of the sympathetic cooling of initially laser-cooled, cold impurity atoms into the ultracold temperature regime of the Rb buffer gas. During the cooling, the confinement of the impurity atom is enhanced to a reduced volume inside the buffer gas, which increases the interspecies collision rate. By analyzing the cooling process, the interspecies scattering cross section is estimated. The lifetime of the resulting hybrid system is limited by three-body recombination of the impurity atom with atoms of the buffer gas. The atomic resolution of the impurity atom number allows the determination of the lifetime atom-by-atom. Additional information is gained from the precisely known fluctuations of the number of lost impurity atoms. This information is exploited to assign the three-body losses unambiguously to a single loss channel. The interaction of an impurity atom in a quantum-mechanical superposition state with the buffer gas is of special interest for future experiments. First experiments into this direction are presented at the end of the thesis.
  • K. Karapetyan
    Single optical microfibre-based modal interferometer, (2012), PhD thesisBibTeXPDF
    In this thesis, I report on the experimental investigation and the computer simulation of optical microfibre-based modal interferometers. An optical microfibre (OMF) can be produced from a commercial single-mode optical fibre by a tapering process consisting in simultaneous heating and pulling the fibre. OMFs have attracted much attention in the recent years due to high light concentration, a strong evanescent field around the OMF waist, and convenience of use thanks to their fibre-coupled nature. It makes them a promising element for both basic research and sensing applications. Interferometers based on OMFs extend possible application areas to dispersive sensing. In a single-OMF modal interferometer (SOMMI), the two interferometer arms share the same path, and interference occurs between two transverse modes excited in the down-taper and recombined in the up-taper. During my work, I have produced OMF samples, characterized them, and used them as SOMMIs. To verify the OMF shape, different approaches have been implemented, including a light scattering method and a newly developed optical harmonic generation-based diameter measurement method [1]. For actual verification of the SOMMI performance, a simple post-production procedure, based on the stretch-interferometry, was realized. In this stretch-test, the experimental samples showed high contrast and very good signal-to-noise ratio making them suitable for sensing applications. Additionally, they were tested using spectral interferometry in air. Furthermore, I have designed and produced SOMMI samples specifically for interferometry in liquids and tested them as a refractive index sensor. Exhibiting a characteristic achromatic fringe, SOMMIs are a promising tool for the absolute refractive index measurement. In this experiment, a sensitivity of 3000 to 4000 nm per refractive index unit was measured. This is the highest sensitivity observed in non-birefringent OMF-based sensors so far. I have also developed a computer model of OMFs and SOMMIs. While the calculation methods for light propagation simulation in usual optical fibres are well established, simulation of OMFs demands many questions to be answered. The main challenge here is the calculation of the taper regions, where the fibre diameter varies from the standard diameter of a commercial fibre of 125 um to the diameter of the OMF waist of several hundred nanometres. Together with the diameter, the light-guidance regime changes from the weak guidance in the untapered fibre to the strong guidance in the waist, requiring different models to be combined. To the best of my knowledge, I have created the first reliably working software code for automatic calculation of all guided modes supported by tapered fibres [2]. I have then used this code to create computer models for stretch- and spectral-interference in SOMMIs. The experimental results confirm the validity of these models.


  • S. John
    Towards Single Atom Aided Probing of an Ultracold Quantum Gas, (2011), PhD thesisBibTeXPDF
    In this thesis, the interactions in an unbalanced Rubidium (Rb)-Caesium (Cs) mixture are studied, where the Cs atoms have been used as probes to study the interspecies interactions. The Rb atoms are trapped and stored in a conservative potential, in an optical dipole trap and cooled to quantum degeneracy. The coherent manipulation of the spin states is realized using a microwave and radio frequency radiation to prepare the Rb atoms in the various Zeeman split hyper ne levels of the ground state. Cs atoms are trapped in a MOT. An overlap of these two entities is obtained via a magnetic transport to study the interspecies interactions. The dynamics of the Cs MOT is studied in the presence of a 600nK thermal cloud of Rb, where a loss in the Cs atoms is observed. Rb remains una ected. Here, a method has been demonstrated, where the interspecies inelastic two- and three-body collisions have been investigated by monitoring the one- and two-atom loss rates in Cs. Each term in the complicated inelastic rate equation has been determined individually without having to solve the rate equation which can not be solved analytically. This is therefore, a nondestructive and simple method to extract information about the interactions and can be performed for future experiments with Cs in a conservative species speci c potential.
  • T. Kampschulte
    Coherently driven three-level atoms in an optical cavity, (2011), PhD thesisBibTeXPDF
    We experimentally realize strong light-matter coupling of a single cesium atom to a single mode of a high-finesse optical cavity. In this regime, the optical properties of one atom change the transmission spectrum of the resonator significantly. The two hyperfine ground states of cesium can be distinguished by the relative transmission of a weak probe beam coupled to the cavity. Here, we coherently couple the two hyperfine ground states via an electronically excited state with two-photon transitions. In the first experimental configuration, two-photon Raman transitions are driven between the two ground states while continuously observing the atomic state. I present a new in-situ spectroscopic technique for the internal hyperfine and Zeeman-sublevel dynamics of an atom inside the cavity mode, using time-dependent Bayesian analysis of quantum jumps. In the second configuration, the three-level atomic structure forms the basis of Electromagnetically Induced Transparency (EIT). The modification of the absorptive and dispersive properties of an atom by destructive interference leads to strong changes in the transmission of the probe beam. Our observations are qualitatively described in a semiclassical picture in the weak-probing limit. I furthermore present a fully quantum mechanical model, where deviations from the weak-probing limit, dephasing effects and other hyperfine states are taken into account to fit our data quantitatively. Moreover, I formulated an extension of the semiclassical model to highlight a conceptual contrast to the quantum model. Additionally, the EIT effect is connected with a strong cooling effect, resulting in a 20-fold increase of the storage time of the atoms inside the cavity. I present further results of investigations of this effect where the atoms are trapped and EIT-cooled outside the cavity. From microwave sideband spectra it can be inferred that almost 80% of the atoms are in the ground state of motion along the trap axis. ------ Copyright notice: Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modified, adapted, performed, displayed, published, or sold in whole or part, without prior written permission from the American Physical Society.
  • U. Wiedemann
    Control of photochromic molecules adsorbed to optical microfibres, (2011), PhD thesisBibTeXPDF
    The high light intensity in an optical microfibre and the resulting nonlinear effects were applied to develop a new method to precisely determine the microfibre diameter. The evanescent field of these optical microfibres was then used to control the internal state of surface-adsorbed photochromic molecules. I start with a brief sketch of the mathematical description of light propagation in step-index optical fibres. From the results the main properties of optical microfibres are derived. Then, I describe the fabrication of optical microfibres with special requirements for the experiments presented later in the thesis. A new technique to measure the submicrometre diameter of optical microfibres with an accuracy of better than 2 % is presented. This method is based on second- and third-harmonic generation. It is found that the fibre diameter can be unambiguously deduced from the peak wavelength of the harmonic light. High-resolution scanning electron microscope imaging is used to verify the results. In the following, the experimental basics for the switching of photochromic molecules adsorbed to optical microfibres are described. I present the technique to deposit and detect the molecules and show their basic behaviour due to light exposure. The internal state of the molecules is measured via their state-dependent light absorption. Repeated switching between the states is achieved by exposure to the evanescent field of a few nanowatts of light guided in the microfibre. The photochromic processes are then quantitatively analysed. Time-resolved photoswitching dynamics are measured and mathematically modelled with a rate equation model. By adjusting the microfibre evanescent field strength the dynamic equilibrium state of the molecules is controlled. I also study how many times the photochromic system can be switched before undergoing significant photochemical degradation.


  • C. Weber
    Controlled few-body interactions in ultracold bosonic mixtures, (2010), PhD thesisBibTeXPDF
    In this thesis two experiments with heteronuclear Bose-Bose mixtures are discussed. The goal of the first experiment is a controlled doping of a rubidium condensate with single caesium atoms. These undertake the task of a non-destructive probe to investigate quantum mechanical phenomena time- and spatially resolved. In this thesis the necessary methods to produce, store, and detect both components, single atoms and the condensate, are realized. In a first experiment up to 10 caesium atoms are stored as a probe in contact with a cold rubidium atomic cloud. The interaction parameters are extracted from the dynamics of the single atoms. This is an important step towards the controlled doping of a condensate. The aim of the second experiment is the production and spectroscopy of ultracold heteronuclear potassium-rubidium molecules with universal properties. Close to two magnetic s-wave Feshbach resonances weakly bound molecules in high vibrational states are created, and their binding energy and the position of the associated Feshbach resonance are determined. These results in combination with two narrow d-wave Feshbach resonances provide the basis for a more precise parametrization of the potassium-rubidium molecular potential. The knowledge of this is important to identify a proper scheme to transfer the molecules via coherent coupling into their rovibrational ground state. These molecules offer a permanent dipole moment and thus are of particular interest for e.g. quantum information processing due to their anisotropic, long-range dipole-dipole interaction.
  • M. Karski
    State-selective transport of single neutral atoms, (2010), PhD thesisBibTeXPDF
    The present work investigates the state-selective transport of single neutral cesium atoms in a one-dimensional optical lattice. It demonstrates experimental applications of this transport, including a single atom interferometer, a quantum walk and controlled two-atom collisions. The atoms are stored one by one in an optical lattice formed by a standing wave dipole trap. Their positions are determined with sub-micrometer precision, while atom pair separations are reliably inferred down to neighboring lattice sites using real-time numerical processing. Using microwave pulses in the presence of a magnetic field gradient, the internal qubit states, encoded in the hyperfine levels of the atoms, can be separately initialized and manipulated. This allows us to perform arbitrary single-qubit operations and prepare arbitrary patterns of atoms in the lattice with single-site precision. Chapter 1 presents the experimental setup for trapping a small number of cesium atoms in a one-dimensional optical lattice. Chapter 2 is devoted to fluorescence imaging of atoms, discussing the imaging setup, numeric methods and their performance in detail. Chapter 3 focuses on engineering of internal states of trapped atoms in the lattice using optical methods and microwave radiation. It provides a detailed investigation of coherence properties of our experimental system. Finally manipulation of individual atoms with almost single-site resolution and preparation of regular strings of atoms with predefined distances are presented. In Chapter 4, basic concepts, the experimental realization and the performance of the state-selective transport of neutral atoms over several lattice sites are presented and discussed in detail. Coherence properties of this transport are investigated in Chapter 5, using various two-arms single atom interferometer sequences in which atomic matter waves are split, delocalized, merged and recombined on the initial lattice site, while the interference contrast and the accumulated phase difference are measured. By delocalizing a single atom over several lattice sites, possible spatial inhomogeneities of fields along the lattice axis in the trapping region are probed. In Chapter 6, experimental realization of a discrete time quantum walk on a line with single optically trapped atoms is presented as an advanced application of multiple path quantum interference in the context of quantum information processing. Using this simple example of a quantum walk, fundamental properties of and differences between the quantum and classical regimes are investigated and discussed in detail. Finally, by combining preparation of atom strings, position-dependent manipulation of qubit states and state-selective transport, in Chapter 7, two atoms are deterministically brought together into contact, forming a starting point for investigating two-atom interactions on the most fundamental level. Future prospects and suggestions are finally presented in Chapter 8.
  • L. Förster
    Microwave control of atomic motion in a spin dependent optical lattice, (2010), PhD thesisBibTeXPDF
    In this thesis I present my results concerning the coherent control of the quantized motional state of trapped neutral Cesium atoms. This is accomplished using microwave radiation in combination with a spin dependent potential con ning the atoms. I present both cooling of atoms close to the motional ground state and the preparation of nonclassical motional states. In total, our apparatus is thus capable to control the spin, the position along the periodic potential and the vibrational state of the atoms. In chapter 1 I give an overview of the experimental apparatus. Our setup is designed to trap and to store on the order of ten atoms in a one dimensional optical lattice. Fluorescence imaging in conjunction with a microscope lens system is used to determine both the number and the position of the atoms. The spin degree of freedom is manipulated using microwave radiation and the trapping potential allows to shift the atoms to the 'left' or to the 'right' along the potential axis, depending on their spin orientation. In chapter 2 I discuss the coupling mechanism between the spin and the motional degree of freedom. A microwave spectrum with a slightly displaced lattice exhibits sideband peaks corresponding to a change of the vibrational quantum number. For the full quantitative understanding I compare the experimental results with a theoretical model, which is also used to quantify possible decoherence mechanisms. Based on this investigations, in chapter 3 I present the results for our ground state cooling scheme, whereby the focuss lies on the peculiarities of our system. A model based on master equations is used to analyze the present cooling limits. In chapter 4, nally, two detection schemes for arbitrary motional states of an atomic ensemble are presented. In particular, they are employed to verify the preparation of nonclassical states.


  • M. Khudaverdyan
    A controlled one and two atom-cavity system, (2009), PhD thesisBibTeXPDF
  • J. Kim
    Efficient sub-Doppler Transverse Laser Cooling of an Indium Atomic Beam, (2009), PhD thesisBibTeXPDF
    In this dissertation, I describe transverse laser cooling of an Indium atomic beam. For efficient laser cooling on a cycling transition, I have built a tunable, continuous-wave coherent ultraviolet source at 326 nm based on frequency tripling. For this purpose, two independent high power Yb-doped fiber amplifiers for the generation of the fundamental radiation at λ = 977 nm have been constructed. I have observed sub-Doppler transverse laser cooling of an Indium atomic beam on a cycling transition of In by introducing a polarization gradient in the linear-perpendicular-linear configuration. The transverse velocity spread of a laser-cooled In atomic beam at full width at half maximum was achieved to be 13.5 ± 3.8 cm/s yielding a full divergence of only 0.48 ± 0.13 mrad. In addition, nonlinear spectroscopy of a 3-level, lambda-type level system driven by a pump and a probe beam has been investigated in order to understand the absorption line shapes used as a frequency reference in a previous two-color spectroscopy experiment. For the analysis of this atomic system, I have applied a density matrix theory providing an excellent basis for understanding the observed line shapes.
  • S. Reick
    Internal and external dynamics of a strongly-coupled atom-cavity system, (2009), PhD thesisBibTeXPDF


  • G. Sagué
    Cold atom physics using ultra-thin optical fibres, (2008), PhD thesisBibTeXPDF
    In this thesis I present experiments concerning the investigation and manipulation of cold neutral atoms using ultra-thin optical fibres with a diameter smaller than the wavelength of the guided light. In such a fibre-field configuration the guided light exhibits a large evanescent field that penetrates into the free-space surrounding the fibre thus enabling to couple laser cooled atoms to the fibre mode. By trapping and cooling caesium atoms in a magneto-optical trap formed around the fibre I investigated the interaction of the atoms with the evanescent field at sub-micrometre distances from the fibre surface. Chapters 1 and 2 provide the theoretical foundations of this work. Chapter 1 describes the propagation of light in optical fibres. The general solution of the Maxwell’s equations in the fibre that complements the description is provided in Appendix A. In Chapter 2, the theory of the interaction of atoms with time-varying electric fields is described. In Chapter 3 the resonant interaction of laser cooled caesium atoms with the evanescent field of a probe laser launched through a 500-nm diameter fibre is studied. A detailed analysis of the atomic absorption at sub-micrometre distances from the fibre surface is given. I have performed Monte-Carlo simulations of atomic trajectories inside the cold atom cloud surrounding the fibre. From the simulations, the atomic density at the vicinity of the fibre is deduced and the absorbance profiles of the atoms measured during the experiments can be modelled. By carefully investigating the linewidths of these profiles, clear evidence of dipole forces, van der Waals interaction, and a significant enhancement of the spontaneous emission rate of the atoms is found. The atomic spontaneous emission into the guided mode of a 500-nm diameter optical fibre is the focus of Chapter 4. Here, I show that the fibre can be used as an efficient tool to collect and guide the spontaneous emission of the atoms. The dipole force induced by the evanescent field on the atoms is the central idea of the experiments performed in Chapter 5. I have built a new version of the experimental setup that opens the route towards atom trapping in the evanescent field in an array of surface microtraps around the fibre. Such traps are created by the combination of two laser fields with opposite sign of the detuning with respect to the excitation frequency of the atoms. The first experimental results reporting the influence of the two-colour evanescent field on the spectral properties of the atoms are presented.


  • I. Dotsenko
    Single atoms on demand for cavity QED experiments, (2007), PhD thesisBibTeXPDF
  • M. Haas
    Sympathetisches Kühlen in einer Rubidium-Cäsium-Mischung: Erzeugung ultrakalter Cäsiumatome, (2007), PhD thesisBibTeXPDF
  • B. Klöter
    Lichtkräfte auf einen Indiumatomstrahl, (2007), PhD thesisBibTeXPDF
  • F. Warken
    Ultra thin glass fibers as a tool for coupling light and matter, (2007), PhD thesisBibTeXPDF
    This thesis presents an examination of ultrathin glass fibers as a novel tool for coupling light and matter. As a basic concept, matter, i.e. atoms, molecules, etc., will be coupled to the evanescent field in the vicinity of the fiber surface, which contains a large portion of the power of the guided light. Here, the effects of forming and microstructuring of these fibers on the field strength at the surface relative to the field strength in the center of the fiber are studied. Chapter 1 reports on the construction and optimization of a pulling system for glass fibers, which can be used to reproducibly manufacture subwavelength diameter fibers of centimeters length from standard glass fibers with high accuracy. The transmission through the whole structure is measured to be up to 97 %. The properties of the evanescent field and its potential for coupling of light and matter are analyzed in chapter 2 by spectroscopy of thin surface adsorbed films of organic molecules (PTCDA). It is theoretically and experimentally shown that the spectroscopic sensitivity can be increased by several orders of magnitude with respect to conventional techniques. This method allows for the first time the observation of sub-monolayer dynamics of structural changes of PTCDA on glass at ambient conditions. In chapter 3 two types of resonators, which can be built from glass fibers, are investigated. Firstly, a Bragg-mirror is integrated into an ultrathin glass fiber and the reflectivity is measured qualitatively. The second resonator type can be formed from a 16 µm thick fiber. Selective coupling of light from an ultrathin fiber into a whispering-gallery mode of the resonator is realized with an efficiency of 99,3% and tuning of the resonance frequency by more than one free spectral range of order 100 GHz is demonstrated. The quality factor of the excited modes has been measured to be of order 105 and limitations are discussed. Finally, a method is developed to determine the quantum numbers of a resonator mode. Thereby, this thesis makes a contribution towards the utilization of ultrathin and structured glass fibers to couple light and matter and opens the route to fiber-based quantum optics experiments.


  • Y. Miroshnychenko
    An atom-sorting machine, (2006), PhD thesisBibTeXPDF
  • V. Leung
    Neutral Atom Interactions at Surfaces, in Mixtures, and Bose-Condensates, (2006), PhD thesisBibTeXPDF
    This thesis presents, through a series of experimental and numerical results, an investigation of the collisional interactions of neutral atoms for topics of technological and scientific interest, namely, atom-surface interactions for lithography, and atom-atom interactions in cold atomic mixtures and Bose-Einstein condensates. In the first chapter I report on an experimental scheme to investigate the interaction of metastable helium atoms with molecular surface monolayers, which act as ultrathin resists for atom lithography. We seek to isolate the interaction between the metastable atom and the monolayer from other possible interactions, such as that of ultraviolet photons, which are also present in significant quantities. Using the characterized properties of a new liquid nitrogen-cooled discharge source, an experimental scheme was implemented which utilizes magnetic manipulation techniques for neutral atoms to create a lithography exposure involving metastable helium atoms alone. In the second chapter, the development of an experiment for the study of ultracold interactions between rubidium and cesium atoms is documented. Starting with an experiment for the Bose-Einstein condensation of Rb-87, modifications were made which allowed the simultaneous confinement of rubidium and cesium atoms in magneto-optical, quadrupole, and Ioffe trapping configurations. By imprinting a temperature gradient between the overlapped atomic clouds through optical molasses, re-thermalization between magnetically trapped rubidium and cesium atoms through s- and p-wave collisions was observed. In order to create precise and variable temperature gradients in the binary mixture, a modular 6.83 GHz source was implemented for species-selective evaporative cooling at the hyperfine transition frequency of rubidium. Bose-Einstein condensates of rubidium was created and the lifetime-limiting losses due to three-body collisions investigated. The third chapter puts forward the results of numerical simulations on the creation and propagation of bright soliton trains in Bose-condensates, based on the experimental observation of soliton trains by Strecker et al.. Using a mean-field approach, numerical solutions of the Gross-Pitaevski equation were obtained which reproduce the key features of the experiment and offer insights into soliton collisions and the determination of soliton number.


  • R. dela Torre
    Laser Manipulation of Indium Atoms, (2005), PhD thesisBibTeXPDF
  • D. Frese
    Bose-Einstein Condensation of Rubidium. Towards Ultracold Binary Bosonic Mixtures, (2005), PhD thesisBibTeXPDF


  • H. Merimeche
    Slow Atomic Beams Manipulation with Magnetic Videotapes, (2004), PhD thesisBibTeXPDF
  • W. Alt
    Optical control of single neutral atoms, (2004), PhD thesisBibTeXPDF
  • M. Mützel
    Erzeugung von Nanostrukturen mit laserpräparierten thermischen Atomstrahlen, (2004), PhD thesisBibTeXPDF
  • J. Wang
    Opto-mechanical manipulation of indium atoms, (2004), PhD thesisBibTeXPDF
  • D. Schrader
    A Neutral Atom Quantum Register, (2004), PhD thesisBibTeXPDF


  • S. Kuhr
    A controlled quantum system of individual neutral atoms, (2003), PhD thesisBibTeXPDF


  • C. Affolderbach
    Dark State Magnetometers and Gradiometers, (2002), PhD thesisBibTeX


  • S. A. Knappe
    Dark Resonance Magnetometers and Atomic Clocks, (2001), PhD thesisBibTeX
  • B. Ueberholz
    Kalte Stöße in einer magnetooptischen Falle mit hohem Magnetfeldgradienten, (2001), PhD thesisBibTeXPDF
    Im Rahmen dieser Arbeit wurde eine neuartige experimentelle Methode zur Untersuchung von inelastischen kalten Stößen zwischen Neutralatomen vorgestellt. Das Experiment wurde in einer speziell präparierten magnetooptischen Falle durchgeführt, in welcher der verwendete hohe Magnetfeldgradient gegenüber herkömmlichen Fallen schon mit wenigen neutralen Atomen eine hohe Dichte erzeugt. Der Einfluss des hohen Magnetfeldgradienten in der MOT auf die Verlusteigenschaften der gespeicherten Atome wurde herausgestellt. Die Dynamik der Atomanzahl konnte dabei mit Hilfe der Detektion der Resonanzfluoreszenz in einem bisher unerreichten Signal-zu-Untergrund-Verhältnis detektiert werden, wodurch eine präzise Zuordnung der momentanen Atomanzahl in der Falle von bis zu 20 Atomen gewährleistet ist. Somit wurde erstmals ermöglicht, Lade- und Verlustereignisse getrennt voneinander zu beobachten, die eine Bestimmung der Verlustkoeffizienten ohne jede Einwirkung auf die Fallenparameter zulässt. Diese neuartige Messmethode erlaubt einen neuen Zugang zur Studie verschiedener Stoßprozesse - im Gegensatz zu Standardexperimenten mit vielen gespeicherten Atomen. Durch die geringe Fallentiefe der magnetooptischen Falle hat man Zugang zu Grundzustandsstößen, deren Unterdrückung durch den Rückpumplaser erstmals beobachtet wurde. Die Abhängigkeit der Stoßkoeffizienten konnte mit einem einfachen semiklassischen Modell sehr gut wiedergegeben werden. Dieser in jeder MOT stattfindende Effekt ist für das ganze Forschungsgebiet von großer Relevanz. Es wurden alle relevanten Verlustmechanismen diskutiert und sorgfältige semiklassische Berechnungen präsentiert.


  • A. Nagel
    Precision Spectroscopy of Coherent Dark States in Thermal Cesium Vapour, (1999), PhD thesisBibTeXPDF
    The first two chapters will give an introduction into the theoretical models developed in order to understand the CPT resonance dependence on a multitude of external parameters for the special experimental configurations used. Chapter three describes the two different experimental configurations. Whereas the initial setup was characterized by the versatility needed for many systematic investigations of CPT resonances, the new set-up, which is based on a novel type diode laser, already constitutes a step towards an extremely simple and robust device as it is needed for practical applications outside the laboratory environment. In chapter four which occupies major parts of this thesis a wide range of systematic investigations is reported and compared with the theoretical models of the first chapters. This also includes the search for possible optimization of experimental parameters. For instance, the use of buffer gas techniques facilitated the reduction of the CPT linewidth observed tb below 50 Hz. The contents of the last chapter consists of two proof-of-principle experiments for the sensitive detection of small magnetic fields and the determination of the g-factor ratio. Although the setup was far from being optimized, oscillating fields in the kHz range with a flux density amplitude of only several pT could already be detected.


  • H. Schadwinkel
    Die Magnetooptische Falle als lichtgebundenes Atomgitter, (1998), PhD thesisBibTeXPDF

    Im Rahmen dieser Arbeit wird die Dynamik von lasergekühlten Cäsiumatomen in dreidimensionalen lichtinduzierten Potentialen untersucht. Die Charakterisierung von Lichtfeldern zeigt, daß im Allgemeinen  sowohl deren Polarisation als auch Intensität räumlich variieren. Das Interfernzmuster hängt dabei entscheidend von den relativen Phasen zwischen den beteiligten Laserstrahlen ab. Für die experimentelle Kontrolle wurde ein neuartiges Konzept entwickelt, das die intrinsische Stabilität der relativen Zeitphasen eines Lichtfeldes gewährleistet. Mit diesem Aufbau wurden erstmals zwei spezielle Konfigurationen studiert, die als reine Polarisationsgitter (NOT) bzw. Intensitätsgitter (MOT00) interessante Grenzfälle darstellen.

  • F. Lison
    Atomlithographie und reflexive Atomoptik mit laserpräparierten Atomen, (1998), PhD thesisBibTeX
  • A. Goepfert
    Führung und Ablenkung eines neutralen Atomstrahls, (1998), PhD thesisBibTeXPDF
    Im Rahmen dieser Arbeit konnten sehr unterschiedliche Methoden vorgestellt werden, mit denen die Ablenkung beziehungsweise Führung von Atomen in konservativen Potentialen erreicht wird. Die bei gegenläufigen kurzen Lichtpulsen auftretende stimulierten Kräfte können die spontane Lichtkraft um Größenordnungen übertreffen. Gleichzeitig wird das mit der spontanen Emission verbundenen statistischen Heizen stark unterdrückt. Die experimentell bestimmte räumlich variierende, rücktreibende Kraft konnte zur Fokussierung eines Atomstrahls genutzt werden. In der Erweiterung auf drei Dimensionen ergibt sich ein Potential, mit dem der Aufbau neuartiger, extrem tiefer Fallen möglich ist. Aufgrund der spektralen Frequenzbandbreite ist die stimulierte Lichtkraft auch in Anwesenheit steiler Magnetfeldgradienten und bei großen Dopplerverschiebungen einsetzbar. Bei Atomen mit einem magnetischen Dipolmoment, wie dem Cäsiumatom, bieten sich magnetische Felder an, um die Bahn der Atome zu verändern. Aus Selten-Erd-Permanentmagneten wurde dazu eine magnetische Quadrupolfeldkonstruktion aufgebaut, die einen Atomstrahl führen kann. Der Aufbau ist sehr kompakt und flexibel gehalten, so daß sich die magnetischen Quadrupolfelder auf einfache Weise den jeweiligen experimentellen Anforderungen anpassen lassen. Aufeinander aufbauende Experimentierabschnitte können so entkoppelt und modulare Aufbauten realisiert werden. Die atomoptischen Eigenschaften werden anhand einer 24°-Ablenkung experimentell untersucht und mit theoretischen Vorhersagen verglichen.

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