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Quantum technologies

Dieter Meschede's research group
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Theses - FCQED

2017

  • L. Ahlheit
    Frequenzvariable Phasenstabilisierung eines Diodenlasers auf einen optischen Frequenzkamm, (2017), Bachelor thesisBibTeXPDF
  • J. C. Gallego
    Strong Coupling between Small Atomic Ensembles and an Open Fiber Cavity, (2017), PhD thesisBibTeXPDF
    ABSTRACT »

    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.

  • M. Ammenwerth
    Analysing a phase-frequency lock of a laser to an optical frequency comb, (2017), Bachelor thesisBibTeXPDF

2016

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

    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.

2013

  • S. Urban
    Optimization of an optical fiber endfacet machine, (2013), Bachelor thesisBibTeX

2012

  • F. Seidler
    Aufbau einer Maschine zur Bearbeitung von Glasfaser-Endflächen, (2012), Bachelor thesisBibTeX
    ABSTRACT »
    Der einfachste Optische Resonator besteht aus zwei einander gegenüber positionierten Spiegeln (ein sogenannter Fabry-Pérot-Resonator). Die Eigenschaften dieser Resonatoren hängen stark von Form, Oberfläche und Positionierung der verwendeten Spiegel ab. Um einen besonders vorteilhafte Satz an Parametern für Experimente auf dem Gebiet der CQED (Cavity Qantum Electro Dynamics) zu erhalten, möchte man die Endflächen von Glasfasern als Spiegel verwenden. Dies setzt insbesondere eine Möglichkeit zur Bearbeitung der Faserendflächen voraus.Grundlegende Zusammenhänge und Eigenschaften dieser Faser-Fabry-Pérot Resonatoren, wie auch theoretische Betrachtungen zur gewählten Bearbeitungsmethode für die Faserendflächen folgen in Kapitel 2. Anschließend wird der Aufbau einer Maschine zur Bearbeitung von Faserendflächen geschildert (Kapitel 3). Erste Tests werden in Kapitel 4 dargelegt. Abschließend wird ein Ausblick auf noch ausstehende Arbeiten gegeben, die schließlich die Herstellung von Faserresonatoren im gewünschten Rahmen ermöglichen sollen (Kapitel 5).

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