Quantum technologies

2022

• J. E. Uruñuela
  Control of Atoms in a High–Bandwidth Cavity for Quantum Nodes, (2022), PhD thesisBibTeX
  ABSTRACT »

  Optical cavities with coupled atoms are a promising platform as the nodes of future quantum networks, enabling interchange of information between single atoms and single photons. In particular, fiber-based high-bandwidth cavities offer convenient and efficient routing of quantum information, in an interesting regime that combines strong coupling with the atom and high-rate information exchange with the quantum channel. However, critical control of coupled atoms for the quantum node operation, is hindered by the strong Purcell effect, the miniaturized geometry and the high-bandwidth property of such cavities. In this thesis, I report on my contributions towards a high level of control of individual atoms coupled to a high-bandwidth cavity. To this end, three new experimental techniques were developed specially adapted to high-bandwidth fiber cavities, with the following specific goals: (i) intracavity ground-state cooling of single atoms; (ii) atom position detection by fluorescence imaging independent from the cavity transition; (iii) cavity loading of small atomic ensembles with increased density.

  In the first part of this work, I present the experimental setup, consisting of a fiber Fabry-Pe ́rot cavity (FFPC) coupled to 87Rb atoms, and the necessary experimental apparatus to operate the system in a stable manner. I start by motivating the advantage of high-bandwidth cavities with a brief discussion on cavity-mediated light-matter interfacing, and the peculiar strong coupling regime. Then, I give an overview of the complete system with emphasis on the recent technical upgrades, such as an improved cavity stabilization, an upgraded Raman laser setup with a linewidth-reduced DBR laser, and a new cavity-compatible imaging system. Lastly, I introduce the basic experimental toolbox for atomic control that we employ to operate the atom-cavity module: (i) cavity-based atom detection; (ii) cooling with a magneto-optical trap (MOT) and trapping with a 3D lattice; (iii) state initialization by optical pumping; (iv) Raman hyperfine manipulation; (v) position detection by imaging. Most of my work was to extend such basic toolbox for an improved atomic control, with the techniques presented in the next chapters.

  In Chapter 3, I report successful cooling of a single 87Rb atom to its one-dimensional motional ground state while coupled to the FFPC, by degenerate Raman sideband cooling (dRSC). We overcome the challenge of cooling in such high-bandwidth atom-cavity modules, by adapting the degenerate dRSC technique to our cavity and lattice geometry. Raman cooling transitions are driven by the trapping lattice and repumping by the intracavity probe field, without the need of additional lasers and activated by the magnetic bias field. The resource-efficient and simple implementation is a highlight.

  In Chapter 4, I present a newly implemented method in our system for successful fluorescence imaging of small atomic ensembles coupled to a high-bandwidth FFPC, that overcomes the inhibiting Purcell effect and the restricted optical access. It is based on techniques from the field of quantum gas microscopes and relies on the detection of repumper fluorescence on the D1 line generated by three-dimensional (3D) continuous Raman sideband cooling (cRSC). Thus, it remains fully independent from the cavity on the D2 line, for simultaneous operation of the atom-cavity node and position detection of the atoms. It requires only a single free-space beam together with intra-cavity fields, ideal for platforms with limited optical access, e.g. miniaturized quantum optical devices. The repumper-induced differential light shifts and the heating by dipole-force fluctuations (DFFs) are also analyzed.

  In Chapter 5, I introduce a novel and simple method to load the intracavity lattice: the drive-through loading. It only relies on the dynamic control of intensity and phase of one lattice arm that works as a conveyor belt between the MOT and the intracavity lattice. I discuss the working principle of the technique, demonstrate that its efficiency, and show its tuning capability of the cavity-coupled atom number. In the last chapter, I summarize the advances presented here that extend the toolbox for control and manipulation of atom-cavity systems, impacting in the development of quantum networks. The three new techniques presented here, with a future implementation of single-atom addressing, pave the way for creating atomic arrays with predefined number and positions in the cavity: a cavity-quantum register.

2020

• F. Hemmann
  Aufbau und Vorbereitungen für Cavity-Ring-Down-Spektroskopie mit monolithischen Faserresonatoren, (2020), Bachelor thesisBibTeX
  ABSTRACT »

  Zu Beginn der Arbeit werden in Abschnitt 2.1 Funktionsweise und Besonderheiten von Faser-Fabry-Perot-Resonatoren erläutert. Eine Einführung in Cavity-Ring-Down-Spektroskopie und eine erste Abschätzung der Genauigkeit, welche in einem miniaturisierten CRDS-Experiment mit Faserresonatoren erreicht werden sollte, folgen in Abschnitt 2.2. Die neu entwickelten Designs für monolithische FFPR werden in Kapitel 3 eingeführt. In den Abschnitten 3.1 und 3.2 wird erläutert, wie hier bei stabiler und kompakter Bauweise die Ausrichtungsgenauigkeit der Faserspiegel gewährleistet werden kann. Anschließend wird das Herstellungsverfahren der monolithischen FFPR beschrieben (Abschnitt 3.3). Kapitel 4 wendet sich schließlich dem Aufbau zu, mit welchem CRDS mit monolithischen FFPR erprobt werden soll. In diesem Aufbau wird das Licht eines durchstimmbaren Lasers (Abschnitt 4.1) in den Resonator gekoppelt, dessen Reflexionssignal auf einem Oszilloskop betrachtet werden kann (Abschnitt 4.2). Zur Durchführung von Spektroskopie-Messungen wird der Resonator in einer Kammer platziert, die mit einem Gasgemisch aus Sauerstoff und Stickstoff gefüllt wird (Abschnitt 4.3). Eine experimentelle Erprobung von CRDS mit diesem Aufbau steht zum Zeitpunkt der Abgabe dieser Arbeit noch aus.

2018

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

  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.

2017

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

• L. Ahlheit
  Frequenzvariable Phasenstabilisierung eines Diodenlasers auf einen optischen Frequenzkamm, (2017), Bachelor thesisBibTeX

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

2016

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