Coupling neutral atoms to nanoscale waveguides is a promising approach for realizing light-matter quantum interfaces. In combination with resonator enhancement of the guided light, such systems provide powerful tools for a broad range of applications. They even allow one to reach and study new paradigms of light-matter interaction such as the multimode strong coupling regime of resonator quantum electrodynamics (QED), where the atom-resonator interaction strength exceeds the free spectral range of the resonator. In my talk, I will present our current experimental status on the realization of such a novel type of resonator QED system, where we interface a fiber-ring-resonator that contains a nanofiber-part with an ensemble of laser-cooled cesium atoms from a magneto-optical trap.
Coupling neutral atoms to nanoscale waveguides is a promising approach for realizing light-matter quantum interfaces. In combination with resonator enhancement of the guided light, such systems provide powerful tools for a broad range of applications. They even allow one to reach and study new paradigms of light-matter interaction such as the multimode strong coupling regime of resonator quantum electrodynamics (QED), where the atom-resonator interaction strength exceeds the free spectral range of the resonator. In my talk, I will present our current experimental status on the realization of such a novel type of resonator QED system, where we interface a fiber-ring-resonator that contains a nanofiber-part with an ensemble of laser-cooled cesium atoms from a magneto-optical trap.
We experimentally realize an optical fiber ring resonator that includes a tapered section with a subwavelength-diameter waist. In this section, the guided light exhibits a significant evanescent field which allows for efficient interfacing with optical emitters. A commercial tunable fiber beam splitter provides simple and robust coupling to the resonator. Key parameters of the resonator such as the out-coupling rate, free spectral range, and birefringence can be adjusted. Thanks to the low taper- and coupling-losses, the resonator exhibits an unloaded finesse of F=75±1, sufficient for reaching the regime of strong coupling for emitters placed in the evanescent field. The system is ideally suited for trapping ensembles of laser-cooled atoms along the nanofiber section. Based on measured parameters, we estimate that the system can serve as a platform for optical multimode strong coupling experiments. Finally, we discuss the possibilities of using the resonator for applications based on chiral quantum optics.
Optical nanofibers provide a versatile platform for exploring strong light-matter interactions at the single-quantum level, a central goal of quantum optics. Their subwavelength diameter supports tightly confined guided modes with strong evanescent fields, enabling efficient coupling between photons and nearby atoms. By incorporating fiber Bragg gratings, nanofibers can form fully fiber-integrated high-finesse cavities, combining strong atom-photon coupling with efficient and low-loss optical input-output interfacing in the setting of cavity quantum electrodynamics (QED). In parallel, nanofiber systems naturally realize waveguide QED, where atoms interact strongly with propagating photons in a one-dimensional geometry. In this review, we introduce the underlying physical principles, summarize key experimental techniques and representative phenomena, and discuss prospects for applications in quantum information processing, quantum networks, and related quantum technologies.
In this thesis, I present the realization of a fiber-optical interface using optically trapped cesium atoms, which is an efficient tool for coupling light and atoms. The basic principle of the presented scheme relies on the trapping of neutral cesium atoms in a two-color evanescent field surrounding a nanofiber. The strong confinement of the fiber guided light, which also protrudes outside the nanofiber, provides strong confinement of the atoms as well as efficient coupling to near-resonant light propagating through the fiber. In chapter 1, the necessary physical and mathematical background describing the propagation of light in an optical fiber is presented. The exact solution of Maxwell’s equations allows us to model fiber-guided light fields which give rise to the trapping potentials and the atom-light coupling in the close vicinity of a nanofiber. Chapter 2 gives the theoretical background of light-atom interaction. A quantum mechanical model of the light-induced shifts of the relevant atomic levels is reviewed, which allows us to quantify the perturbation of the atomic states due to the presence of the trapping light-fields. The experimental realization of the fiber-based atom trap is the focus of chapter 3. Here, I analyze the properties of the fiber-based trap in terms of the confinement of the atoms and the impact of several heating mechanisms. Furthermore, I demonstrate the transportation of the trapped atoms, as a first step towards a deterministic delivery of individual atoms. In chapter 4, I present the successful interfacing of the trapped atomic ensemble and fiber-guided light. Three different approaches are discussed, i.e., those involving the measurement of either near-resonant scattering in absorption or the emission into the guided mode of the nanofiber. In the analysis of the spectroscopic properties of the trapped ensemble we find good agreement with the prediction of theoretical model discussed in chapter 2. In addition, I introduce a non-destructive scheme for the interrogation of the atoms states, which is sensitive to phase shifts of far-detuned fiber-guided light interacting with the trapped atoms. The inherent birefringence in our system, induced by the atoms, changes the state of polarization of the probe light and can be thus detected via a Stokes vector measurement.
Trapping and optically interfacing laser-cooled neutral atoms are essential requirements for their use in advanced quantum technologies. Here we simultaneously realize both of these tasks with cesium atoms interacting with a multicolor evanescent field surrounding an optical nanofiber. The atoms are localized in a one-dimensional optical lattice about 200 nm above the nanofiber surface and can be efficiently interrogated with a resonant light field sent through the nanofiber. Our technique opens the route towards the direct integration of laser-cooled atomic ensembles within fiber networks, an important prerequisite for large scale quantum communication schemes. Moreover, it is ideally suited to the realization of hybrid quantum systems that combine atoms with, e.g., solid state quantum devices.
This project aims to develop scientific as well as technical foundations towards realization of a novel platform for light-matter interaction based on tapered optical nanofibers that enable to interface neutral atoms in their vicinity. A setup for magneto-optical trap (MOT) has been built. Tapered nanofibers (TNFs) of sub-wavelength diameter, with transmission of 95 percent have been fabricated and characterized. Next, a nanofiber is transferred into the MOT setup and installed inside the vacuum chamber. The MOT cloud and nanofiber are made to spatially overlap inside ultra-high vacuum (UHV). Coupling of fluorescence of MOT into the guided mode of TNF has been observed. This clearly shows the coupling of atoms with the fiber-guided mode and that our interface works. In addition, 65-70 percent absorption of a free space beam by the atomic cloud has been observed and the relevant MOT parameters are obtained from it. Our efforts are directed towards observing interaction of atoms with the fiber-guided light via evanescent field. Attempts have been made to observe absorption of a probe beam through the nanofiber by the MOT cloud and techniques for switching on-off the MOT beams, the probe beam and the magnetic field are developed for that purpose. Ultimately, the goal will be to trap atoms in the vicinity of a nanofiber enabling to reach a large optical depth accompanied by the ability to reach high intensities for low optical power. I will present my work on the development of this MOT-nanofiber interface - a new promising platform for research in nonlinear quantum optics, quantum information science and technology, quantum communication and hybrid quantum system.
An exciting frontier in quantum information science is the integration of otherwise "simple" quantum elements into complex quantum networks. The laboratory realization of even small quantum networks enables the exploration of physical systems that have not heretofore existed in the natural world. Within this context, there is active research to achieve nanoscale quantum optical circuits, for which atoms are trapped near nano-scopic dielectric structures and "wired" together by photons propagating through the circuit elements. Single atoms and atomic ensembles endow quantum functionality for otherwise linear optical circuits and thereby enable the capability of building quantum networks component by component. Toward these goals, we have experimentally investigated three different systems, from conventional to rather exotic systems : free-space atomic ensembles, optical nano fibers, and photonics crystal waveguides. First, we demonstrate measurement-induced quadripartite entanglement among four quantum memories. Next, following the landmark realization of a nanofiber trap, we demonstrate the implementation of a state-insensitive, compensated nanofiber trap. Finally, we reach more exotic systems based on photonics crystal devices. Beyond conventional topologies of resonators and waveguides, new opportunities emerge from the powerful capabilities of dispersion and modal engineering in photonic crystal waveguides. We have implemented an integrated optical circuit with a photonics crystal waveguide capable of both trapping and interfacing atoms with guided photons, and have observed the collective effect, superradiance, mediated by the guided photons. These advances provide an important capability for engineered light-matter interactions, enabling explorations of novel quantum transport and quantum many-body phenomena.