Cold atom-nanophotonic systems constitute a powerful research platform for the exploration of new regimes of light-matter interaction. The guided light in nanophotonic systems are tightly confined, and often used to trap and manipulate cold atoms at subwavelength distance away from the surface. While the initialization of atomic internal degrees of freedom in these systems has been achieved, a full control of the atomic quantum state also requires manipulating and preparing the atomic motional state at the quantum level. In this thesis, we explore and characterize the coupling between motional and spin degrees of freedom in nanofiber-trapped atoms. This coupling originates from the strong polarization gradient which occurs naturally in spatially confined light fields, i.e., the guided light field in a nanophotonic system. We demonstrate that the spin-motion coupling can be utilized to implement degenerate Raman cooling and prepare atoms close to the three-dimensional motional ground state.We obtain mean numbers of motional quanta of nanofiber-trapped atoms using heterodyne fluorescence spectroscopy. Building on this work, we then use the spin-motion coupling for nanofiber-trapped atoms to realize a mechanical analogue of the Dicke model. We infer the energy spectrum of the system from transitions observed in the fluorescence spectrum. We show that our system reaches the ultrastrong coupling regime. Moreover, we demonstrate that the coupling strength can be readily tuned using an additional nanofiber-guided light field. Furthermore, we achieve imaging of single nanofiber-trapped atoms. Taking advantage of degenerate Raman cooling, we cool atoms near the motional ground state while collecting atomscattered light using a camera. We show single atoms can be detected by imaging at an integration time far less than the trapping lifetime. To demonstrate the potential of this technique, we perform two proof of principle experiments. First, we measure the extinction of a nanofiberguided light atom by atom, and verify the Beer-Lambert Law in the few atoms regime. Second, we detect the atom-scattered light that is coupled to the nanofiber. We observe interference of scattered light fields as a function of the distance between two trapped atoms. By controlling atomic motional states and imaging single atoms, our work in this thesis adds to the toolbox for manipulating and detecting cold atoms interfaced to nanophotonic systems, and paves the way for realizing the bottom-up approach to explore new regimes of light-matter interaction atom by atom.
Cold atom-nanophotonic systems constitute a powerful research platform for the exploration of new regimes of light-matter interaction. The guided light in nanophotonic systems are tightly confined, and often used to trap and manipulate cold atoms at subwavelength distance away from the surface. While the initialization of atomic internal degrees of freedom in these systems has been achieved, a full control of the atomic quantum state also requires manipulating and preparing the atomic motional state at the quantum level. In this thesis, we explore and characterize the coupling between motional and spin degrees of freedom in nanofiber-trapped atoms. This coupling originates from the strong polarization gradient which occurs naturally in spatially confined light fields, i.e., the guided light field in a nanophotonic system. We demonstrate that the spin-motion coupling can be utilized to implement degenerate Raman cooling and prepare atoms close to the three-dimensional motional ground state.We obtain mean numbers of motional quanta of nanofiber-trapped atoms using heterodyne fluorescence spectroscopy. Building on this work, we then use the spin-motion coupling for nanofiber-trapped atoms to realize a mechanical analogue of the Dicke model. We infer the energy spectrum of the system from transitions observed in the fluorescence spectrum. We show that our system reaches the ultrastrong coupling regime. Moreover, we demonstrate that the coupling strength can be readily tuned using an additional nanofiber-guided light field. Furthermore, we achieve imaging of single nanofiber-trapped atoms. Taking advantage of degenerate Raman cooling, we cool atoms near the motional ground state while collecting atomscattered light using a camera. We show single atoms can be detected by imaging at an integration time far less than the trapping lifetime. To demonstrate the potential of this technique, we perform two proof of principle experiments. First, we measure the extinction of a nanofiberguided light atom by atom, and verify the Beer-Lambert Law in the few atoms regime. Second, we detect the atom-scattered light that is coupled to the nanofiber. We observe interference of scattered light fields as a function of the distance between two trapped atoms. By controlling atomic motional states and imaging single atoms, our work in this thesis adds to the toolbox for manipulating and detecting cold atoms interfaced to nanophotonic systems, and paves the way for realizing the bottom-up approach to explore new regimes of light-matter interaction atom by atom.
Single particle-resolved fluorescence imaging is an enabling technology in cold-atom physics. However, so far, this technique has not been available for nanophotonic atom-light interfaces. Here, we image single atoms that are trapped and optically interfaced using an optical nanofiber. Near-resonant light is scattered off the atoms and imaged while counteracting heating mechanisms via degenerate Raman cooling. We detect trapped atoms within 150 ms and record image sequences of given atoms. Building on our technique, we perform two experiments which are conditioned on the number and position of the nanofiber-trapped atoms. We measure the transmission of nanofiber-guided resonant light and verify its exponential scaling in the few-atom limit, in accordance with Beer-Lambert's law. Moreover, depending on the interatomic distance, we observe interference of the fields that two simultaneously trapped atoms emit into the nanofiber. The demonstrated technique enables postselection and possible feedback schemes and thereby opens the road toward a new generation of experiments in quantum nanophotonics.
Laser-cooled atoms coupled to nanophotonic structures constitute a powerful research platform for the exploration of new regimes of light-matter interaction.While the initialization of the atomic internal degrees of freedom in these systems has been achieved, a full preparation of the atomic quantum state also requires controlling the center-of-mass motion of the atoms at the quantum level.Obtaining such control is not straightforward because of the close vicinity of the atoms to the photonic system at ambient temperature.Here, we demonstrate cooling of individual neutral cesium atoms that are optically interfaced with light in an optical nanofiber, preparing them close to their three-dimensional motional ground state.The atoms are localized less than 300 nm away from the hot fiber surface.Ground-state preparation is achieved by performing degenerate Raman cooling, and the atomic temperature is inferred from the analysis of heterodyne fluorescence spectroscopy signals.Our cooling method can be implemented either with externally applied or guided light fields.Moreover, it relies on polarization gradients, which naturally occur for strongly confined guided optical fields.Thus, this method can be implemented in any trap based on nanophotonic structures.Our results provide an ideal starting point for the study of novel effects such as light-induced self-organization, the measurement of novel optical forces, and the investigation of heat transfer at the nanoscale using quantum probes.
Techniques to controllably interface atoms with quantum optical fields form the basis for many applications in quantum information science. For example, photons are convenient to relay information over large quantum networks, while atoms naturally are physical systems that can process and store this information. Thus far, the available techniques to efficiently couple single photons with atomic media fall into one of the following, mostly independent, categories: i) cavity quantum electrodynamics (QED), where atomic interactions with light are enhanced via a high-finesse cavity, ii) coherent coupling with atomic ensembles exhibiting large optical depths, and iii) the use of fields tightly focused to dimensions smaller than or approaching the scattering cross-section of a single atom. Although remarkable achievements have been made with all of these approaches, a robust, scalable technique remains elusive. Recently, several groups have successfully demonstrated that cold atoms can be trapped near and interfaced with nanophotonic systems, such as hollow-core photonic crystal fibers [1] and tapered nanofibers [2–4]. The traps are well-characterized [2, 3, 5], and the nearly diffraction-limited transverse confinement of optical fields enables ∼ 10% coupling efficiency of a single atom to the fiber [2, 3]. This has already led to remarkable observations of strong light-matter interactions using relatively few atoms and low powers [1]. It has also been proposed that such techniques can be extended to highly configurable photonic crystal waveguides [6]. Together, these efforts raise the intriguing possibility for future nanophotonic systems with tremendous figures of merit, wherein atom-light interactions can be tailored nearly at will. Here, we discuss recent efforts to develop novel techniques to realize strong, controllable atom-photon and photon-photon interactions, which take full advantage of the parameter space afforded by nanophotonic interfaces and are not based upon the extension of existing techniques. As a specific example, we discuss a protocol to achieve “all-atomic” cavity QED [7], as briefly described below. First, we show that although the single-atom coupling to guided modes of a nanophotonic waveguide might be relatively weak, there exist collective modes of a trapped atomic ensemble whose coupling to light is enhanced by the square root of the atom number, √ NA. While collective effects are generally well-known, special consequences emerge in the nanofiber system when the atoms are trapped in a lattice. In particular, collective effects cause such a lattice to act as a near-perfect mirror for an incident field close to resonance. In analogy to cavity QED, we then demonstrate that two sets of atomic mirrors can form an effective cavity, which can greatly enhance the coupling of a single, specially chosen “impurity” atom (or a few impurity atoms) positioned inside. We introduce a novel quantum spin model to describe the atom-light coupling, which allows one to exactly map the atom-nanofiber interface onto the simple and elegant Jaynes-Cummings model of cavity QED [8]. A unique feature of our atomic mirrors compared to conventional cavities is that they have long relaxation times and are highly dispersive. Remarkably, even with very low mirror finesse (F ∼ 10 2 ), this property allows one to attain the “strong coupling” regime of cavity QED, where vacuum Rabi oscillations [9–12] occur between an excited impurity atom and a single “photon” stored in the cavity (or more precisely, in the atomic mirrors). Furthermore, as quantum mechanical objects, these atom mirrors can be used to store quantum information and transfer this information into propagating waveguide modes. These various features can be combined to realize all of the building blocks for scalable quantum information processing.
The evanescent field surrounding nanoscale optical waveguides offers an efficient interface between light and mesoscopic ensembles of neutral atoms. However, the thermal motion of trapped atoms, combined with the strong radial gradients of the guided light, leads to a time-modulated coupling between atoms and the light mode, thus giving rise to additional noise and motional dephasing of collective states. Here, we present a dipole force free scheme for coupling of the radial motional states, utilizing the strong intensity gradient of the guided mode and demonstrate all-optical coupling of the cesium hyperfine ground states and motional sideband transitions. We utilize this to prolong the trap lifetime of an atomic ensemble by Raman sideband cooling of the radial motion which, to the best of our knowledge, has not been demonstrated in nano-optical structures previously. This Letter points towards full and independent control of internal and external atomic degrees of freedom using guided light modes only.
The study of atom-light interaction is a key element of quantum optics and a central part of atomic physics. Systems composed of atoms interacting with each other through the electromagnetic field can be used for studies from fundamental research to practical applications. Experimental realizations of these systems benefit from three distinct attributes: large atom-light coupling, trapping and control of atomic ensembles, and engineering and manipulation of the electromagnetic field. Optical waveguides provide a platform that achieves these three goals. In particular, optical nanofibers are an excellent candidate. They produce a high confinement of the electromagnetic field that improves atom-light coupling, guiding the field that mediates the interactions between atoms, while allowing trapping of the atoms close to it. This thesis describes the uses of an optical nanofiber for quantum optics experiments, demonstrating its possibilities for enabling special atom-light interactions. We trap atoms near the optical nanofiber surface, and characterize the trap in a non-destructive manner. We show how the presence of the nanofiber modifies the fundamental atomic property of spontaneous emission, by altering the electromagnetic environment of the atom. Finally, we use the nanofiber to prepare collective states of atoms around it. These states can radiate faster or slower than a single atom (superradiance and subradiance). The observation of subradiance of a few atoms, a rather elusive effect, evidences nanofibers as a strong candidate for future quantum optics experiments. Moreover, we show how the guided field mediates interaction between atoms hundreds of wavelengths apart, creating macroscopically delocalized collective states.
We have recently demonstrated a new experimental platform for trapping and optically interfacing laser-cooled cesium atoms. The scheme uses a two-color evanescent field surrounding an optical nanofiber to localize the atoms in a one-dimensional optical lattice 200 nm above the nanofiber surface [1]. In order to use this fiber-coupled ensemble of trapped atoms for applications in the context of quantum communication and quantum information processing, non-classical states of the atomic spins have to be prepared and should live long enough to allow one to apply successive quantum gates. The close proximity of the trapped atoms to the nanofiber surface and the strong polarization gradients of nanofiber-guided light fields are potentially important sources of decoherence. Here, we present our latest experimental results on the coherence properties of atomic spins in our nanofiber-based trap. Using a microwave field to drive the clock transition, we determine inhomogeneous and homogeneous dephasing times by Ramsey and spin echo techniques, respectively. Our results constitute the first measurement of the coherence properties of atoms trapped in the vicinity of a nanofiber and represent a fundamental step towards establishing nanofiber-based traps for cold atoms as a building block in a quantum network.