Strong interactions between light and atoms at the single-quantum level are an important ingredient for quantum technologies, as well as for studies of fundamental effects in quantum optics. This thesis describes the development of a novel experimental platform that allows for trapping a single rubidium atom in the evanescent mode of a nano-fabricated optical cavity with sub-wavelength dimensions. By virtue of their small size, these cavities provide extremely large atom-photon coupling strengths and good prospects for scalability and integration into complex quantum optical circuits. Positioning the atom near the nano-structure is accomplished using a scanning optical tweezer dipole trap. As a first application, we have demonstrated a coherent optical switch, where a single gate photon controls the propagation of many subsequent signal photons, with the interaction mediated by the atom and cavity. We have also shown that the optical response of the combined atom-cavity system is nonlinear at the level of one or two photons.
Strong interactions between light and atoms at the single-quantum level are an important ingredient for quantum technologies, as well as for studies of fundamental effects in quantum optics. This thesis describes the development of a novel experimental platform that allows for trapping a single rubidium atom in the evanescent mode of a nano-fabricated optical cavity with sub-wavelength dimensions. By virtue of their small size, these cavities provide extremely large atom-photon coupling strengths and good prospects for scalability and integration into complex quantum optical circuits. Positioning the atom near the nano-structure is accomplished using a scanning optical tweezer dipole trap. As a first application, we have demonstrated a coherent optical switch, where a single gate photon controls the propagation of many subsequent signal photons, with the interaction mediated by the atom and cavity. We have also shown that the optical response of the combined atom-cavity system is nonlinear at the level of one or two photons.
Underlying matter and light are their building blocks of tiny atoms and photons. The ability to control and utilize matter-light interactions down to the elementary single atom and photon level at the nano-scale opens up exciting studies at the frontiers of science with applications in medicine, energy, and information technology. Of these, an intriguing front is the development of quantum networks where N >> 1 single-atom nodes are coherently linked by single photons, forming a collective quantum entity potentially capable of performing quantum computations and simulations. Here, a promising approach is to use optical cavities within the setting of cavity quantum electrodynamics (QED). However, since its first realization in 1992 by Kimble et al., current proof-of-principle experiments have involved just one or two conventional cavities. To move beyond to N >> 1 nodes, in this thesis we investigate a platform born from the marriage of cavity QED and nanophotonics, where single atoms at ~100 nm near the surfaces of lithographically fabricated dielectric photonic devices can strongly interact with single photons, on a chip. Particularly, we experimentally investigate three main types of devices: microtoroidal optical cavities, optical nanofibers, and nanophotonic crystal based structures. With a microtoroidal cavity, we realized a robust and efficient photon router where single photons are extracted from an incident coherent state of light and redirected to a separate output with high efficiency. We achieved strong single atom-photon coupling with atoms located ~100 nm near the surface of a microtoroid, which revealed important aspects in the atom dynamics and QED of these systems including atom-surface interaction effects. We present a method to achieve state-insensitive atom trapping near optical nanofibers, critical in nanophotonic systems where electromagnetic fields are tightly confined. We developed a system that fabricates high quality nanofibers with high controllability, with which we experimentally demonstrate a state-insensitive atom trap. We present initial investigations on nanophotonic crystal based structures as a platform for strong atom-photon interactions. The experimental advances and theoretical investigations carried out in this thesis provide a framework for and open the door to strong single atom-photon interactions using nanophotonics for chip-integrated quantum networks.
Hybrid quantum devices, in which dissimilar quantum systems are combined in order to attain qualities not available with either system alone, may enable far-reaching control in quantum measurement, sensing, and information processing. A paradigmatic example is trapped ultracold atoms, which offer excellent quantum coherent properties, coupled to nanoscale solid-state systems, which allow for strong interactions. We demonstrate a deterministic interface between a single trapped rubidium atom and a nanoscale photonic crystal cavity. Precise control over the atom's position allows us to probe the cavity near-field with a resolution below the diffraction limit and to observe large atom-photon coupling. This approach may enable the realization of integrated, strongly coupled quantum nano-optical circuits.
Realizing an efficient quantum optical interface for multi-qubit systems would enable their quantum network integration and the scaling-up of the qubit number via distributed quantum computing. This thesis presents the building blocks for integrating a quantum processor based on a Rydberg atom array with a nanophotonic cavity as an optical interface. In our approach, the integration would be achieved by coherently transporting tweezer-trapped atoms between the cavity near-field and free space. To that end, we first characterize the strong coupling between single photons and atoms enabled by the nanophotonic device. Two 87Rb atoms are trapped in optical tweezers and brought to the cavity near-field, while the cavity is probed in reflection. We observe Purcell enhancement and cavity-mediated interactions between the atoms, corresponding to a single atom cooperativity of C = 70 in the mode center. The qubits are encoded in atomic hyperfine ground states, which have good coherence properties even when the atoms are trapped in the cavity near-field. We create an atomic Bell state by pairing up the coherent manipulation of atomic qubits with a quantum nondemolition measurement, implemented via a single photon reflection from the cavity. Crucial to the integration, we show that single atom coherence and two-atom entanglement can be preserved as the atoms are moved away from the cavity. The nanophotonic dielectric surface can host charges which affect highly polarizable Rydberg states, and this influence is studied by measuring the n = 70 Rydberg state coherence at different distances from the cavity. Our measurements suggest that a Rydberg array may be placed ∼ 200 μm away from a nanophotonic device while retaining high quantum gate fidelities. Our methods can be applied to other atomic species and device geometries, paving the path to quantum networks of Rydberg array processors.
New paradigms for optical physics emerge with lattices of atoms trapped in one and two-dimensional photonic crystals [1-4]. Exemplary experimental platforms include photonic crystal waveguides [5-7] and cavities [8, 9]. Owing to their small optical loss and tight field confinement, these nanoscale dielectric devices are capable of mediating long-range atom-atom interactions using photons propagating in their guided modes. In a complimentary fashion, long-range interactions between photons can be mediated by an underlying lattice of atoms. Such systems have the potential to provide new tools for scalable quantum networks, quantum phases of light and matter, and quantum metrology. However, bringing this future of atom nanophotonics to fruition requires the creation of an interdisciplinary toolkit for the control, manipulation, and interaction of atoms and photons with a complexity and scalability not currently possible. I will give an overview of the theoretical prospects for new physics and review experimental progress in this nascent field at the interfaces of nano-photonics, atomic physics, and quantum optics.
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.