Atoms with narrow-line resonances play a major role in high precision measurements like magnetometry and atomic clocks. Due to their long inherent coherence time, atoms can serve as quantum memories as well. Moreover, as they possess well-defined electronic levels, coherent interactions with the photon fields can be used to manipulate their quantum states very precisely. Besides, the capability of the optical excitation and read out, increase the spatial resolution of the atomic sensors. Within the last couple of decades interfacing atoms with engineered confined light fields has been a proper playground for investigating various quantum-electrodynamical effects. So far different strategies have been utilized successfully to integrate atoms with a confined light field, for example in high-finesse optical cavities, hollow core fibers, and tapered nanofibers. While cold atom setups provide ideal conditions and controllability to explore different coupling regimes, the large setups required to cool and trap the atoms have hindered their scalability for any realistic quantum networks. Thermal vapors, on the other hand, allow for less precision and control, but their low technical complexity and suitable compatibility with miniaturization and integration make them a promising candidate for realizing scalable networks.
Atoms with narrow-line resonances play a major role in high precision measurements like magnetometry and atomic clocks. Due to their long inherent coherence time, atoms can serve as quantum memories as well. Moreover, as they possess well-defined electronic levels, coherent interactions with the photon fields can be used to manipulate their quantum states very precisely. Besides, the capability of the optical excitation and read out, increase the spatial resolution of the atomic sensors. Within the last couple of decades interfacing atoms with engineered confined light fields has been a proper playground for investigating various quantum-electrodynamical effects. So far different strategies have been utilized successfully to integrate atoms with a confined light field, for example in high-finesse optical cavities, hollow core fibers, and tapered nanofibers. While cold atom setups provide ideal conditions and controllability to explore different coupling regimes, the large setups required to cool and trap the atoms have hindered their scalability for any realistic quantum networks. Thermal vapors, on the other hand, allow for less precision and control, but their low technical complexity and suitable compatibility with miniaturization and integration make them a promising candidate for realizing scalable networks.
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.
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.
The advances in laser cooling and trapping techniques allow samples of ultra-cold atoms to be created and controlled for the study of quantum physics. In this thesis, we focus on using ensembles of cold atoms to investigate novel quantum effects and their applications in quantum optics. Realization of a scalable intercontinental quantum system, e.g. a global quantum internet, relies on quantum memories with long lifetimes on the order of seconds. We have reduced the differential light shifts of the atomic ground states to a sub-Hz level and achieved a lifetime of 16 seconds. Achieving fast and efficient quantum operations, on the other hand, requires strongly-interacting systems. We explore highly excited Rydberg atoms towards this goal and demonstrate Rydberg excitation blockade, many-body Rabi oscillations, trapping of Rydberg atoms, deterministic single photon source, and atom-photon entanglement. Finally, we integrate the two essential capabilities for quantum information processing, fast quantum state generation and long-term storage, by simultaneously exploiting Rydberg levels for interactions and ground atomic levels for the storage of quantum state. Our work advances the control of coherence properties and interactions in cold atomic ensembles to a new level and opens new opportunities for studies of complex quantum systems.
Devices that harness the surprising properties of quantum systems at scale hold the possibility of revolutionary advances in information technology. Algorithms for quantum computers can take advantage of quantum parallelism to solve problems that are intractable with classical hardware, including prime factorization and simulating chemical processes. A quantum communication network that carries qubits instead of bits can leverage the irreversibility of projective measurement to guarantee secure communication. Because quantum states are fragile, building such devices requires exquisite control over many isolated quantum systems. One of the most critical capabilities is the interaction of stationary and flying qubits. In this thesis we consider how to engineer atom-light interactions as an interface for networked quantum systems. Quantum atom-light couplers require the efficient and reversible interaction of atomic and optical qubits, but the natural interaction of atoms and photons is weak. It is this very lack of interaction between light and its environment that makes light such an excellent carrier of quantum information. There are three promising strategies for stronger coherent atom-light interactions. First, the light field may be confined to a cavity so that it interacts with the atom over a longer time. Second, the light field may be focussed to a small spot with high aperture optics and matched to the natural atomic radiation pattern. Third, we may pass the light field through an ensemble of many atoms and store the qubit in a distributed state. In practice, atom-light couplers employ some combination of all three strategies. We will focus our research on the second two approaches: first by coupling one and two trapped ions with high aperture optics, and then operating a quantum optical memory with an ensemble of approximately 10 billion neutral atoms. Atoms that have been ionized and trapped in a common electromagnetic potential are an advanced few-qubit computation platform. Qubit states are associated with an electronic excitation at each atom, controlled coherently with lasers, and made to interact in two (or many) qubit gates by mutual Coulomb repulsion. One path to quantum advantage with ion-trap quantum processors is to scale small qubit registers by distributing entanglement with photonic interconnects. We consider the feasible efficiency of free-space atom-light couplers for atomic dipole transitions, and derive the atomic image by common free-space couplers. We couple a trapped-ion register with high-aperture lenses and operate a single trapped ion as a photon source. We model the dynamic character of the source and show spin-orbit coupling in the single-photon spatial mode. Although the total collection efficiency is only approximately 0.01, the source has exceptional photon-number purity A = (1.9 +/- 0:2) * 10^-3 such that the higher-order quantum nature of the field persists even after most of the field has been discarded. We derive an efficient quantum non-Gaussian witness and surpass it, the first such demonstration for a trapped-atom single-photon source. We entangle two trapped atoms by single-photon detection, and observe an interference pattern in the spatial mode of the bipartite state. Constructive interference between the entangled components enhances the emission probability by up to 29%, an example of collective enhancement by a two-atom ensemble. The efficiency of free-space atom-light couplers is limited by our capability to engineer high-precision, high-aperture optics. We fabricate ultra-precise hemispheric mirrors with numerical aperture 0.996 by single point diamond turning. The mirrors are amongst the smoothest hemispheric surfaces ever manufactured with root-mean-square (RMS) errors consistently below 25 nm. The smoothest of our mirrors has a RMS error of 14 nm and peak-to-valley error of 88 nm. A mirror with this surface is capable of suppressing or enhancing the spontaneous emission of an atom into free space by 96% in a proposed quantum electrodynamics experiment. We show how these mirrors, with a simple modification, can shape the spatial mode of a trapped atom by similar vacuum-mode engineering. We derive a near-hemispheric mirror coupling scheme that should be 72% efficient with mirrors as precise as ours. We design an ion trap for use with such high-aperture optics. With careful conditioning and control, ensembles of many atoms can be made to store and release photonic qubits on demand. Such optical quantum memories leverage the collective interaction of a light field with many billions of atoms to achieve storage and recall efficiencies approaching unity. We show how stationary light fields for photonic phase gates can be generated in optically deep ensembles, an effect which has since been observed. We implement the gradient echo memory (GEM) scheme in an ultra-high optical depth cold-atom ensemble. The performance of a practical quantum memory is contingent on the chosen qubit encoding. We extend the GEM protocol to allow the simultaneous storage of frequency separated signals and demonstrate that this `dual-rail' memory is suitable for high-fidelity frequency qubits. Dual-rail signals are recalled with 35% efficiency, 82% interference fringe visibility and 6 degrees of phase stability. We describe how the fidelity of the scheme is limited by frequency-dependent polarization rotation and how this may be addressed in an improved configuration. Finally, we demonstrate single-rail storage by GEM with 87% efficiency and 1 ms memory lifetime. Our memory surpasses the no-cloning limit for up to 600 us of storage, out-performing an optical fibre delay line by a factor of six. This is the first quantum memory to beat this important benchmark.
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.
Precision metrology and quantum measurement often demand that matter be prepared in well-defined quantum states for both internal and external degrees of freedom. Laser-cooled neutral atoms localized in a deeply confining optical potential satisfy this requirement. With an appropriate choice of wavelength and polarization for the optical trap, two electronic states of an atom can experience the same trapping potential, permitting coherent control of electronic transitions independent of the atomic center-of-mass motion. Here, we review a number of recent experiments that use this approach to investigate precision quantum metrology for optical atomic clocks and coherent control of optical interactions of single atoms and photons within the context of cavity quantum electrodynamics. We also provide a brief survey of promising prospects for future work.