Cavity QED with atomic mirrors

A promising approach to merge atomic systems with scalable photonics has emerged recently, which consists of trapping cold atoms near tapered nanofibers. Here, we describe a novel technique to achieve strong, coherent coupling between a single atom and photon in such a system. Our approach makes use of collective enhancement effects, which allow a lattice of atoms to form a high-finesse cavity within the fiber. We show that a specially designated “impurity” atom within the cavity can experience strongly enhanced interactions with single photons in the fiber. Under realistic conditions, a “strong coupling” regime can be reached, wherein it becomes feasible to observe vacuum Rabi oscillations between the excited impurity atom and a single cavity quantum. This technique can form the basis for a scalable quantum information network using atom-nanofiber systems. 1 ar X iv :1 20 1. 06 43 v3 [ qu an tph ] 1 8 Fe b 20 12 Techniques to controllably interface atoms with quantum optical fields form the basis for many applications in quantum information science [1, 2]. 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) [3–5], where atomic interactions with light are enhanced via a high-finesse cavity, ii) coherent coupling with atomic ensembles exhibiting large optical depths [6], and iii) the use of fields tightly focused to dimensions smaller than or approaching the scattering cross-section of a single atom [7–13]. Although remarkable achievements have been made with all of these approaches, a robust, scalable technique that can be easily integrated with photonics remains elusive. Here, we describe a hybrid strategy that combines appealing attributes of each of the methods described above, and which can be implemented with relatively modest resources. Our approach utilizes a promising atom-light interface developed in recent years, which consists of cold atoms trapped near tapered nanofibers [14, 15]. The traps are wellcharacterized [14–16] and can potentially be used to transport and couple atoms to other systems, such as dielectric optical cavities [17–19] and nanomechanical resonators [20, 21]. The nearly diffraction-limited transverse confinement of optical fields thus far enables ∼ 10% coupling efficiency of a single atom to the fiber [14, 15], which has allowed for observations of strong light-matter interactions using relatively few atoms and low powers [22–24]. Our hybrid approach is based upon the following principles. First, we show that although the single-atom coupling in this system 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 [6]. 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 [25]. A unique feature of our atomic mirrors compared to conventional