Nanofiber Ring Resonator

Implementing future photonic information and communication architectures require precise control on the flow of light in integrated optical devices. When this control reaches the level of the fundamental quantum processes such as the absorption and emission of single photons, this enables one then to introduce quantum features into future applications. Devices that provide this fundamental quantum control are so-called lightmatter quantum interfaces. Their realization in an efficient and scalable fashion promises to introduce quantum technologies into the modern information age. The goal of this project is to realize a novel physical platform that provides an efficient and fully fiber- integrated quantum interface. The key element is a new type of optical resonator: A fiber ring resonator that contains an optical nanofiber part, i.e. an optical fiber with a diameter of a few 100 nanometer smaller than the wavelength of the guided light. In this nanofiber section, a large amount of the light travels outside the fiber as evanescent field. Nonetheless, the light propagation is essentially loss-less. At the same time, we can efficiently couple large number of trapped atoms to the evanescent field of the nanofiber part. In this way, we will realize a quantum interface that has small photon losses and, at the same time, very large light-matter interaction strengths. This allows us to realize and investigate in detail novel physical situations that so far could not be demonstrated such as the so-called regime of multimode strong light-matter coupling. Another goal of the project is to realize a new experimental platform for quantum information processing. For this purpose, we will combine our resonator with an atom-chip a chip-integrated platform for the cooling and manipulation of atoms. In this combined system the resonator mediates long-distance interactions between ultra-cold atoms provided by the atom-chip. This is the key ingredient that we will use for implementing an integrated hybrid quantum computer. Establishing this novel experimental platform for quantum information processing will pave the way towards realizing next-generation computers and will ensure the continuing success story of the modern information age.

The implementation of future photonic information and communication architectures requires precise control of the light flow in integrated optical devices. When this control reaches the level of fundamental quantum processes, such as the absorption and emission of single photons, it becomes possible to introduce quantum features into future applications. Devices that provide this fundamental quantum control are called light-matter quantum interfaces, and realizing them in an efficient and scalable way will make photonic quantum technologies available for commercial applications. To achieve the required level of control in such systems, photons are typically confined to small volumes in so-called optical cavities or optical resonators. In this way, their interaction strength with the emitter can be significantly enhanced and their absorption and reemission by the emitter can be controlled deterministically. The goal of the NanoFire ("nanofiber ring resonator") project was to study resonator-enhanced atom-photon interactions in a new regime where, in contrast to the standard approach of using very short optical cavities, the photons are stored in an extremely long optical ring resonator with a roundtrip length of about 40 m. The resonator consists of a long optical fiber loop and contains a 1 cm long nanofiber section. In this section, the fiber diameter is only about 450 nm, which is smaller than the wavelength of the propagating photons. As a result, a large fraction of the light propagates outside the nanofiber as an evanescent field. This makes it possible to couple laser-cooled atoms to the light field of the resonator. In this way, we realize a new type of coupled atom-resonator system that allows light-matter coupling in the so-called multimode strong or superstrong coupling regime. Here, the collective atom-resonator coupling strength exceeds the free spectral range of the resonator, and thus the atoms simultaneously interact strongly with photons in different longitudinal resonator modes. In the NanoFire project, we experimentally demonstrated light-matter interaction in this new regime by coupling an ensemble of about 1000 caesium atoms to the fiber ring resonator. We performed detailed theoretical and experimental studies of the physical properties of the atom-light interaction in this new regime, and also investigated the transition to the traditional regime of strong light-matter coupling using conventional (short) resonators. Our results advance the understanding of the nature of atom-light coupling and lay the groundwork for future studies and applications that exploit the advantageous features of this new coupling regime.