Cold-Atom Loading of Hollow-Core Photonic Crystal Fibre for Quantum Technologies

Abstract

Ultra-strong light-atom interaction is a key resource for numerous applications in quantum-information processing, nonlinear optics, and quantum sensing. Maximising the strength of the interaction requires optimising the combination of light-atom coherent interaction time, spatial overlap between the optical mode and the atomic cross section, and the number of participating atoms. An exciting approach to achieving these goals is to use a collection of laser-cooled atoms inside a hollow-core photonic crystal fibre. Here the tight transverse confinement provided by fibre guarantees overlap between the atomic sample and guided optical modes over an arbitrarily long distance. Laser cooling improves the effective atom number of the sample by increasing the fraction that participate in the interaction and significantly improves the coherent interaction time by reducing the spatial decoherence rate of the ensemble. This project focuses around the development of an apparatus that realises the lasercooling, trapping, and loading of atoms into a kagome-lattice hollow-core fibre. In this thesis we describe the development of the elements required to realise this task, including the vacuum system, laser sources, computer oversight, and theoretical models employed. The resulting platform is capable of achieving the ultra-high optical depths required for exciting quantum-optics applications such as long-lived coherent optical pulse storage. We have demonstrated high-efficiency transport of cold rubidium atoms from a magneto-optical trap into a hollow-core fibre, measuring a peak optical depth of 600 with only 3£106 atoms. These experiments were guided by a Monte-Carlo simulation that has been shown to have excellent agreement with the physical system. The results show that this platform is in an excellent position to investigate coherent optical phenomena at the few-photon level. Along the way we investigated the application of light-shift engineering to both measure and compensate for the perturbative effects the strong light fields present in the experiment have on atomic states. We extend the ‘magic-wavelength’ technique used in the atomic lattice clock community to nullify the lineshape broadening of the target ensemble by introducing an additional light field. This allows the technique to be implemented in a broad range of atomic species and transitions, where the original technique was only accessible for limited species with specific energy-level structures. We also take advantage of light-shift engineering to extract a detailed model of the spatial distribution of an optically-trapped ensemble through a simple spectroscopic technique. We use this model to infer the temperature, coherence time, and number of atoms in the trap in addition to the depth of the trap itself. Experimentally we demonstrate this on our cold-atom-filled fibre platform, showing that this information can be extracted from a system with limited optical access and where conventional techniques cannot be applied. The apparatus and experimental techniques we have developed place this project in an excellent position to perform cutting-edge research in the fields of quantum information processing and nonlinear optics.