Spin-motion coupling of nanofiber-trapped atoms and its applications

Abstract

Cold atom-nanophotonic systems constitute a powerful research platform for the exploration of new regimes of light-matter interaction. The guided light in nanophotonic systems are tightly confined, and often used to trap and manipulate cold atoms at subwavelength distance away from the surface. While the initialization of atomic internal degrees of freedom in these systems has been achieved, a full control of the atomic quantum state also requires manipulating and preparing the atomic motional state at the quantum level. In this thesis, we explore and characterize the coupling between motional and spin degrees of freedom in nanofiber-trapped atoms. This coupling originates from the strong polarization gradient which occurs naturally in spatially confined light fields, i.e., the guided light field in a nanophotonic system. We demonstrate that the spin-motion coupling can be utilized to implement degenerate Raman cooling and prepare atoms close to the three-dimensional motional ground state.We obtain mean numbers of motional quanta of nanofiber-trapped atoms using heterodyne fluorescence spectroscopy. Building on this work, we then use the spin-motion coupling for nanofiber-trapped atoms to realize a mechanical analogue of the Dicke model. We infer the energy spectrum of the system from transitions observed in the fluorescence spectrum. We show that our system reaches the ultrastrong coupling regime. Moreover, we demonstrate that the coupling strength can be readily tuned using an additional nanofiber-guided light field. Furthermore, we achieve imaging of single nanofiber-trapped atoms. Taking advantage of degenerate Raman cooling, we cool atoms near the motional ground state while collecting atomscattered light using a camera. We show single atoms can be detected by imaging at an integration time far less than the trapping lifetime. To demonstrate the potential of this technique, we perform two proof of principle experiments. First, we measure the extinction of a nanofiberguided light atom by atom, and verify the Beer-Lambert Law in the few atoms regime. Second, we detect the atom-scattered light that is coupled to the nanofiber. We observe interference of scattered light fields as a function of the distance between two trapped atoms. By controlling atomic motional states and imaging single atoms, our work in this thesis adds to the toolbox for manipulating and detecting cold atoms interfaced to nanophotonic systems, and paves the way for realizing the bottom-up approach to explore new regimes of light-matter interaction atom by atom.