Coherent light-matter interactions with potassium atoms

Abstract

This thesis presents experimental work undertaken towards realising a system composed of magneto-optically-trapped potassium-39 atoms (39K MOT) enclosed in a ring cavity. The length of the cavity is 9.51 (5) cm and consequently its volume is big compared to cavities used for cavity quantum electrodynamics experiments, operating in the single-atom regime. As such, in order to enter a regime where the light-matter interactions are strong, thousands of atoms have to collectively interact with the mode of the cavity. We achieve strong interactions with potassium atoms for the first time, after 4.36(3) x 104atoms were overlapped with the cavity mode. The signature of our system lying in the collective strong coupling regime was the observation of the splitting and avoided crossing of the normal modes, revealing a coupling strength constant G = 2π x 19.1(1.6) MHz. \n \nThe thesis also discusses the application of the eltectromagnetically-induced transparency (EIT) technique to control the group index of refraction (ng) of atomic ensembles. We firstly apply EIT in the Λ configuration, in a heated vapour cell of potassium. This helped us to better understand the underlying fundamentals of the technique, before ultimately applying it in the (main) cold atoms experiment. Deeply sub-natural EIT features have been achieved, for which we calculate ng as high as 6330(160). Furthermore, by performing a novel measurement of the circular dichroism (and thus birefringence). We observed greater anisotropy when the weak probe and strong coupling beams, which participate in the formation of the EIT system. have lin∥ lin rather than \nlin ⊥ linpolarisation. \n \nBy optimising the temperature and optical depth of the 39K MOT, EIT investigations were made possible with the cold atoms. While a splitting of the absorption line was observed at high coupling beam powers, no EIT resonances were observed when the coupling power was reduced. Our measurements imply that the EIT signal is compromised by decoherence factors, such as the inhomogeneity of the magnetic field across the atomic cloud. Eliminating decoherence and achieving EIT in the strongly interacting atom-cavity system will enable studies with applications in metrology and sensing.