High-resolution optical spectroscopy in a hollow-core photonic crystal fiber

In this paper, we present detailed high-resolution spectroscopy of rubidium (Rb) vapor confined within a hollow-core photonic crystal fiber (HC-PCF). We find a very low level of additional frequency broadening associated with this confinement, with spectral features being only 1 MHz broader than the natural linewidth of the excited state. We show that this additional broadening is consistent solely with the atoms' transit across the fiber's optical mode. This low level of decoherence opens the door to a wide variety of applications including compact frequency standards and new types of quantum optical devices based on alkali-metal-loaded HC-PCFs. We highlight the low level of decoherence through observation of electromagnetically induced transparency in the confined vapor.