Experimental realization of strong coupling between a cold atomic ensemble and an optical fiber microcavity

Cavity quantum electrodynamics (QED) system is a promising platform for quantum optics and quantum information experiments. And its core is the strong coupling between atoms and optical cavity, which causes difficulty in the overlap for the atoms and the antinode of optical cavity mode. Here, we use a programmable movable optical dipole trap to load a cold atomic ensemble into an optical fiber microcavity and realize the strong coupling between the atoms and the optical cavity in which the coupling strength can be improved by polarization gradient cooling and adiabatic loading. By the measurement of vacuum Rabi splitting, the coupling strength can be as high as g N = 2 π × 400 MHz, which means the effective atom number is N ef f = 16 and the collective cooperativity is C N = 1466. These results show this experimental system can be used for cold atomic ensemble and cold molecule based cavity QED research.