Single atoms coupled to a near-concentric cavity

Concentric cavities can lead to strong photon-atom coupling without a need for high finesse or small physicalcavity volume. In this proof-of-principle experiment we demonstrate coupling of single Rb atoms to an 11-mm-long near-concentric cavity with a finesse F = 138(2). Operating the cavity 1.7(1) mu m shorter than the critical length, we observe an atom-cavity coupling constant g(0) = 2 pi x 5.0(2) MHz which exceeds the natural dipole decay rate. by a factor of g(0)/gamma = 1.7(1).