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 physical-cavity volume. In a proof-of-principle experiment of this concept we demonstrate coupling of single Rb atoms to a 11mm long near-concentric cavity with a finesse F=138(2). Operating the cavity 1.65(1)$\mu$m shorter than the critical length, we observe an atom-cavity coupling constant $g_0=2\pi \times 5.0(2)\,$MHz which exceeds the natural dipole decay rate $\gamma$ by a factor $g_0/\gamma=1.7(1)$.