Interferometric thermometry of a single sub-Doppler-cooled atom

Efficient self-interference of single photons emitted by a sideband-cooled barium ion is demonstrated. First, the technical tools for performing efficient coupling to the quadrupolar transition of a single ¹³⁸Ba⁺ ion are presented. We show efficient Rabi oscillations of the internal state of the ion by using a highly stabilized 1.76-μm fiber laser resonant with the S1/2-D5/2 transition. We then show sideband cooling of the ion's motional modes and use it as a means to enhance the interference contrast of the ion with its mirror image to up to 90$%$. Last, we measure the dependence of the self-interference contrast on the mean phonon number, thereby demonstrating the potential of the setup for single-atom thermometry close to the motional ground state.