Microlaser: A laser with One Atom in an Optical Resonator

We have demonstrated laser oscillation with one atom in an optical resonator. In our experiment a beam of ${}_{}{}^{138}{}_{}{}^{}\mathrm{Ba}$ atoms traverses a single-mode cavity with a finesse of 8 × ${10}^5$. The atoms are excited by a $\pi$ pulse from the ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ ground state to the ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}\mathrm{}(m=0\mathrm{})\mathrm{}$ excited state before they enter the cavity. Laser oscillation at 791 nm (${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}\to {}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$) has been observed, with the mean number of atoms inside the mode ranging from 0.1 to 1.0, resulting in the mean number of photons inside the cavity changing from 0.14 to 11. The results are in good agreement with a fully quantized micromaser theory adapted to our experiment.