Hétet, Blatt, and colleagues replaced the second mirror with a single atom—actually a barium ion. To focus the light on the atom and collect the light bouncing off it, they put a 1.5-centimeter-wide lens between it and the mirror. To hold the ion steady 14 millimeters away from the mirror, they captured it in an electronic trap and used other laser beams to cool it so that it jiggled no more than 20 nanometers from the trap's center. Finally, they tuned the wavelength of the light entering the interferometer so that it could "excite" the atom from a particular low-energy state to a higher-energy one. Without such a light-atom interaction, the atom can't affect the light.
You should read the rest of the article to also figure out what possible applications and studies that can come out of this.
An amazing piece of experimental work.