It's not quite that simple though; a single atom will oscillate too fast to be read. So the researchers basically split a cesium atom in half, and had one half move back and forth. Because this movement dilated the oscillation of that half, and the scientists knew exactly how they'd disrupted the atom, they were able to use all the data to calculate the oscillation of the original atom, and essentially make a single atom clock out of it.
A one-atom clock is awesome, but not exactly great at its job. It's only about as accurate as the first atomic clocks, and one billion times less so than the nuclear clocks we have today. Still, it's an impressive accomplishment. More here.