An atomic clock actually uses a hunk of quartz, not a cesium atom, to tell time.
You're probably a bit puzzled by this, given that we've spent all this time telling you how much more accurate an atomic clock is due to its use of the oscillation of cesium. But the part of the clock that actually keeps time is a standard quartz crystal oscillator, which subjects a piece of the crystal to electrical current to make it vibrate. The difference is that in most ordinary quartz clocks, the oscillator is tuned accurately when the clock is built, but its frequency is never checked or adjusted after that, which means that over time, slight variations develop that make the clock a little fast or a little slow. In an atomic clock, however, the oscillation of the cesium is used to check the frequency of the quartz device, which is what gives the clock such amazing accuracy [source: Sciencemuseum.org.uk].