The Engineering Book: From the Catapult to the Curiosity Rover: 250 Milestones in the History of Engineering

1949

Atomic Clock

Louis Essen (1908–1997)

What is time? Scientists are not really sure. But we do know how to measure time, and engineers have been doing so with increasing precision for several centuries. We measure time with clocks.

If you think about a clock, it has two main parts: an oscillator and a counter. The oscillator is something that happens at a known frequency. So a pendulum in a pendulum clock swings back and forth once every second, say. That is a simple oscillator. A simple counter uses gears to move the hands on the face of a clock.

If you want a more accurate clock, you make the oscillator more and more precise. A quartz crystal oscillates more precisely than a pendulum, and much faster. You will never measure 1/1000^th of a second with a pendulum, but you can with a quartz crystal.

Even faster and more precise are the oscillations of a known atom. The official definition of a second is 9,192,631,770 oscillations of a Cesium-133 atom, established in 1967. These atomic oscillations are the basis of humankind’s most accurate clocks. They have an obvious name: atomic clocks. The first was built by the US National Board of Standards in 1949. English physicist Louis Essen developed the first accurate atomic clock later, in 1955.

How does an engineer make an atom oscillate precisely? And then count those oscillations? Many ways have been invented. One way is to excite a cloud or a stream of atoms with microwave energy. By determining the exact frequency of the microwaves that excite the most atoms to the right level, the engineers create a super-accurate oscillator.

The first atomic clocks were huge. But they have been shrinking and getting more reliable and precise. The newest, smallest commercial atomic clocks are about the size of a matchbook. They are called “chip scale atomic clocks.”

At the time of writing, the most accurate atomic clock uses ytterbium atoms excited by a laser. Clocks like these might lose a second over the course of billions of years. Compared with pendulum clocks, which can gain or lose seconds per day, the accuracy is nearly perfect.

The first atomic clock, built 1949 at the US National Bureau of Standards (now National Institute of Standards and Technology) by Harold Lyons and associates.