Atomic Clocks

Very accurate clocks can be constructed by locking an electronic oscillator to the frequency of an atomic transition. The frequencies associated with such transitions are so reproducible that the definition of the second is now tied to the frequency associated with a transition in cesium-133:

The two most widely used atomic clocks in recent years have been the cesium beam atomic clock and the rubidium clock. Such clocks have provided the accuracy necessary to test general relativity and to track variations in the frequencies of pulsars. Atomic clocks are integral parts of the Global Positioning System since extreme accuracy in timing is necessary for the triangulation involved.

Cesium Atomic Clock

The current time standard for the United States is a cesium atomic frequency standard at the National Institute of Standards and Technology in Boulder, Colorado. In 1967 a standard second was adopted based on the frequency of a transition in the Cs-133 atom:

The frequency of this atomic clock is in the microwave region of the electromagnetic spectrum and is a convenient one for locking a microwave oscillator. Cesium clocks have demonstrated stability to 1 part in 10^13, or one second in 300,000 years.

Set your watch by it: (303) 499-7111 . Time signals based on it are available by short wave radio (WWV and WWVH).

Rubidium Atomic Clock

The two most commonly used atomic clocks in recent years have been the cesium clock and the rubidium clock. Both involve the locking of an electronic oscillator to the atomic transition. The rubidium clock has had the advantage of portability, achieving an accuracy of about 1 in 10^12 in a transportable instrument. This has made it useful for carrying from one cesium clock to another to synchronize the clocks.