What is atomic clock?
An atomic clock is a time keeping device that uses the frequency of an atomic transition as the standard of time. This frequency is measured by measuring the probability of state transition after the atoms interact with an electromagnetic field of frequency . The probability peaks when . This frequency is set as the standard of 9,192,631,770 Hz for Cs-133 clock.
The measurement of the probability is done periodically determined by the time of flight of the atoms in the setup (going across the chamber in the beam clock, the time for the atom to travel up and down the path in the atomic fountain). is set to different frequency each time.
What is the precision?
The precision of this method is equivalent to a shift of 1 second in 300 million years for cesium atomic fountain ( Hz) .
Properties of transition and sources of noise
For a transition to be suitable for being a frequency standards, there are some properties  such as
- The linewidth of the transition has to be small. (Linewidth introduces noise in the frequency standard.)
- The interaction time between the atoms and light has to be long. (The signal is average over a long time.)
- The laser has small linewidth. (Again, linewidth introduces noise.)
- The signal-to-noise ratio for the transition is high.
- The transition should not be affected by electromagnetic field.
- Velocity of atoms has to be low. (Minimize the frequency error due to Doppler effect.)
Why do we need this precision?
The importance of atomic clocks is not in daily time keeping, but in applications like GPS where precise time keeping translate into a measure distance. With large conversion factor (the speed of light), the precision of the atomic clock is crucial in getting the position on the surface of the earth correctly.
- Bauch, Andreas. “Caesium atomic clocks: function, performance and applications.” Measurement Science and Technology 14.8 (2003): 1159.
- Laura Ost “NIST Launches a New U.S. Time Standard: NIST-F2 Atomic Clock.” (2014) Retrieved from http://www.nist.gov/pml/div688/nist-f2-atomic-clock-040314.cfm