Invention of most precise clocks and their precision

During my studies, I knew about Cesium Atomic clock which is used as World Standard to define duration of one second. Today I ready about some more clocks in the science magazine. Here is the summary. Hope this will be helpful to me in keeping my memories and may help others as well.

Cesium Clock: First suggested by Lord Kelvin in 1879.  The first atomic clock was built in 1949 at the U.S. National Bureau of Standards.

• Principal: It uses photons oscillations(absorption and emission of energy by electrons)
• Frequency: 9192631770 Hz
• Accuracy: +/- 1 Second every 100 Million Yrs
• Number of Atoms used: 10 Million

Strontium Clock: Developed in 2009 at Joint Institute for Laboratory Astrophysics (JILA).

• Principal:It uses blue lasers to excite Strontium (neutral) atoms in a vacuum chamber.
• Frequency: 45,000 times higher than Cesium Clock ( 450 Trillion Hz)
• Accuracy: +/- 1 Second every 300 Million Yrs
• Number of Atoms used: 2000

Mercury Logic Clock: Developed in 2009 at National Institute of Standards and Technology.

• Principal: It uses lasers to excite mercury atom.
• Frequency: 4,000 times higher the Cesium Clock (~ 40 Billion Hz)
• Accuracy: +/- 1 Second every 1 Billion Yrs
• Number of Atoms used: 1

Quantum Logic Clock: Developed in February 2010 at National Institute of Standards and Technology. Currently this is considered as the most precise clock in the world.

• Principal: It uses lasers to excite Aluminium atom and Beryllium to report at absolute Zero temperature.
• Frequency: 100,000 times higher the Cesium Clock (~ 1 Quadrillion Hz)
• Accuracy: +/- 1 Second every 4 Billion Yrs
• Number of Atoms used: 2

Scientists are continuously working on to redefine the precision of the measurable time. The results are not only fascinating but helpful too. They help in many great ways in learning Cosmology and understanding the Physics even better than before.