Atomic clock is a clock device that uses an electron transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Only a few countries in the world can mass produce atomic clocks. The cesium atomic clock is mainly produced by Symmetricom and OSA; the cesium clock is currently the largest and most widely used atomic clock product in mass production, and the countries that can produce cesium clock in batches include the United States, Switzerland, Russia and China.
 
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