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PTB's first caesium clock turns 50


As of now, the CS1 atomic clock – a major quantum standard of PTB – has been ticking for 50 years. 

CS1, PTB᾽s first caesium clock

The clock was completed in PTB᾽s atomic clock hall in 1969, and the first measurement values were published the same year. The relative uncertainty of CS1 was 4.5 · 10–13 in 1970. Today, the uncertainty of the best primary clocks is slightly higher than 1 · 10-16. Until 1992, legal time in Germany was based on CS1᾽s seconds. Then, CS1 was replaced by the CS2 atomic clock, which is more modern, more stable and more precise and which was built using the same principle. CS1, however, still belongs to PTB᾽s ensemble of four primary atomic clocks. ”Primary“ as used here denotes complete uncertainty estimation, whereas for commercial devices, merely summarized information is provided on ”accuracy“ without further explanation.