Logo of the Physikalisch-Technische Bundesanstalt
Panoramic view of the clock hall at PTB with the four caesium clocks CS1, CS2, CSF1 and CSF2.

Since when has the first atomic clock been working at PTB?

Brief chronology of the atomic clock era at PTB


Jan. 1, 1959

Official start of the emission of standard frequency and time signals via the transmitter DCF77 of the Deutsche Bundespost near Frankfurt.
Signal generation based on highly precise quartz clocks, steered according to astronomical observations.
Late 60ies quartz clocks steered towards atomic clocks operated in Braunschweig.
On June 5, 1973, introduction of a time code for the signals emitted (still valid today).


Application for the construction of an "atomic clock house" at PTB.


Start of construction work.

March 1967

Move of the PTB "Laboratory for the Unit of Time" to the new Atomic Clock House.

October 1967

International redefinition of the second by the 13th General Conference for Weights and Measures.
Definition of the atomic second: "The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom."


The atomic clock CS1 (caesium-one) becomes operational.


With the Time Act of July 25, 1978, PTB becomes responsible for legal time.


First introduction of Daylight Saving Time after World War II.


The atomic clock CS2 becomes operational. From 1991 onward, it furnishes the seconds of legal time.


Two more atomic clocks (CS3 and CS4) become operational.


With CSF1, an atomic clock of the most recent generation (caesium fountain clock) starts ticking at the PTB. It is immediately by far the best clock in Germany, initially with a rate uncertainty of one second in thirty million years

June 2005

The CS4 atomic clock (the "twin sister" quasi of the CS3, which is still "ticking") is being sorted out and is moving on to the Braunschweigisches Landesmuseum at the Burgplatz. There it will be placed in the midst of an exhibition on the subject of "time" (open daily, except Mondays, admission free.)


All these atomic clocks except CS4 are still operating. They contribute substantially to the making of International Atomic Time.

The four primary caesium clocks of the PTB