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

Official start of the emission of time signals via the transmitter DCF77 of the Deutsche Bundespost near Frankfurt (time measurement with highly precise quartz clocks); second defined on an astronomical basis.
Late 60ies (cf. below): transition to the atomic time scale. 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.

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

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.

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) starts ticking at the PTB. It is by far the best clock in Germany, with a rate uncertainty of one second in thirty million years, or three millions of a second over a human lifetime.

The atomic clock CS4 is put out of service and taken to the State Museum of Braunschweig (Braunschweigisches Landesmuseum) at Burgplatz.

The atomic clock CS3 is taken out of operation.

PTB's second caesium fountain clock CSF2 is put into service. CSF2 is a further development of CSF1 and along with CSF1 is one of the most accurate clocks in the world.

The atomic clocks CS1 and CS2 are still in operation and along with the fountain clocks CSF1 and CSF2 make important contributions to International Atomic Time.