The main tasks of the department are the realization and dissemination of the base unit of time (second) and the dissemination of legal time in the Federal Republic of Germany.
The second is defined as 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 133Cs atoms. For realizing and disseminating the unit of time, caesium atomic clocks are developed and operated as primary standards of time and frequency. In the past decades, these have significantly contributed to the international atomic time scale TAI.
Currently PTB is operating four primary Cs clocks that realize the unit of time: two thermal beam clocks (CS1 and CS2) and two clocks with laser cooled atoms (the caesium fountain clocks CSF1 and CSF2) whose uncertainty is approximately ten times smaller than that of the thermal beam clocks ( the relative frequency uncertainty is smaller than 1×10-15). These clocks are the basis of legal time in Germany. Legal time is disseminated to the population and users in industry, commerce, and research via the low-frequency transmitter DCF77, the internet, the telephone network, and via satellite links.
Future atomic clocks will most likely be based on transitions in laser-cooled stored ions in the optical frequency range. Such optical standards are being developed in the department along with the means to precisely relate optical frequencies to radiofrequency or one-pulse-per-second signals.
One expects that transitions in atomic nuclei are even better shielded from environmental perturbations than transitions in the electron shell which have been used so far as atomic clock references. The department conducts basic research aiming at the use of a transition in the ultraviolet range in the 229Th nucleus for a future generation of atomic clocks.
The work of the department is complemented by research into precision time transfer techniques and tests of fundamental physical theories through highly accurate frequency measurements. These activities are being funded in the frame of several international joint research projects and by the Deutsche Forschungsgemeinschaft (DFG), here in particular jointly with Leibniz Universität Hannover in the frame of the Cluster of Excellence QUEST.
The department participates in the test phase of the European satellite navigation system Galileo and in the preparation of the ESA mission ACES (Atomic Clock Ensemble in Space).
