The main task of the Department Time and Frequency is the realization and dissemination of the base unit time (the 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 the 133Cs atom. For realizing and disseminating the unit of time, the department develops and operates caesium atomic clocks as primary standards of time and frequency. In the past decades, these have served as the world's most accurate clocks and they contribute substantially to the international atomic time scale TAI.
To realize the unit of time, PTB operates four primary Cs clocks: two clocks based on a thermal atomic beam (CS1 and CS2) and two clocks that use laser cooled atoms (fountain clocks CSF1 and CSF2) whose uncertainty is approximately a factor of 10 lower than that of the thermal Cs clocks (relative frequency uncertainty below 1·10-15). These clocks provide the basis for German legal time. The legal time is disseminated to the public and to various users in industry, commerce and research through the low-frequency radio transmitter DCF77, the internet, a fixed-line telephone service, and satellite links.
The next generation of atomic clocks will most likely be based on transitions of atoms and ions in the optical range. Optical frequency standards based on single laser-cooled ytterbium ions are presently being developed in the Department together with the means to convert their optical frequencies without error to the radiofrequency range or 1-pulse-per-second signals. Moreover the Department coordinates the research project. "High-accuracy optical clocks with trapped ions", which is financed by the EU in the frame of EMRP.
One expects that transitions between the energy levels of the atomic nucleus are even better shielded from external perturbations than the transitions in the electron shell which have been used so far as atomic clock references. For a future generation of atomic clocks we therefore investigate a transition in the nucleus of 229Th that lies in the ultraviolet spectral range.
The work of the Department is complemented by research into precision time transfer techniques and by tests of fundamental physical theories through precise frequency measurements. This research is funded in the frame of several international projects and nationally through the Deutsche Forschungsgemeinschaft (DFG).