Physikalisch-Technische Bundesanstalt

Higher accuracy and higher stability are the aims of the development of novel frequency standards. Compared with the conventional caesium clock scheme, these objectives can be achieved by

• reducing systematic shifts of the observed atomic frequency with respect to the value one would expect in the ideal case of undisturbed atoms,
  
  
• raising the ratio of transition frequency to resonance linewidth, the so-called line quality factor.
  

Both objectives can be reached more easily if the kinetic energy of the atoms used as the reference is reduced. For this purpose, the technique of laser cooling - influencing the motion of the atoms by the interaction with laser radiation - is used.

One expects that atomic frequency standards with reference transitions in the optical range (10 ... 1000 THz) can reach accuracies which are several orders of magnitude higher than those of the present microwave standards.