Optical atomic clocks, like optical lattice clocks or single-ion clocks, have outperformed the best caesium atomic clocks that realize the base unit of time in the International System of Units (SI) with respect to accuracy and stability. With fractional uncertainties in the 10-18 regime and fractional instabilities of a few 10-16 in one second, optical atomic clocks are presently the most accurate measuring devices of all.
The status of optical clocks and the different means to compare remote clocks will be reviewed. Recent experiments to address the question of the constancy of fundamental constants or their application in the novel field of relativistic geodesy will be discussed. All these developments will eventually ask for a new definition of the unit of time based on optical frequency standards, thereby replacing the caesium standard in the SI. In the second part of this talk, I will address the question: "what will be the necessary pre-requisites and challenges for such a redefinition." The current status of the discussion will be reported and an attempt for a tentative roadmap towards such a redefinition of the second in the SI will be given.
Entanglement can be witnessed by the Fisher information. This proves the deep relation between entanglement and the distinguishability of quantum states and it provides the basic framework to develop quantum technologies based on metrology and interferometric phase estimations. Further applications on the characterization of quantum phase transitions and in foundational problems like Bell nonlocality and the quantum Zeno paradox will be discussed.