Optical Atomic Clocks for Redefining SI Units of Time and Frequency

Nowadays, most of the standards of measurement are based on fundamental constants, and among all, the SI units of time and frequency are realized with the highest precision. The SI unit of time interval, i.e. second, is realized on the basis of a hyperfine transition of ground state of 133Cesium atom in the microwave region. Atomic clocks operating at the optical frequencies have potential of providing better accuracy and higher stability than the microwave atomic clocks, and it is expected that SI second will be redefined on the basis of an optical transition. In this article, we focus on different atomic frequency standards operating in the optical domain of the spectrum by interrogating neutral atoms in optical lattice or a single ion within a radiofrequency ion trap. Recent worldwide developments along with activities at CSIR-National Physical Laboratory (CSIR-NPL) towards building optical atomic clock or optical frequency standard have also been presented.