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.

如今，大多数测量标准都基于基本常数，其中，时间和频率的SI单位可以最高精度地实现。时间间隔的SI单位（即秒）是在基态¹³³的超精细跃迁的基础上实现的微波区域中的铯原子。以光频率工作的原子钟具有提供比微波原子钟更好的准确性和更高的稳定性的潜力，并且期望将基于光跃迁重新定义SI秒。在本文中，我们通过询问光学晶格中的中性原子或射频离子阱中的单个离子，着眼于在光谱的光学域中运行的不同原子频率标准。还介绍了最近的全球发展以及CSIR国家物理实验室（CSIR-NPL）朝着建立光学原子钟或光学频率标准的活动。