Optical Atomic Clocks for Redefining SI Units of Time and Frequency
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ORIGINAL PAPER
Optical Atomic Clocks for Redefining SI Units of Time and Frequency L. Sharma1,2, H. Rathore1,2, S. Utreja1,2, Neelam1,2, A. Roy1,2,3, S. De4 and S. Panja1,2* 1
CSIR-National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012, India 2
Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
3
Max Planck Institute for the Science of Light, Staudtstr. 2, 91058 Erlangen, Germany
4
Inter-University Centre for Astronomy and Astrophysics (IUCAA), Post Bag 4, Ganeshkhind, Pune 411007, India Received: 26 August 2020 / Accepted: 15 September 2020 Ó Metrology Society of India 2020
Abstract: 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. Keywords: Atomic clocks; Optical clocks; Precision measurements; Frequency standards; Ion trap; Magneto-optical trap; Laser cooling; Systematic shifts; Frequency comb 1. Introduction In 1955, Louis Essen and J. V. L. Parry realized the first atomic standard of frequency and time interval, which was based on the microwave ground-state hyperfine transition of 133Cesium (133Cs) atoms [1]. In 1967, the Comite International des Poids et Mesures (CIPM) adopted the Cs standard as the SI unit of time and frequency. The SI unit of time interval, i.e. second, was defined via an unperturbed hyperfine transition of atomic 133Cs interrogated with microwave at frequency of 9,192,631,770 Hz [2]. Technological development over more than five decades, in particular development of laser cooling techniques [3], has helped to reduce the measurement uncertainty to which the centre frequency of the hyperfine transition of ground-state 133 Cs atom can be measured to * 10–16 [4–6]. The SI definition of time and frequency got amendment in 1997 where the transition frequency of the cold Cs atoms at
*Corresponding author, E-mail: [email protected]
nearly 0 K, realized through a Cs atomic fountain, has been considered as the primary standard. However, clocks with even better accuracy remain as an essential requirement for many of the applied research studies as well as to address many advanced physics problems related to communications, surveillanc
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