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The second is the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.
The 13th Conférence Générale des Poids et Mesures (CGPM) in 1967, Resolution 1
The second is defined by the frequency of the microwave, absorbed or emitted by cesium atoms. It is actually very difficult to obtain the microwave, whose frequency strictly corresponds to the defined one.
Members in the Atomic Frequency Standards Project are conducting research to develop atomic clocks with ultra-high accuracy, aiming for the establishment of the standard of time and frequency with ultra-low uncertainty.
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