Abstract: An atomic beam tube for frequency standard which employs either counter propagating optical beams or counter propagating atomic beams and Coherent Population Trapping (CPT) is disclosed. Atoms selected from the group consisting of the alkali metal family (Cesium, Rubidium, Potassium, Sodium and Lithium) are emitted from one or two sources to form a single or double atomic beams. The atoms interact with the optical beams at two crossing points. The optical beams are generated by a laser and are modulated at half the hyperfine frequency. The optical beam is splitted into two counter propagating beams in round paths which interact with the atomic beam at two interaction regions. The interaction with the light causes the atoms to enter a CPT state. A dark line in the fluorescence at the second crossing is used to lock an RF oscillator to the atomic hyperfine transition.