An apparatus for an atomic clock includes first and second distinctive substrates, each having at least a planar surface substantially parallel therebetween. The apparatus also includes a medium having particles capable of undergoing energetic transition between at least two energy levels, said medium being located in the space defined between the planar surfaces. It further includes a magnetic device arranged to the first substrate and generating at least in the volume of the medium a predetermined static magnetic field B the direction of which is substantially parallel or perpendicular to the planar surfaces and an excitation device arranged to the second substrate and generating an excitation magnetic field H at, at least an excitation frequency, the direction of said excitation magnetic field H in the volume of the medium being substantially orthogonal to said direction of the static magnetic field B.