Superhigh-resolution spectroscopy based on interference of states

We describe the current methods of superhigh-resolution spectroscopy based on interference of atomic states (quantum beats and level crossing) and compare them with the methods of Doppler-free laser spectroscopy. We pay major attention to the quantum-beat method, which has been developed in recent years in connection with progress in the technique of tunable lasers, and also to the phenomenon of alignment of atoms in a gasdischarge plasma. We treat examples of application of these methods for solving problems of superhigh-resolution spectroscopy.