Magnetic-field measurements based on quantum superpositions

The authors report on their investigations on nonlinear Faraday effect (NFE), i.e. light-intensity dependent rotation of the polarization plane of linearly polarized light upon its propagation through a medium placed in a magnetic field. The phenomenon is associated with quantum superpositions of atomic or molecular levels of particles constituting the medium. In this context it is based on the same grounds as electromagnetically induced transparency, coherent population trapping, storage and retrieval of light pulse from a medium. The paper also discusses the mechanisms of generation and detection of the superpositions with light and means of controllable tailoring of a quantum state in long-living (lifetimes >10 ms) atomic state.