We have investigated some possibilities to use effect of Electromagnetically Induced Transparency (EIT), which is based on quantum interference, for measuring magnetic fields. Multi-frequency laser light is driving atomic transitions (e.g. in a Λ-type excitation scheme), and the atoms are decaying into a super positional dark state, in which the population is trapped, if the frequencies of the applied light fulfills certain conditions. Then the atomic medium becomes transparent for the exciting electromagnetic fields. The observed line width of this "transparency window" is not limited by the natural line width of the involved optical transitions and can be decreased down to a few hundredths of Hertz. As a consequence, this narrow feature can be used as a sensitive probe if the frequencies fulfill the required conditions, which are dependent on the energy positions of the involved atomic levels. These energy positions are influenced by external magnetic fields, thus the observation of EIT can be used to determine external magnetic fields strengths.
In our approach a so called Hanle-configuration is used, where only one laser frequency is used to simultaneously excite σ+- and σ--transitions within a single hyperfine transition of the Rb87 Dl-line. In this configuration a resonant Λ-system is formed at zero magnetic field. The detuning from two-photon resonance is caused by a magnetic field scanned in laser propagation direction. Any additional probe magnetic field characteristically changes the width and shape of the EIT transparency window resonance; hence the probe magnetic field can be determined directly from the shape of this resonance.