The schematic diagram of the optical compass. A linearly polarized bichromatic optical pump is sent through the atomic cell placed in a homogeneous magnetic field . The transmission of the cell is measured as a function of the polarization direction , set with a polarization rotation () wave plate. There are two maxima corresponding to the mutually orthogonal magnetic field and the optical polarization, where ]). The measurement determines the plane defined by the direction of the light propagation and the magnetic field vector (). The direction of the magnetic field (the method does not allow one to distinguish between and ) is obtained by the repetition of the measurement using optical pump propagating in a different direction ).