Credit: Alan Stonebraker

Figure 1: The schematic diagram of the optical compass. A linearly polarized bichromatic optical pump is sent through the $87Rb$ atomic cell placed in a homogeneous magnetic field $B$. The transmission of the cell is measured as a function of the polarization direction $E$, set with a polarization rotation ($λ/2$) wave plate. There are two maxima corresponding to the mutually orthogonal magnetic field and the optical polarization, $E||n$ where $n=[k×B$]). The measurement determines the plane defined by the direction of the light propagation and the magnetic field vector ($k,B$). The direction of the magnetic field $±B$ (the method does not allow one to distinguish between $B$ and $-B$) is obtained by the repetition of the measurement using optical pump propagating in a different direction $k1(k1≠k$).