报告摘要：It was predicted more than a decade ago that the spectropolarimetry arising from ground state alignment (GSA) traces 3D magnetic field in diffuse medium. Nonetheless, the observationally it remained unexplored territory until recently. Here I will review briefly the theory before reporting our first detection of GSA in ISM. The comparison between different absorption line polarimetry confirms the prediction from the GSA theory. 3D magnetic tomography is revealed. In addition, magnetic strength is constrained to be around 100muG, and thus improving the accuracy by 2 orders of magnitude compared to earlier. The applicability of GSA in various environments will be exhibited.