  Optical random scattering is generally considered to be a nuisance of
microscopy that limits imaging depth and spatial resolution. Wavefront shaping
techniques have recently enabled optical imaging at unprecedented depth, but a
remaining problem is also to attain super-resolution within complex media. To
address this challenge, we introduce a new technique to focus inside of complex
media by the use of a quantum reference beacon (QRB), consisting of solid-state
quantum emitters with spin-dependent fluorescence. This QRB provides
subwavelength guidestar feedback for wavefront shaping to achieve an optical
focus below the microscope's diffraction limit. We implement the QRB-guided
imaging approach using nitrogen-vacancy centers in diamond nanocrystals, which
enable optical focusing with a subdiffraction resolution below 186 nm ($\approx
\lambda/3.5\mbox{NA}$), where the microscope's NA=0.8. This QRB-assisted
wavefront shaping paves the way for a range of new applications, including
deep-tissue quantum enhanced sensing and individual optical excitation of
magnetically-coupled spin ensembles for applications in quantum information
processing.
