Marcus’ research is characterised by the application of first-principles understanding of optical defects in diamond to the innovation of quantum technologies. In his Ph.D. thesis, Marcus developed the first self-consistent theory of the nitrogen-vacancy (NV) colour centre in diamond, which is a model system for many quantum technologies. Marcus’ theory resolved issues that limited the performance of the NV centre at the time and is the basis for the ongoing innovation of NV technologies and the growing effort to systematically identify similar optical defects. For this significant contribution, Marcus was awarded the University of Melbourne Chancellor's Prize for Excellence in the PhD thesis and a nomination for the Bragg Gold Medal of the Australian Institute of Physics.