Low temperature photo-physics of single NV centers in diamond*

We present the magnetic field dependent photo-physics of individual Nitrogen-Vacancy (NV) color centers in diamond under cryogenic conditions. At distinct magnetic fields, we observe significant reductions in the NV photoluminescence rate, which indicate a marked decrease in the optical readout efficiency of the NV's ground state spin. We assign these dips to excited state level anti-crossings, which occur at magnetic fields that strongly depend on the effective, local strain environment of the NV center. Our results offer new insights into the structure of the NVs' excited states and a new tool for their effective characterization. Using this tool, we observe strong indications for strain-dependent variations of the NV's orbital g-factor and obtain new insights into NV charge state dynamics. Additional investigations show strain-dependent photo-physics for the transition from cryogenic to room temperature. This result is in excellent agreement with an extensive theoretical model which advances our understanding of orbital averaging due to NV-phonon interactions. Next to the fundamental insights our results bring into orbital averaging dynamics in the NV's excited state, they inform on suitable regimes in magnetic field and temperature, where NV centers can be efficiently applied in quantum information processing and quantum sensing.