Ab-initio study of nickel defects in diamond acting as qubits

Nickel is a typical contaminant during HPHT and CVD diamond synthesis, that paved the way to observe various nickel related optical signals in diamond during the past decades. However, various assignments of optical signals to defect structures are only tentative assumptions. For example, the 1.4-eV optical center was originally attributed to positively charged interstitial Ni, but our very recent results indicate that it should be associated with the negatively charged nickel-vacancy defect which was falsely associated with the 1.72-eV (NE4) center. Our study resolves a few decades controversy by means of ab-initio DFT simulations to predict optical fine structures and vibronic sidebands, zero field splitting tensors and EPR g-tensors. Furthermore, qubit protocols (Λ-scheme) for the 1.4-eV center are proposed towards quantum technology applications.