The excited state structure of the nitrogen-vacancy center in diamond
  Optical and microwave double resonance techniques are used to obtain the excited state structure of single nitrogen-vacancy centers in diamond. The excited state is an orbital doublet and it is shown that it can be split and associated transition strengths varied by external electric fields and by strain. A group theoretical model is developed. It gives a good account of the observations and contributes to an improved understanding of the electronic structure of the center. The findings are important for quantum information processing and other applications of the center. 
Ph. Tamarat, N. B. Manson, R. L. McMurtie, N. Nitsovtsev, C. Santori,   P. Neumann, T. Gaebel, F. Jelezko, P. Hemmer, and J....