  Fluorescent defects in non-cytotoxic diamond nanoparticles are candidates for
qubits in quantum computing, optical labels in biomedical imaging and sensors
in magnetometry. For each application these defects need to be optically and
thermodynamically stable, and included in individual particles at suitable
concentrations (singly or in large numbers). In this letter, we combine
simulations, theory and experiment to provide the first comprehensive and
generic prediction of the size, temperature and nitrogen-concentration
dependent stability of optically active NV defects in nanodiamonds.
