State-dependent phonon-limited spin relaxation of nitrogen-vacancy centers

Understanding the limits to the spin-coherence of the nitrogen-vacancy (NV) center in diamond is vital to realizing the full potential of this quantum system. We show that relaxation on the |m_s=-1rangle leftrightarrow |m_s=+1rangle transition occurs approximately twice as fast as relaxation on the |m_s=0rangle leftrightarrow |m_s=pm 1rangle transitions under ambient conditions in native NVs in high-purity bulk diamond. The rates we observe are independent of NV concentration over four orders of magnitude, indicating they are limited by spin-phonon interactions. We find that the maximum theoretically achievable coherence time for an NV at 295 K is limited to 6.8(2) ms. Finally, we present a theoretical analysis of our results that suggests Orbach-like relaxation from quasilocalized phonons or contributions due to higher-order terms in the spin-phonon Hamiltonian are the dominant mechanism behind |m_s=-1rangle leftrightarrow |m_s=+1rangle relaxation, motivating future measurements of the temperature dependence of this relaxation rate.