Longitudinal spin relaxation in nitrogen-vacancy ensembles in diamondMariusz Mrózek, Daniel Rudnicki, Pauli Kehayias, Andrey Jarmola, Dmitry Budker and Wojciech GawlikEPJ Quantum Technol., 2 1 (2015) 22DOI: https://doi.org/10.1140/epjqt/s40507-015-0035-z