Longitudinal spin relaxation in nitrogen-vacancy ensembles in diamond Mariusz Mrózek, Daniel Rudnicki, Pauli Kehayias, Andrey Jarmola, Dmitry Budker and Wojciech Gawlik EPJ Quantum Technol., 2 1 (2015) 22 Published online: 29 October 2015 DOI: 10.1140/epjqt/s40507-015-0035-z