“…The density operator is expanded in 16 basis operators: T 00 (the identity), T 10 ϭ 1/ ͌ 5I z (proportional to longitudinal magnetization), T 11 (a) ϭ 1/ ͌ 5I x and T 11 (s) ϭ Ϫi/ ͌ 5I y (proportional to the xand y-magnetization, respectively), T 20 (quadrupolar spin polarization), T 21 (s) and T 21 (a) (rank-two singlequantum coherences), T 22 (s) and T 22 (a) (rank-two double-quantum coherences), T 30 (octopolar spin polarization), T 31 (s) and T 31 (a) (rank-three single-quantum coherences), T 32 (s) and T 32 (a) (rank-three double-quantum coherences), and T 33 (s) and T 33 (a) (rank-three triple-quantum coherences).…”