Multiple decoherence-free states in multi-spin systems
  A numerical procedure is presented for mapping the vicinity of the null-space of the spin relaxation superoperator. The states populating this space, i.e. those with near-zero eigenvalues, of which the two-spin singlet is a well-studied example, are long-lived compared to the conventional T1 and T2 spin-relaxation times. The analysis of larger spin systems described herein reveals the presence of a significant number of other slowly relaxing states. A study of coupling topologies for n-spin systems (n = 4 - 8) suggests the symmetry requirements for maximising the number of long-lived states. 
H. J. Hogben, P. J. Hore and I. Kuprov
physics.chem-ph