Sensitivity to small perturbations in systems of large quantum spins

We investigate the sensitivity of nonintegrable large-spin quantum lattices to small perturbations with a particular focus on the time reversal experiments known in statistical physics as Loschmidt echoes and in nuclear magnetic resonance (NMR) as magic echoes. Our numerical simulations of quantum spin-71/2 clusters indicate that there is a regime where Loschmidt echoes exhibit nearly exponential sensitivity to small perturbations with characteristic onstant approximately equal to twice the value of the largest Lyapunov exponent of the corresponding classical spin clusters. The above theoretical results are verifiable by NMR experiments on solids containing large-spin nuclei.