Measuring lifetime of correspondence with classical decay of correlation   in quantum chaos
  A very weakly coupled linear oscillator is proposed as a detector for observing time-irreversible characteristics of a quantum system, and it is used to measure the lifetime during which a classically chaotic quantum system shows decay of correlation. Except for a particular case where the lifetime agrees with the conventional Heisenberg time, which is proportional to the Hilbert space dimension $N$, it is in general much longer: the lifetime increases in proportion to the product of $N$ and the number of superposed eigenstates, and is proportional to $N^2$ in the case of full superposition. 
Fumihiro Matsui, Hiroaki S. Yamada, Kensuke S. Ikeda
cond-mat.stat-mech