M. Merkli, G.P. Berman, I.M. Sigal
Dynamics of Collective Decoherence and Thermalization
(403K, pdf)

ABSTRACT.  We analyze the dynamics of $N$ interacting spins (quantum 
register) collectively coupled to a thermal environment. Each spin 
experiences the same environment interaction, consisting of an 
energy conserving and an energy exchange part. 
We find the decay rates of the reduced density matrix elements in 
the energy basis. We show that if the spins do not interact among each other, 
then the fastest decay rates of off-diagonal matrix 
elements induced by the energy conserving interaction is of order 
$N^2$, while that one induced by the energy exchange interaction 
is of the order $N$ only. Moreover, the diagonal matrix 
elements approach their limiting values at a rate independent of 
$N$. 
For a general spin system the decay rates depend 
in a rather complicated (but explicit) way on the size $N$ and the interaction between the spins. 
Our method is based on a dynamical quantum resonance theory valid 
for small, fixed values of the couplings. We do not make Markov-, 
Born- or weak coupling (van Hove) approximations.