Extremal spacings between eigenphases of random unitary matrices and their tensor products

Extremal spacings between eigenphases of random unitary matrices of size N pertaining to circular ensembles are investigated. Explicit probability distributions for the minimal spacing for various ensembles are derived for N=4. We study ensembles of tensor product of k random unitary matrices of size n which describe independent evolution of a composite quantum system consisting of k subsystems. In the asymptotic case, as the total dimension N=n^{k} becomes large, the nearest neighbor distribution P(s) becomes Poissonian, but statistics of extreme spacings P(s_{min}) and P(s_{max}) reveal certain deviations from the Poissonian behavior