Entanglement of truncated quantum states

We investigate the impact of Hilbert-space truncation upon the entanglement of an initially maximally entangled m x m bipartite quantum state, after propagation under an entanglement-preserving n x n (n >= m) unitary. Truncation- physically enforced, e.g., by a detector's finite cross section-projects the state onto an s x s-dimensional subspace (3 <= s <= n). For a random local unitary evolution, we obtain a simple analytical formula that expresses the truncation-induced entanglement loss as a function of n, m and s.