Time-invariant negativity and persistent bound entanglement in a cyclic two-qutrit family

Abstract

We study the entanglement dynamics of two qutrits under collective dephasing for a family of states obtained by mixing a maximally entangled state that lives in the decoherence-free subspace with three nonstationary two-term states sharing a common amplitude ratio $r$. We obtain exact PPT and separability regions along the whole trajectory, a specialization of known results on cyclic sign invariant states, and we supply explicit certificates: a finite product decomposition, three fixed positive-map witnesses, and a closed realignment formula that detects every PPT-entangled state, so that no state is left undecided. The negativity is computed in closed form. It shows that the entanglement is time-invariant, positive and constant from the initial time although the state changes, exactly when the weight of the maximally entangled state exceeds the threshold $U_r=r/(1+r+r^2)$. To our knowledge, this is the first demonstration of time-invariant entanglement for qutrit--qutrit systems under collective dephasing. For smaller weights the entanglement decays and vanishes at a finite time: the state either becomes separable (entanglement sudden death) or, for $r\neq1$ in an intermediate range, loses distillability suddenly and remains bound entangled at all later times. Every state with negative partial transpose is one-copy distillable by a fixed local projection. The same trajectories arise from the global dephasing map and from a correlated local random-unitary semigroup, although the two channels differ on product inputs. Exact rational benchmarks and embeddings of the Horodecki and Ha--Kye--Park families are provided.