Correlated and critical phenomena in multipartite quantum non-Markovianity

We study non-Markovianity in the exact dynamics of two two-level atoms in a resonant cavity. We find a critical behavior in the form of a Markovian to non-Markovian transition at a finite interatomic distance and a discontinuity in the limit of infinitely close atoms. Actually, in this highly correlated regime, we find that non-Markovianity always arises if the system size exceeds a critical threshold. To this end, a nonsingular, additive non-Markovian measure with appropriate asymptotic behavior is formulated. These results show how inter-subsystem distance and system size can act as precise controls for modulating memory effects in multipartite open quantum systems.