  By investigating information flow between a general parity-time (PT)
-symmetric non-Hermitian system and an environment, we find that the complete
information retrieval from the environment can be achieved in the PT-unbroken
phase, whereas no information can be retrieved in the PT-broken phase. The
PT-transition point thus marks the reversible-irreversible criticality of
information flow, around which many physical quantities such as the recurrence
time and the distinguishability between quantum states exhibit power-law
behavior. Moreover, by embedding a PT-symmetric system into a larger Hilbert
space so that the entire system obeys unitary dynamics, we reveal that behind
the information retrieval lies a hidden entangled partner protected by PT
symmetry. Possible experimental situations are also discussed.
