  We address the framework of analysing quantum metrology in the
information-theoretic picture. Firstly we show how to extract the maximum
amount of information in general via suitable state initialization of the
probes at the beginning and a quantum measurement at the end. Our analysis can
apply to both the single-parameter and the multi-parameter estimation
procedures as well as to any other quantum information processing procedures.
We then establish a direct connection between the information-theoretic picture
of quantum metrology and its conventional variance-covariance picture, by
showing that any estimation procedure achieves Heisenberg limit in
variance-covariance picture can also reach the information-theoretic Heisenberg
limit in the asymptotic sense. As a direct consequence, we argue that the
entangled measurement is not necessary for achieving Heisenberg limit in the
information-theoretic pictures, which is explicitly illustrated for the
Quantum-Classical parallel strategy of quantum metrology with a separable
measurement employed and the Heisenberg limit saturated in both pictures.
