  One of the main quests in quantum metrology, and quantum parameter estimation
in general, is to find out the highest achievable precision with given
resources and design schemes that attain that precision. In this article we
present a general framework for quantum parameter estimation which relates the
ultimate precision limit directly to the underlying dynamics. With this
framework we present systematical methods for computing the ultimate precision
limit and optimal probe states. We further demonstrate the power of the
framework by deriving a sufficient condition on when ancillary systems are not
useful for improving the precision limit.
