  We present a new proof of the quantum Cramer-Rao bound for precision
parameter estimation [1-3] and extend it to a more general class of measurement
procedures. We analyze a generalized framework for parameter estimation that
covers most experimentally accessible situations, where multiple rounds of
measurements, auxiliary systems or external control of the evolution are
available. The proof presented demonstrates the equivalence of these more
general metrology procedures to the simplest optimal strategy for which the
bound is proven: a single measurement of a two-level system interacting with a
time-independent Hamiltonian.
