  The widely used quantum Cramer-Rao bound (QCRB) sets a lower bound for the
mean square error of unbiased estimators in quantum parameter estimation,
however, in general QCRB is only tight in the asymptotical limit. With a
limited number of measurements biased estimators can have a far better
performance for which QCRB cannot calibrate. Here we introduce a valid lower
bound for all estimators, either biased or unbiased, which can serve as
standard of merit for all quantum parameter estimations.
