  We theoretically derive the lower and upper bounds of quantum Fisher
information (QFI) of an SU(1,1) interferometer whatever the input state chosen.
According to the QFI, the crucial resource for quantum enhancement is shown to
be large intramode correlations indicated by the Mandel $Q$-parameter. For a
photon-subtracted squeezed vacuum state with high super-Poissonian statistics
in one input port and a coherent state in the other input port, the quantum
Cram\'{e}r-Rao bound of the SU(1,1) interferometer can beat
$1/\langle\hat{N}\rangle$ scaling in presence of large fluctuations in the
number of photons, with a given fixed input mean number of photons. The
definition of the Heisenberg limit (HL) should take into account the amount of
fluctuations. The HL considering the number fluctuation effect may be the
ultimate phase limit.
