  Coherent-state-based phase estimation is a fruitful testbed for the field of
precision measurements since coherent states are robust to decoherence when
compared with exotic quantum states. The seminal work done by Caves
(https://doi.org/10.1103/PhysRevD.23.1693 , Phys. Rev. D 23, 1693 (1981))
stated that the phase sensitivity of a U(2) interferometer fed with a coherent
state is limited by the shot-noise limit (SNL). In this Letter, we demonstrate
that this bound is not conclusive sensitivity limit and can be broken when the
measurement includes an external phase reference. The SNL can be surpassed by a
factor of $\sqrt{2}$ and the validity is supported through the calculation of
quantum Fisher information. Additionally, we discuss other single-mode Gaussian
inputs of which sensitivities are beyond the SNL. Our work shows potential
applications for many metological scenarios, particularly when the measured
samples immersed in great lossy environments or can withstand bright
illumination.
