  Quantum metrology pursues high-precision measurements of physical quantities
by using quantum resources. However, the decoherence generally hinders its
performance. Previous work found that the metrological error tends to diverge
in the long-encoding-time regime due to the Born-Markovian approximate
decoherence, which is called the no-go theorem of noisy quantum metrology. Here
we propose a Gaussian quantum metrology scheme using bimodal quantized optical
fields as the quantum probe. It achieves the precision of a sub-Heisenberg
limit in the ideal case. However, the Markovian decoherence causes the
metrological error contributed by the center-of-mass mode of the probe to be
divergent. A mechanism to remove this ostensible no-go theorem is found in the
non-Markovian dynamics. Our result gives an efficient way to realize
high-precision quantum metrology in practical continuous-variable systems.
