  This Letter verifies the potential of several classes of entangled coherent
state in well known quantum metrology which includes detection of classical
external force, and shows that there is a class of entangled coherent state for
the external force detection system without the quantum limit in the detection
of the light. In the case of the precision measurement of continuos parameter
like phase measurement, the entangled coherent state with perfect entanglement
does not provide remarkable benefit, but we provide a concrete example that
certain class of entangled coherent state gives a remarkable sensitive
detection scheme in the discrimination of digital signal affected by external
force.
