  Modular values are quantities that described by pre- and postselected states
of quantum systems like weak values but are different from them: The associated
interaction is not necessary to be weak. We discuss an optimal
modular-value-based measurement with a spin coherent pointer: A quantum system
is exposed to a field in which strength is to be estimated through its modular
value. We consider two cases, with a two-dimensional and a higher-dimensional
pointer, and evaluate the quantum Fisher information. The modular-value-based
measurement has no merit in the former case, while its sensitivity can be
enhanced in the latter case. We also consider the pointer under a phase-flip
error. Our study should motivate researchers to apply the modular-value-based
measurements for quantum metrology.

Quantum Physics