  KCuCl$_3$ is known to show a quantum phase transition from the disordered to
antiferromagnetically ordered phases by applying pressure. There is a
longitudinal excitation mode (Higgs amplitude mode) in the vicinity of the
quantum critical point in the ordered phase. To detect the Higgs amplitude
mode, high-pressure ESR measurements are performed in KCuCl$_3$. The
experimental data are analyzed by the extended spin-wave theory on the basis of
the vector spin chirality. We report the first ESR detection of the Higgs
amplitude mode and the important role of the electric dipole described by the
vector spin chirality.
