  The unusual quantum-disordered magnetic ground state intertwined with
superconductivity and electronic nematicity in FeSe has been a research focus
in iron-based superconductors. However, the intrinsic spin excitations across
the entire Brillouin zone in detwinned FeSe, which forms the basis for a
microscopic understanding of the magnetic state and superconductivity, remain
to be determined. Here, we use inelastic neutron scattering to map out the spin
excitations of FeSe dewtinned with a uniaxial-strain device. We find that the
stripe spin excitations (Q=(1, 0)/(0, 1)) exhibit the $C_2$ symmetry up to
$E\approx120$ meV, while the N{\'e}el spin excitations (Q=(1, 1)) retain their
$C_4$ symmetry in the nematic state. The temperature dependence of the
difference in the spin excitations at Q=(1, 0) and (0, 1) for temperatures
above the structural phase transition unambiguously shows the establishment of
the nematic quantum disordered state. The similarity of the N\'eel excitations
in FeSe and NaFeAs suggests that the N\'eel excitations are driven by the
enhanced electron correlations in the $3d_{xy}$ orbital. By determining the key
features of the stripe excitations and fitting their dispersions using a
Heisenberg Hamiltonian with biquadratic interaction ($J_1$-$K$-$J_2$), we
establish a spin-interaction phase diagram and conclude that FeSe is close to a
crossover region between the antiferroquadrupolar, N\'eel, and stripe ordering
regimes. The results provide an experimental basis for establishing a
microscopic theoretical model to describe the origin and intertwining of the
emergent orders in iron-based superconductors.
