  We study the quantum dynamics of a non-interacting spin ensemble under the
effect of a reservoir by applying the framework of the non-Hermitian
Hamiltonian operators. Theoretically, the two-level model describes the quantum
spin system and the Bloch vector to establish the dynamical evolution.
Experimentally, phosphorous ($^{31}$P) nuclei with spin $I=1/2$ are used to
represent the two-level system and the magnetization evolution is measured and
used to compare with the theoretical prediction. At room temperature, the
composite dynamics of the radio-frequency pulse plus field inhomogeneities (or
unknown longitudinal fluctuations) along the $z$-axis transform the initial
quantum state and drives it into a mixed state at the end of the dynamics. The
experimental setup shows a higher accuracy when compared with the theoretical
prediction (>98\%), ensuring the relevance and effectiveness of the
non-Hermitian theory at a high-temperature regime.
