  Exceptional points (EPs) are exotic degeneracies of non-Hermitian systems,
where the eigenvalues and the corresponding eigenvectors simultaneously
coalesce in parameter space, and these degeneracies are sensitive to tiny
perturbations on the system. Here we report an experimental observation of the
EP in a hybrid quantum system consisting of dense nitrogen (P1) centers in
diamond coupled to a coplanar-waveguide resonator. These P1 centers can be
divided into three subensembles of spins, and cross relaxation occurs among
them. As a new method to demonstrate this EP, we pump a given spin subensemble
with a drive field to tune the magnon-photon coupling in a wide range. We
observe the EP in the middle spin subensemble coupled to the resonator mode,
irrespective of which spin subensemble is actually driven. This robustness of
the EP against pumping reveals the key role of the cross relaxation in P1
centers. It offers a novel way to convincingly prove the existence of the
cross-relaxation effect via the EP.
