  The antiferromagnet is a closed Hermitian system, we find that its
excitations, even in the absence of dissipation, can be viewed as a
non-Hermitian system with dissipative coupling. Consequently, the
antiferromagnetic resonance spectrum does not show the typical level repulsion,
but shows the level attraction -- a characteristic behavior often observed in
non-Hermitian systems. Such behavior is because the antiferromagnetic ground
state is $\mathcal{PT}$-symmetric. This new understanding on antiferromagnetic
resonance also explains the mysterious enhancement of antiferromagnetic damping
rate. Being effectively non-Hermitian, antiferromagnetic magnons can be used
for quantum entanglement generation without introducing a third party like
external pumping.
