  Continuum of spin excitations observed in inelastic neutron scattering
experiments are often considered as a strong evidence of quantum spin liquid
formation. When quantum spin liquid is indeed the ground state of a
disorder-free magnetic compound, the elementary excitation is no longer the
conventional spin waves (magnons). Instead, the magnons fractionalize into
spinons, leaving only a two-spinon continuum detectable in inelastic neutron
scattering experiments. For a clean ordered antiferromagnet, it was unclear if
we can observe a continuous spectrum similar to the ones in a quantum spin
liquid state. Here we show that the magnetically ordered state in
Na$_2$BaCo(PO$_4$)$_2$ is able to host a spin excitation continuum induced by
strong quantum fluctuations. Thus, a second thought is necessary when
concluding such continuum as signature of quantum spin liquid in new material
explorations.
