  Entanglement constitutes a main feature that distinguishes quantum from
classical physics and is a key resource of quantum technologies. Here we show,
however, that entanglement may also serve as the essential ingredient for the
emergence of classical behavior in a composite nonlinear radiating system. We
consider the radiation from a macroscopic spin emitter, such as the collective
radiation from an atomic ensemble. We introduce a new class of macroscopic spin
states, the coherently radiating spin states (CRSS), defined as the asymptotic
eigenstates of the SU(2) lowering operator. We find that a spin emitter in a
CRSS radiates classical-like coherent light, although the CRSS itself is a
quantum entangled state exhibiting spin squeezing. We further show that CRSS
are naturally produced in Dicke superradiance and underlie the dissipative
Dicke phase transition. Our CRSS theory thus provides new concepts for studying
the quantum physics of radiation, with applications in current platforms
involving collections of atoms or spins, their consideration in quantum
technologies such as metrology and lasing, and the many-body theory of spin
systems.
