  We demonstrate symmetry protected bundle emission in the cavity QED system
under the ultrastrong coupling regime. Parity symmetry of Rabi model only
permits the super-Rabi oscillations with periodic generation of even photons in
cavity, which is realized by the laser driven flip of qubit and the parity
conserved transitions induced by Rabi interaction. Combined with dissipation,
only 2$n$-photon correlated emissions are allowed, and the corresponding
purities are significantly enhanced by the parity symmetry, due to the almost
perfect suppression of bundle emissions with odd correlated photons. The
switching of bundle emissions between even and odd correlated photons can be
controlled by manipulating the parity symmetry of system with an added magnetic
field. This work extends multi-photon bundle emission to the ultrastrong
coupling regime, and offers the prospect of exploring symmetry-protected
multi-quanta physics.
