  We study the two-qubit asymmetric quantum Rabi model (AQRM) and find another
hidden symmetry related to its dark-state solution. Such a solution has at most
one photon and constant eigenenergy in the whole coupling regime, causing level
crossings in the spectrum, although there is no explicit conserved quantity
except energy, indicating another hidden symmetry. We find a symmetric operator
in the eigenenergy basis to label the degeneracy with its eigenvalues, and
compare it with the well-known hidden symmetry which exists when bias parameter
$\epsilon$ is a multiple of half of the resonator frequency. Extended to the
multimode case, we find not only hidden symmetries mentioned above, but also
symmetries related with conserved bosonic number operators. This provides a new
perspective for hidden symmetry studies on generalized Rabi models.

Quantum Physics