  We propose the simulation of quantum-optical systems in the
ultrastrong-coupling regime using a variational quantum algorithm. More
precisely, we introduce a short-depth variational form to prepare the
groundstate of the multimode Dicke model on a quantum processor and present
proof-of-principle results obtained via cloud access to an IBM device. We
moreover provide an algorithm for characterizing the groundstate by Wigner
state tomography. Our work is a first step towards digital quantum simulation
of quantum-optical systems with potential applications to the spin-boson, Kondo
and Jahn-Teller models.
