  Multi-photon bundle states are crucial for a broad range of applications such
as quantum metrology, quantum lithography, quantum communication, and quantum
biology. Here we propose a scheme that generates multi-photon bundles via
virtual excitations in a quantum Rabi model. Our approach utilizes a {\Xi}-type
three-level atom, where the upper two levels are coupled to a cavity field to
form a quantum Rabi model with ultrastrong coupling, and the transition between
the lower two levels is driven by two sequences of Gaussian pulses. We show
that the driving pulses induce deterministic emission of multiple photons from
the eigenstates of the quantum Rabi model via the stimulated Raman adiabatic
passage technique, and hence can create bundles of multiple photons on-demand
in the cavity output field. We calculate the generalized second-order
correlation functions of the output photons, which reveal that the emitted
photons form antibunched multi-photon bundles.
