  Deterministic single-photon sources are important and ubiquitous in quantum
information protocols. However, to the best of our knowledge, none of them work
in the ultrastrong light-matter coupling regime, and each excitation process
can only emit one photon. We propose a deterministic single-photon source in
circuit QED which can work in the ultrastrong coupling regime. Here, two qubits
are excited simultaneously in one process and two deterministic single photons
can be sequentially emitted with an arbitrary time separation. This happens
through two consecutive adiabatic transfers along the one-photon solutions of
the two-qubit Rabi and Jaynes-Cummings model, which has constant eigenenergy in
the whole coupling regime. Unlike the stimulated Raman adiabatic passage, the
system goes back to the initial state of another period automatically after
photon emission. Our scheme can approach unity single-photon efficiency,
indistinguishability, and purity simultaneously. With the assistance of the
Stark shift, a deterministic single photon can be generated within a time
proportional to the inverse of the resonator frequency.
