  We evaluate the rates of energy and phase relaxation of a superconducting
qubit caused by stray photons with energy exceeding the threshold for breaking
a Cooper pair. All channels of relaxation within this mechanism are associated
with the change in the charge parity of the qubit, enabling the separation of
the photon-assisted processes from other contributions to the relaxation rates.
Among the signatures of the new mechanism is the same order of rates of the
transitions in which a qubit looses or gains energy.
