  Superconducting qubits are resonant absorbers of pair-breaking radiation. The
metal pads that form the qubit capacitance support standing wave modes at
frequencies of order 100~GHz; these modes are strongly coupled to free-space
impedance through their electric dipole moment. While the antenna mode of the
3D transmon qubit is easily seen to be a resonant dipole, other 2D qubit types
can be understood as the aperture duals of wire loop antennas or folded
dipoles. For typical Josephson junction parameters, the junction provides a
reasonable conjugate match to the fundamental antenna mode. We calculate the
contribution to quasiparticle poisoning from resonant absorption of blackbody
radiation. We extend our analysis to dissipation at the qubit frequency, where
radiative losses provide an ultimate limit to qubit energy relaxation time. A
clear understanding of the spurious antenna modes of qubits will allow designs
that are insensitive to pair-breaking radiation and that display reduced
radiative losses at the qubit frequency.
