  The ideal superconductor provides a pristine environment for the delicate
states of a quantum computer: because there is an energy gap to excitations,
there are no spurious modes with which the qubits can interact, causing
irreversible decay of the quantum state. As a practical matter, however, there
exists a high density of excitations out of the superconducting ground state
even at ultralow temperature; these are known as quasiparticles. Observed
quasiparticle densities are of order 1~$\mu$m$^{-3}$, tens of orders of
magnitude larger than the equilibrium density expected from theory.
Nonequilibrium quasiparticles extract energy from the qubit mode and induce
discrete changes in qubit offset charge, a potential source of dephasing. Here
we show that a dominant mechanism for quasiparticle poisoning in
superconducting qubits is direct absorption of high-energy photons at the qubit
junction. We use a Josephson junction-based photon source to controllably dose
qubit circuits with millimeter-wave radiation, and we use an interferometric
quantum gate sequence to reconstruct the charge parity on the qubit island. We
find that the structure of the qubit itself acts as a resonant antenna for
millimeter-wave radiation, providing an efficient path for photons to generate
quasiparticle excitations. A deep understanding of this physics will pave the
way to realization of next-generation superconducting qubits that are robust
against quasiparticle poisoning and could enable a new class of quantum sensors
for dark matter detection.
