Quasiparticle Poisoning Mediated by Spurious Antenna Modes of the Transmon Qubit

Superconducting qubits are a leading candidate for realization of a fault-tolerant quantum computer. However, nonequilibrium quasiparticles can seriously degrade device fidelity. We find that the dominant mechanism for quasiparticle poisoning is direct absorption of pair-breaking photons at the qubit junction. The island of the qubit acts as a resonant antenna for millimeter-wave blackbody radiation, providing an efficient impedance match from the qubit tunnel junction to the impedance of free space. We describe a series of experiments involving different qubit geometries corresponding to fundamental antenna resonances spanning a broad range from 100 GHz to 500 GHz. We use broadband blackbody radiators and coherent Josephson mm-wave sources to characterize the resonant absorption of pair-breaking photons by the qubit. Photon absorption events induce a change in the quasiparticle parity of the qubit island, which we detect using a parity-sensitive Ramsey gate sequence. In addition, we characterize spurious upward and downward qubit transitions induced by photon-assisted quasiparticle poisoning. A deep understanding of this physics will pave the way to realization of a new class of quantum sensors for dark matter detection and to next-generation superconducting qubits that are robust against quasiparticle poisoning.