Quasiparticle poisoning in superconducting qubit arrays from controlled gamma irradiation*

When high-energy radiation, such as a gamma ray or muon, impacts a superconducting qubit chip, large numbers of electron-hole pairs and phonons are created. The ensuing dynamics of the electrons and holes changes the local offset charge environment for qubits near the impact site. The phonons that are produced have energy above the superconducting gap in the device layer, leading to excitations above the superconducting ground state known as quasiparticles. An elevated density of quasiparticles degrades qubit coherence, leading to errors in qubit arrays. Because these pair-breaking phonons spread throughout much of the chip, these errors can be correlated across a large portion of the array, posing a significant challenge for error-correction schemes. In order to study the dynamics of gamma ray impacts on superconducting qubit arrays, we use a gamma-ray source outside the dilution refrigerator to controllably dose our chip within. By using charge-sensitive transmon qubits, we can measure quasiparticle poisoning and offset charge jumps due to the gamma irradiation at different doses.