Investigating the Susceptibility of Niobium- and Tantalum-Based Superconducting Qubits to Infrared Radiation

Improvements in the coherence of superconducting qubits stem from exploring new qubit designs, new materials and fabrication processes and improvements in experimental setups. Recently, the adoption of tantalum has extended qubit coherence times to the millisecond range, attributed to reduced dielectric losses at the metal-air interface, compared to niobium. However, the base layer material of Josephson-junction-based qubits also affects their susceptibility to quasi-particle tunneling-induced decoherence, where energy dissipation occurs through single-electron tunneling across junctions. In this study, we assess cryogenic setups in terms of quasi-particle tunneling rates for both niobium and tantalum-based qubits. By creating out-of-equilibrium quasi-particles using a source of infrared radiation, we identify an increased susceptibility of tantalum qubits to this noise source, which we attribute, at least in part, to the lower superconducting gap energy. In testing various methods to protect the qubits from environmental electromagnetic radiation, we differentiate between radiation predominantly propagating towards the sample through cryogenic wiring or other paths. We succeed in reducing observed quasi-particle tunneling rates by a factor of two to three for niobium and by more than two orders of magnitude for tantalum-based qubits by improvements to our setup.