Beta tantalum transmons with quality factors exceeding five million

Tantalum-based transmon qubits are a promising platform for building large-scale quantum processors. So far, these qubits have been made from tantalum films grown exclusively in the alpha phase (α-Ta) at elevated temperatures. While beta-phase tantalum (β-Ta) readily nucleates at room temperature (RT), β-Ta is believed to be detrimental to qubit performance as it has a higher normal resistivity and a lower superconducting gap than α-Ta. We challenge this prevailing belief by fabricating transmons using a base layer of β-Ta on sapphire that yield time-averaged quality factors exceeding 5 million over 50 hours. Resonator studies show that these quality factors are limited by two-level systems (TLS) losses from the surface and bulk dielectrics, with the surface loss contribution for β-Ta being about twice that of α-Ta. Notably, the dominant loss mechanism is surface TLSs, not kinetic inductance or thermal quasiparticles - loss channels that arise from the bulk material properties of β-Ta. Our work establishes β-Ta on sapphire as a low-loss, RT-compatible material stack for transmons and other superconducting devices such as kinetic inductance detectors and on-chip quasiparticle traps.