Millisecond lifetimes and coherence times in 2D transmon qubits

Materials improvement is a powerful approach to reducing loss and decoherence in superconducting qubits, because such improvements can be readily translated to large-scale processors. Recent work improved transmon coherence by using tantalum as a base layer and sapphire as a substrate¹. The losses in these devices are dominated by two-level systems with comparable contributions from both the surface and bulk dielectrics², indicating that both must be tackled to achieve substantial improvements in the state of the art. Here we show that replacing the substrate with high-resistivity silicon markedly decreases the bulk substrate loss, enabling 2D transmons with time-averaged quality factors (Q^avg) of 9.7 × 10⁶ across 45 qubits. For our best qubit, we achieve a Q^avg of 1.5 × 10⁷, reaching a maximum Q of 2.5 × 10⁷, corresponding to a lifetime (T₁) up to 1.68 ms. This low loss also allows us to observe decoherence effects related to the Josephson junction, and we use an improved, low-contamination junction deposition to achieve Hahn echo coherence times (T_2E) exceeding T₁. We achieve these materials improvements without any modifications to the qubit architecture, allowing us to readily incorporate standard quantum control gates. We demonstrate single-qubit gates with 99.994% fidelity. The tantalum-on-silicon platform comprises a simple material stack that can potentially be fabricated at the wafer scale and therefore can be readily translated to large-scale quantum processors.