Millisecond lifetimes and coherence times in 2D transmon qubits

Materials improvements are 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 utilizing tantalum (Ta) as a base layer and sapphire as a substrate [1]. The losses in these devices are dominated by two-level systems (TLSs) with comparable contributions from both the surface and bulk dielectrics [2], indicating that both must be tackled to achieve major improvements in the state of the art. In this talk, I will present that replacing the substrate with high-resistivity silicon (Si) dramatically 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. We additionally conduct a systematic investigation on various Si substrates with a wide range of resistivities and defect levels to identify the underlying limiting loss mechanism arising from the Si substrates.

[1]Nature Communications 12, 1779 (2021)

[2]Physical Review X 13, 041005 (2023)

[3] arXiv:2503.14798 (2025), Nature, in press