2D transmons with lifetimes and coherence times exceeding 1 millisecond (2503.14798v1)

Abstract: 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. The losses in these devices are dominated by two-level systems (TLSs) with comparable contributions from both the surface and bulk dielectrics, indicating that both must be tackled to achieve major improvements in the state of the art. Here we show that replacing the substrate with high-resistivity silicon (Si) dramatically decreases the bulk substrate loss, enabling 2D transmons with time-averaged quality factors (Q) exceeding 1.5 x 10^7, reaching a maximum Q of 2.5 x 10^7, corresponding to a lifetime (T_1) of up to 1.68 ms. This low loss allows us to observe decoherence effects related to the Josephson junction, and we use improved, low-contamination junction deposition to achieve Hahn echo coherence times (T_2E) exceeding T_1. We achieve these material 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 Ta-on-Si platform comprises a simple material stack that can potentially be fabricated at wafer scale, and therefore can be readily translated to large-scale quantum processors.

Enhancements in 2D Transmon Qubit Lifetimes via High-Resistivity Silicon Substrates

This paper presents a significant advancement in the realization of high-coherence 2D transmon qubits, achieving lifetimes and coherence times that surpass 1 millisecond. Traditionally, superconducting qubit performance has been limited by losses dominated by two-level systems (TLSs), with contributions from both surface and bulk dielectrics. The authors propose a methodological shift by replacing the substrate material in transmon construction, transitioning from sapphire to high-resistivity silicon. This modification markedly decreases bulk substrate loss, enabling qubits with unprecedented quality factors and coherence times.

Key Findings:

• The paper showcases transmon qubits with time-averaged quality factors (Q) exceeding $1.5 \times 10^{7}$, reaching a peak of $2.5 \times 10^{7}$. This quality factor corresponds to a maximum measured lifetime ($T_1$) of up to 1.68 ms.
• High-resistivity silicon significantly reduces dielectric loss compared to sapphire, specifically noting no saturation of Q with device size on silicon, unlike sapphire-based devices.
• The qubit architecture remains unchanged, allowing incorporation into existing quantum control schemes. Single qubit gate fidelities of 99.994% were demonstrated.
• Innovations in fabrication include optimized junction deposition under ultra-high vacuum conditions, which mitigates contaminant-induced decoherence.

Implications:

• The improved coherence times and lifetimes directly impact quantum error correction efforts, wherein halving physical error rates could enhance the logical performance of surface codes by four orders of magnitude.
• Future developments should focus on understanding and reducing surface-related TLS losses, potentially through material encasements that prevent oxide formation.
• The paper highlights silicon's role in scaling quantum processors, leveraging existing semiconductor infrastructure to facilitate larger-scale qubit array fabrication.

Speculative Future Directions:

• Continued refinement of Josephson junction fabrication processes to further reduce decoherence sources associated with material impurities.
• Research into alternative substrate materials with similarly low dielectric losses could expand the toolkit available for qubit fabrication.
• Exploration of qubits' sensitivity to environmental factors such as quasiparticles, vortex motion, and cosmic rays, using the framework laid out in this work, may yield insights into broader quantum processor design and resilience.

This research fortifies the understanding of material impacts in superconducting qubit performance, laying a solid foundation for future innovations in quantum processor development and scale-up.