New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds (2003.00024v1)

Abstract: The superconducting transmon qubit is a leading platform for quantum computing and quantum science. Building large, useful quantum systems based on transmon qubits will require significant improvements in qubit relaxation and coherence times, which are orders of magnitude shorter than limits imposed by bulk properties of the constituent materials. This indicates that relaxation likely originates from uncontrolled surfaces, interfaces, and contaminants. Previous efforts to improve qubit lifetimes have focused primarily on designs that minimize contributions from surfaces. However, significant improvements in the lifetime of two-dimensional transmon qubits have remained elusive for several years. Here, we fabricate two-dimensional transmon qubits that have both lifetimes and coherence times with dynamical decoupling exceeding 0.3 milliseconds by replacing niobium with tantalum in the device. We have observed increased lifetimes for seventeen devices, indicating that these material improvements are robust, paving the way for higher gate fidelities in multi-qubit processors.

• The paper demonstrates that replacing niobium with tantalum significantly extends qubit coherence times, achieving over 0.3 milliseconds.
• The study employs high-temperature tantalum deposition on sapphire to form a stable α-phase that reduces microwave losses.
• Advanced spectroscopy and microscopy confirm a uniform tantalum oxide layer and consistent grain structure, enhancing overall qubit reliability.

Analysis of Tantalum-Based Superconducting Transmon Qubits for Quantum Computing

This paper explores the development of a new material platform for superconducting transmon qubits, particularly focusing on replacing niobium with tantalum. The paper demonstrates a notable enhancement in qubit coherence times, reaching over 0.3 milliseconds with the application of dynamical decoupling, marking a significant improvement over previous standards in the field of two-dimensional (2D) transmon qubits.

Key Insights and Methodological Advances

The authors address the persistent challenge of qubit relaxation and decoherence, which are significantly shorter than theoretical limits imposed by the bulk material properties. By substituting niobium with tantalum, the research uncovers a pathway for extending qubit lifetimes, suggesting that the oxide stoichiometry of niobium contributes to additional microwave loss.

1. Material Selection: The transition to tantalum is based on its improved dielectric properties and reduced complexities in its surface stoichiometry compared to niobium. The paper hypothesizes that the insulating oxide layer of tantalum mitigates losses, enhancing qubit performance.
2. Fabrication Process: The research employs a detailed fabrication method involving the deposition of tantalum on sapphire substrates at 500°C. This high-temperature process ensures the growth of the stable body-centered cubic $\alpha$ phase of tantalum, corroborated by resistance measurements consistent with a critical temperature around 4.3 K.
3. Performance Metrics: The achieved results indicate a time-averaged lifetime ($T_1$) exceeding 0.3 milliseconds in the best device, with an average $T_1$ of 0.23 milliseconds across all devices. This marks a substantial progression from previous niobium devices, underscoring the effectiveness of the material substitution.
4. Spectroscopy and Microscopy: X-ray photoelectron spectroscopy (XPS) reveals the composition and thickness of the native tantalum oxide layer, estimated at approximately 2-3 nm. High-resolution scanning transmission electron microscopy (STEM) depicts a consistent, tightly-oriented grain structure without intervening oxide growth, reinforcing the material's uniformity and quality.
5. Preservation of Surface Quality: The paper emphasizes the importance of clean fabrication environments, detailing aggressive sapphire cleaning processes to minimize particulate and carbon contamination. These steps contribute to the overall enhancement of device coherence and reliability.

Implications and Future Directions

The research affirms that leveraging material science advances is a formidable approach to enhancing quantum hardware. By demonstrating consistent improvements in coherence characteristics, this paper provides a framework for further explorations in material substitutions, including potential applications to all-tantalum qubits and other superconducting quantum systems. The systematic methodology adopted here may also translate to improvements across other quantum platforms constrained by surface and interface limitations, including trapped ions and semiconductor quantum dots.

Future investigations could focus on systemic studies of material properties such as grain size and oxide thickness on qubit performance. Additionally, exploring the interplay of tantalum film deposition and substrate choice could unveil further optimizations. There is considerable interest in understanding the role of material quality in reducing qubit-to-qubit variations within multi-qubit devices, a key factor for scalable quantum computing architectures.

Conclusion

This paper presents a significant step forward in superconducting qubit technology by introducing tantalum as a viable material to extend qubit coherence times. The meticulous focus on material selection, fabrication processes, and advanced characterization techniques serves not only to enhance current quantum computing capabilities but also paves the way for future innovations in quantum device engineering.