Aluminum Grain Size Linked to Improved Qubit Coherence Times

Researchers at King Abdullah University of Science and Technology and the National Company of Telecommunications and Information Security in Jeddah have established a direct link between aluminum film grain size and non-Two-Level System (non-TLS) losses, a connection previously overlooked in efforts to improve qubit performance. The work moves beyond conventional loss mitigation strategies focused on material interfaces to examine the intrinsic properties of the superconducting metal itself. Specifically, the study identifies that the profile and roughness of dielectric trimming contribute to TLS loss, meaning the shaping of a material’s edges impacts qubit coherence. By engineering aluminum microstructure with controlled heating during deposition to increase grain size, and employing a two-step etching technique, Tropic etching, the team demonstrated a two-orders-of-magnitude improvement in resonator quality factor. These results lay out a fabrication pathway for lower-loss aluminum resonators and can enhance the performance of superconducting quantum devices.

Aluminum Microstructure Impacts Non-TLS Losses

Most loss mitigation strategies have historically focused on the interfaces between materials, rather than the intrinsic properties of the metal itself, making this finding significant. The team’s work centers on engineering the microstructure of aluminum during deposition. By applying heat to the substrate during the aluminum film creation process, researchers successfully increased grain size and reduced the density of grain boundaries within the metal. This manipulation of the aluminum’s internal architecture directly addresses non-TLS losses, a critical factor limiting qubit coherence times and resonator quality factors. Previous investigations reported either an improvement in quality factor at a single frequency or no clear correlation between aluminum microstructure and resonator quality factor, leaving the underlying relationship poorly understood. Here, researchers address this gap by studying the resulting improvement in average quality factor across resonators spanning a 1, 10 GHz frequency range.

The study quantifies the impact of this microstructural control, showing an improvement of two-orders-of-magnitude in quality factor. The authors write that this two-pronged approach, controlling both the metal’s internal structure and the smoothness of the surrounding dielectric, represents a fundamental shift in how scientists are tackling the challenge of improving superconducting quantum devices, potentially unlocking pathways to more stable and powerful qubits.

Recent advances in fabrication techniques are tackling dielectric losses, a critical limitation on qubit coherence, to improvements in superconducting materials themselves. Researchers are now focusing on the precise profile and roughness of dielectric trimming, recognizing that the way materials are shaped at their edges significantly impacts qubit performance. This moves beyond conventional loss mitigation strategies that historically prioritized material interfaces, instead addressing the surface quality of the dielectric material surrounding superconducting circuits. Conventional silicon etching leaves behind sidewall and bottom roughness due to silicon redeposition during the process. This roughness, the researchers found, creates defect sites that act as two-level systems (TLS), reintroducing dielectric loss despite efforts to remove lossy material. The study reports the use of a two-step etching technique, Tropic etching, to combat this. These results lay out the fabrication pathway for aluminum resonators with lower loss, demonstrating two-orders-of-magnitude improvement in quality factor. Since dielectric edges are inherent to most common device geometries, this technique promises broad applicability across various superconducting quantum devices, enhancing their overall performance and paving the way for more stable and reliable quantum computations.

The team’s work demonstrates a shift in focus, moving beyond simply addressing boundaries between materials to actively engineering the aluminum’s internal structure. This adds a new dimension to loss control, extending beyond material purity and interface quality. Initial investigations had yielded inconsistent results regarding aluminum microstructure and resonator quality factor, but this work clarifies the relationship by examining average quality factor improvements across a broad frequency range. “We address the two gaps identified above, aluminum grain microstructure and silicon dielectric smoothness, by examining their impact on the resonator quality factor and isolating their respective contributions to non-TLS and TLS loss,” the paper states.

Superconducting resonator quality factor serves as a remarkably sensitive probe of material properties, directly impacting the performance of quantum devices. Researchers are increasingly leveraging this metric not simply to identify losses, but to dissect their origins within the complex layers of superconducting circuits. This approach has yielded significant improvements in resonator performance, demonstrating a pathway toward more stable and reliable qubits.

Conventional approaches to enhancing superconducting qubit coherence have largely focused on refining material interfaces. The team’s work centers on manipulating aluminum grain boundaries, defects within the metal’s crystalline structure. This is particularly significant given aluminum’s widespread use as a foundational material in Josephson junctions and resonator fabrication. To address this, the team implemented a two-step etching technique, Tropic etching. These results lay out the fabrication pathway for aluminum resonators with lower loss, demonstrating two-orders-of-magnitude improvement in quality factor. Since aluminum is the basis for most high-coherence Josephson junctions and dielectric edges are inherent to all common device geometries, the improvements in aluminum microstructure and edge profiling, presented here, can enhance the performance of superconducting quantum devices.

A surprising link between aluminum grain size and microwave losses is reshaping the pursuit of higher-performance superconducting qubits. Researchers have long focused on minimizing imperfections at material interfaces to boost qubit coherence. The investigation highlights that larger aluminum grain sizes correlate with reduced non-TLS losses. Previous attempts to improve aluminum films through varying deposition temperatures yielded inconsistent results; however, this research directly links grain boundary density to performance. This nuanced understanding of aluminum’s internal structure opens new avenues for material optimization, and the study also sheds light on the impact of dielectric edge smoothness on qubit coherence.