Enhancing the performance of superconducting qubits, the building blocks of quantum computers, relies heavily on the quality of their base materials. Whether it involves baking a cake, constructing a house, or developing a quantum device, the ingredients or base materials play a vital role in determining the quality of the final product. Researchers have been tirelessly experimenting with different base materials in their quest to extend the coherent lifetimes of qubits.
The coherence time, which measures the duration for which a qubit retains quantum information, serves as a primary indicator of performance. Recently, a breakthrough was made as scientists uncovered tantalum's ability to significantly improve the functionality of superconducting qubits. However, until recently, the underlying reasons for this improvement remained elusive.
A collaborative effort involving the Center for Functional Nanomaterials (CFN), the National Synchrotron Light Source II (NSLS-II), the Co-design Center for Quantum Advantage (C2QA), and Princeton University aimed to unravel the fundamental aspects that contribute to the enhanced performance of tantalum-based qubits. By decoding the tantalum's chemical profile, these scientists successfully shed light on the mystery.
The findings of this study, recently published in the esteemed journal Advanced Science, hold immense value in guiding the design of future qubits with even greater capabilities. The CFN and NSLS-II, both US Department of Energy (DOE) Office of Science User Facilities located at DOE's Brookhaven National Laboratory, collaborated with C2QA, a quantum information science research center led by Brookhaven, where Princeton University serves as a prominent partner.
Finding the right ingredient
Tantalum stands out as a remarkable and versatile metal, offering a range of advantageous properties. It boasts high density, hardness, and excellent workability. Furthermore, tantalum possesses an impressive melting point and remarkable corrosion resistance, rendering it valuable in numerous commercial applications. Moreover, tantalum exhibits superconductivity, enabling it to conduct electric current without any resistance when cooled to extremely low temperatures.
Tantalum-based superconducting qubits have astounded researchers with their prolonged lifetimes, surpassing half a millisecond. In comparison, qubits fabricated from niobium and aluminum—currently utilized in large-scale quantum processors—fall significantly shorter in terms of lifespan.
Given its exceptional characteristics, tantalum proves itself as a prime candidate for constructing superior qubits. However, the pursuit of enhancing superconducting quantum computers has been impeded by a lack of comprehensive understanding regarding the factors that limit qubit lifetimes, a phenomenon referred to as decoherence. While noise and microscopic sources of dielectric loss are generally believed to contribute, the exact mechanisms and underlying reasons remain uncertain.
Nathalie de Leon, an associate professor of electrical and computer engineering at Princeton University and the materials thrust leader for C2QA, elaborated on the significance of the research presented in this paper. Addressing a significant challenge in qubit fabrication, the study aims to establish a microscopic, atomistic model that elucidates the nature of loss, accounting for all observed behaviors and providing insights into specific device limitations. Accomplishing this requires precise and quantitative measurement techniques coupled with advanced data analysis.
Surprising results
To gain deeper insights into the root cause of qubit decoherence, scientists from Princeton University and CFN conducted experiments involving the growth and chemical processing of tantalum films on sapphire substrates. These tantalum samples were then analyzed at NSLS-II's Spectroscopy Soft and Tender Beamlines (SST-1 and SST-2) using X-ray photoelectron spectroscopy (XPS). By employing XPS, which leverages X-rays to dislodge electrons from the sample, the researchers were able to gather crucial information about the chemical properties and electronic state of the tantalum oxide layer formed on the surface.
The scientists proposed that the thickness and composition of the tantalum oxide layer played a significant role in determining qubit coherence, particularly since tantalum exhibits a thinner oxide layer compared to the more commonly used niobium in qubits.
Andrew Walter, a lead beamline scientist in NSLS-II's soft X-ray scattering & spectroscopy program, elaborated on the experimental approach: “Our aim was to gain a better understanding of the underlying processes by studying these materials at the beamlines. Initially, there was an assumption that the tantalum oxide layer exhibited uniformity; however, our measurements revealed its non-uniform nature. It is always intriguing when unexpected answers emerge, as that is when true learning occurs.”
The research team made a noteworthy discovery, identifying multiple tantalum oxide variations on the tantalum surface. This finding has given rise to a fresh set of inquiries in the quest for enhanced superconducting qubits. Can these interfaces be modified to enhance overall device performance, and which specific modifications would yield the greatest benefits? Furthermore, what types of surface treatments can be employed to minimize loss?
Unraveling the complexities of tantalum oxide layers and their impact on qubit coherence opens up new avenues for exploration and optimization in the pursuit of superior superconducting qubits.
Embodying the spirit of codesign
Mingzhao Liu, a materials scientist at CFN and the materials subthrust leader in C2QA, expressed admiration for the collaborative efforts displayed by experts from diverse backgrounds in tackling a shared problem. The study embodied a highly cooperative endeavor that pooled together the facilities, resources, and expertise available across all participating institutions. Liu found it particularly exhilarating, from a materials science perspective, to contribute to the creation of samples and play a pivotal role in the research.
Walter emphasized that this work exemplified the collaborative nature on which C2QA was built. The electrical engineers from Princeton University played a crucial role in device management, design, data analysis, and testing. The materials group at CFN, on the other hand, focused on growing and processing samples and materials. Walter's team at NSLS-II specialized in characterizing these materials and studying their electronic properties.
Bringing together these specialized groups not only facilitated a smooth and efficient research process but also provided the scientists with a broader understanding of their work within a larger context. Students and postdocs had the opportunity to gain invaluable experience in multiple areas and make meaningful contributions to the research.
De Leon highlighted the collaborative nature of the team, emphasizing that rather than traditional hand-offs of materials between materials scientists and physicists, the team worked hand-in-hand, developing new methods along the way that could have broader applications at the beamline in the future.
Source: Brookhaven National Laboratory