Transmon Qubit Exploiting Superconductor-Insulator Transition Circumvents Millikelvin Temperature Limitations

Superconducting qubits represent a leading technology in the pursuit of practical quantum computation, but current designs face challenges in scalability and performance. Researchers led by C. G. L. Bøttcher, E. Önder, and T. Connolly, alongside colleagues including J. Zhao, C. Kvande, and D. Q. Wang, now demonstrate a significant advance by creating a transmon qubit using a novel superconducting weak link. This innovative design overcomes limitations of traditional Josephson junctions by exploiting the superconductor-insulator transition in a single film of niobium nitride, achieving a measured anharmonicity of MHz and a linewidth of, and paving the way for qubits that operate at higher temperatures and with reduced complexity. The team’s approach, relying on atomic layer deposition and etching, offers a fully planar geometry that minimises unwanted capacitance and dissipation, and promises to deepen understanding of the fundamental physics governing the superconductor-insulator transition itself.

Niobium Nitride Josephson Junction Fabrication and Characterization

Scientists have meticulously investigated the fabrication and properties of superconducting weak links, essential components for building advanced quantum devices and superconducting circuits. These weak links, created from niobium nitride, exhibit unique characteristics that influence their performance and suitability for various applications. The research focuses on two distinct fabrication methods, designated S1 and S2, and details how each impacts the resulting junction properties. The team aimed to understand how subtle changes in the fabrication process affect critical parameters like critical current, nonlinearity, and response to magnetic fields.

Josephson junctions, the core of these weak links, allow supercurrent, electrical current with zero resistance, to flow via quantum tunneling. A crucial property is the critical current, the maximum current the junction can carry before switching to a resistive state. When exposed to a magnetic field, these junctions exhibit a pattern known as Fraunhofer interference, revealing information about their geometry and uniformity. The shape of this pattern, specifically the number and height of its lobes, provides insights into the junction’s internal structure. The team discovered that the fabrication method significantly influences the characteristics of the resulting junctions.

Method S1 produced junctions with higher nonlinearity, a desirable trait for certain quantum computing applications. Conversely, method S2 yielded junctions with a higher quality factor, indicating lower energy loss, and a current flow concentrated along the edges of the junction. Both methods, however, resulted in some degree of inhomogeneity, as evidenced by the presence of residual current even when interference should theoretically cancel it out. The research demonstrates that careful control over the fabrication process is crucial for tailoring the properties of these weak links. The ability to create junctions with specific characteristics opens up possibilities for optimizing device performance and exploring new quantum circuit designs. The team’s findings contribute to a deeper understanding of the underlying physics governing these devices and pave the way for advancements in superconducting technology.

Niobium Nitride Weak Links Fabricated by Atomic Layering

Scientists have developed a novel method for creating superconducting weak links using niobium nitride, addressing limitations hindering the scalability of quantum computing. The team successfully fabricated these links by locally thinning a single film of niobium nitride, exploiting its transition between superconducting and insulating states. This innovative approach utilizes atomic layer deposition and etching techniques, eliminating the need for lift-off procedures and ensuring robust device quality. Two fabrication sequences, S1 and S2, were employed, differing only in the order of etching steps, but both yielded similar weak link behavior.

The team meticulously controlled the dimensions of the weak links, fabricating devices with widths of 1 micrometer and lengths down to 20 nanometers. Measurements revealed that the relationship between critical current and resistance deviated from expected theoretical values, suggesting that electrons within the link spend a longer time within the device than predicted. Analysis of these deviations allowed the team to determine the mean free path of electrons within the niobium nitride, revealing extremely short distances comparable to the spacing between atoms, indicating electron localization rather than conventional diffusion. The team fabricated a transmon qubit, dubbed the ‘planaron’, using a weak link with dimensions of 1 micrometer by 30 nanometers. This device exhibited a measured anharmonicity of 235MHz. By directly coupling the planaron qubit to a transmission line, the team confirmed qubit operation with an anharmonicity significantly larger than the decoherence rate, demonstrating a highly-lumped resonator.

Precise Control of Superconducting Transition in Niobium Nitride

Scientists have achieved precise control over the transition between superconducting and insulating states in niobium nitride films, enabling the creation of superconducting weak links with tailored properties. The team locally thinned a single film of niobium nitride, exploiting its thickness-driven transition, utilizing both atomic layer deposition and etching to achieve remarkable precision. Experiments revealed a critical thickness of between 2. 5 and 3 nanometers that separates superconducting and insulating regimes. The team meticulously investigated the sheet resistance of the films as a function of thickness and temperature, observing a dramatic change in resistance over a narrow thickness range.

Measurements confirm that the system transitions directly between superconducting and insulating states, without exhibiting an anomalous metallic phase often observed in similar materials. This direct transition is crucial for achieving improved coherence properties in future devices. For sufficiently short weak links, a supercurrent branch was observed in the current-voltage characteristics, indicating robust superconducting behavior. These findings demonstrate the creation of planar weak links with precisely controlled properties, paving the way for new qubit designs. Measurements of the current-voltage characteristics show that the critical current is highly sensitive to the link’s geometry, offering a pathway to tune qubit parameters. The team’s ability to fabricate these links without observing an intervening metallic phase is particularly significant, as it suggests improved coherence properties for future devices. The work establishes a robust and scalable method for creating superconducting circuits with tailored characteristics, advancing the development of practical quantum computers.

Niobium Nitride Transmons Via Film Thickness Control

Scientists have developed a new method for creating superconducting weak links and, consequently, transmon qubits, by carefully controlling the thickness of niobium nitride films. The team successfully exploited the superconductor-insulator transition to fabricate planar weak links without requiring oxide layers or dielectric barriers, simplifying current fabrication processes. This approach yielded a transmon qubit exhibiting a measured anharmonicity, demonstrating the viability of the technique. The team’s work addresses key limitations of existing superconducting qubits, notably by utilizing niobium nitride, a material with a higher superconducting gap than aluminum, potentially enabling operation at elevated temperatures.

Furthermore, the planar geometry of the fabricated weak links minimizes unwanted capacitance, a crucial factor for high-frequency applications. While the fabricated qubit showed moderately reduced anharmonicity, the results confirm the potential of this method for advancing qubit technology. The authors acknowledge that the transmon qubit exhibited relatively large dissipation, a phenomenon not fully understood and a focus for future investigation. They also note that further research is needed to explore the fundamental physics of the superconductor-insulator transition in finite-sized samples. This work opens avenues for applying these weak links to a broader range of superconducting circuits and for developing qubits suitable for millimeter wave regimes and temperatures above 1 Kelvin.