Junction Ageing Affects Superconducting Processor

Researchers are increasingly focused on mitigating degradation in superconducting circuits as a critical challenge for realising scalable quantum processors. Rangga P. Budoyo and Rasanayagam S. Kajen from the Centre for Quantum Technologies, National University of Singapore, working with Bing Wen Cheah, Long H. Nguyen, and Rainer Dumke from the Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, have characterised the aging processes and annealing behaviour of Josephson junctions under varied environmental conditions. Their study, spanning up to three months post-fabrication, reveals a logarithmic aging curve significantly influenced by both fabrication and storage environments, demonstrating faster degradation under ambient conditions compared to nitrogen or vacuum storage. Furthermore, the team observed contrasting annealing effects: nitrogen annealing consistently reduced the critical current, while ambient annealing showed an initial increase at 200°C before decreasing at 250°C, establishing a lower limit on achievable junction tuning and offering vital insights to improve the stability and longevity of quantum computing hardware. Understanding how these junctions degrade over time is central as the field progresses towards building larger and more complex quantum circuits. Their effort centres on characterising junction aging under varied storage conditions and assessing the impact of thermal annealing, a process used to fine-tune junction properties post-fabrication. Initial junction aging follows a predictable logarithmic curve, where the rate of degradation is heavily influenced by storage environment, while the overall extent of aging is determined by the initial fabrication process. Storage in ambient laboratory air accelerates aging compared to storage within a nitrogen atmosphere or under vacuum, demonstrating a clear link between environmental factors and junction stability. Scientists can now modulate the degradation rate by controlling the storage environment, offering a pathway to preserve junction performance for longer periods.

The team investigated thermal annealing, a technique used to adjust the junctions’ critical current, by comparing nitrogen and ambient air environments. The response to annealing differs markedly depending on the surrounding atmosphere. Under a nitrogen environment, resistance consistently decreased at all tested temperatures, aligning with previous observations in low-oxygen settings. By contrast, annealing in ambient air yielded a more complex pattern, with resistance increasing at 200°C before decreasing again at 250°C — at no point was it possible to reduce the junction resistance below its initial fabricated value. Indicating a fundamental limit to the degree of tuning achievable through these methods, and for fabrication, the junctions exhibited a 4% wafer-scale critical current (Ic) coefficient-of-variation (CV) and 1% chip-scale Ic CV. Highlighting the precision required in the manufacturing process. By monitoring junctions for up to 2-3 months revealed that the aging amplitude is primarily determined by fabrication conditions, a finding consistent with recent reports. Since junction design and fabrication, including dimensions and oxidation parameters, influence aging, several methods have been developed to inhibit it, such as plasma cleaning or modified oxidation steps. To reduce aging appears to also reduce the junction response to annealing — a necessary balance between these two properties for large-scale quantum processor manufacturing. Here, the effort also examined the effects of voltage annealing and thermal annealing on junctions with differing storage histories. Finding no significant differences based on prior storage conditions. Voltage annealing does not accelerate aging but rather alters the internal structure of the junctions themselves. Josephson junction degradation rates depend on storage and thermal annealing environments Detailed Josephson junction aging follows a logarithmic curve, with storage conditions governing the rate of aging and fabrication conditions influencing the amplitude. Initially, junctions exhibited a wafer-scale critical current (Ic) coefficient-of-variation (CV) of 4%, alongside a chip-scale Ic CV of 1%, demonstrating the precision achieved during fabrication. Investigations into the effects of varying storage environments showed that junctions kept in ambient laboratory conditions aged more rapidly than those stored in nitrogen or under vacuum. A discernible change in aging speed occurred when the storage environment was altered. Further work compared thermal annealing, a process used to restore junction performance, performed under nitrogen and ambient conditions up to 250°C. Under a nitrogen atmosphere, junction resistance consistently decreased across all tested temperatures. Conversely, annealing in ambient air resulted in increased resistance at 200°C, followed by a decrease at 250°C, indicating a more complex thermal behaviour. Attempts to reduce junction resistance below its initial fabricated value were unsuccessful. A fundamental lower limit to the achievable tuning range. In turn, the logarithmic aging curve was quantified using a phenomenological fit, R(t)/R(t≈0) = 1 + a log(t/τ) + b, where ‘a’ represents the fractional aging amplitude, and ‘τ’ defines the aging speed. This model accounted for the initial resistance, denoted by parameter ‘b’, and provided a framework for understanding the observed changes in junction characteristics over time. Analysis of five chips revealed initial resistance CV values between 3 and 6%, while a sixth chip, with a lower fabrication yield, exhibited a higher CV. Voltage annealing experiments, utilising an alternating bias assisted annealing method with ±0.9V pulses, did not induce accelerated aging but instead altered the internal structure of the junctions. Thermal annealing, conducted in a reflow oven, involved a process of 8 minutes, comprising 3 minutes of heating, 2 minutes of holding, and 3 minutes of cooling, followed by a thermalization period. These findings offer practical guidance for optimising both storage protocols and annealing procedures, enabling manufacturers and researchers to better control the performance of superconducting quantum processors. Josephson junction degradation mechanisms and their influence on qubit coherence Detailed analysis of Josephson junction ageing has provided a clearer picture of the challenges in building dependable quantum computers for years, manufacturers have struggled with inconsistencies in qubit performance, often tracing back to subtle changes in the junctions themselves. Still, the fundamental building blocks of superconducting circuits. This project provides a valuable, granular understanding of how these junctions degrade over time. How storage and post-fabrication treatment impact that process. Yet the team’s logarithmic ageing curve suggests a predictable, if persistent, decline in junction quality. Fabrication precision, as evidenced by the low coefficient of variation in critical current measurements, clearly sets an initial performance baseline. Meanwhile, storage conditions dictate the rate of deterioration. Alternative approaches to junction stabilisation exist, including laser and electron-beam annealing, alongside techniques like plasma cleaning intended to inhibit oxidation. Unlike these methods, which often focus on immediate performance boosts, this effort concentrates on understanding the underlying mechanisms of ageing, offering a longer-term perspective. Even so, the effort’s timeframe, spanning only a few months, limits its ability to predict behaviour over the years required for practical quantum devices. At present, the inability to restore junctions to their original fabricated state represents a significant hurdle. Future investigations should extend the observation period to assess long-term stability and explore novel materials or fabrication techniques that might further slow the aging process, or even reverse it. In the end, a deeper understanding of these subtle effects will be essential as the field moves towards larger, more complex quantum processors. 👉 More information 🗞 Characterization of Josephson Junction Aging and Annealing Under Different Environments 🧠 ArXiv: https://arxiv.org/abs/2602.23888 Tags: