TECHNICAL BLOG
Tuning of Josephson junctions – the effects of depinning physics
- We develop a model for junction tuning based on depinning theory which gives some insight into how the junction may be modified in this process.
- Junctions are interrogated before and after tuning, an increase in breakdown voltage after tuning suggests that the weakest points in the barrier have been modified in this process.
- Cryogenic measurements of higher excited states in qubits, which have previously been linked to the nano-scale structure in barriers, show no difference between tuned and untuned junctions.
Oscar Kennedy
HEAD OF DEVICE SCALING
Oscar has a background in quantum devices, RF instrumentation and materials science. Before joining OQC Oscar was a UCLQ fellow working on spin-based quantum memories. He now leads a team running projects spanning next generation sample packaging, superconducting device design and measurement.
Connor Shelly
DIRECTOR OF MATERIALS SCIENCE & DEVICE ENGINEERING
As the Director of Materials Science and Device Engineering, Connor is responsible for the manufacturing of OQC’s qubits, QPUs, and superconducting devices, as well as being responsible for areas including qubit coherence, cryogenic packaging, materials science, and nanofabrication. Prior to joining OQC, Connor worked at the National Physical Laboratory (NPL) where he was the lead researcher for the Superconducting Electronics team and worked on the development of quantum limited amplifiers and carried out research into the physics of superconducting nanobridges as Josephson elements.
Quantum computers are set to bring new computational protocols with diverse economic benefits to the world. These machines will need high qubit counts, low error rates and error correcting protocols. Building processors made from superconducting qubits at scale and with high quality comes with challenges: many of which lie within the processor manufacturing. The component studied in this work is the Josephson junction, also known as a JJ, which poses its own manufacturing difficulties.
What challenges do we face with JJs?
When manufacturing a qubit, most of the imprecision in the manufacturing process occurs in the parts of the process relating to the JJ. An example of this can be seen in the creation of the aluminium oxide barrier layer where manufacturing on nanometer length scales is challenging. You can learn more about Josephson junctions and the analysis of JJ barrier variation in our recent preprint released on arXiv.
In order to facilitate higher fidelity quantum computers however, we must manufacture elements of quantum computers with tight tolerances. This requires extensive efforts in finding solutions to manufacturing challenges.
One solution for addressing imprecisions in the manufacturing process of JJs is, of course, to improve manufacturing steps: a solution which is being explored by many across the superconducting community.
Tuning Josephson junctions
Another approach is to modify the JJs after they have been fabricated. There are a variety of processes that have been developed which allow for post-manufacture parameter adjustment including; use of laser irradiation1, electrical tuning2, as well as a previous development by the Materials Science & Device Engineering team here at OQC using electron beams3. Today, in their latest preprint: ‘Tuning of Josephson junctions – the effects of depinning physics’, OQC’s Materials Science & Device Engineering team have introduced a modified electrical tuning protocol, an adapted technique using alternating bias at elevated temperatures2.
What does ‘tuning Josephson junctions’ mean?
The key properties of a Josephson junction are its critical current which is determined by the size of the junction and how strongly the superconducting electrodes couple across it. In our JJs this is defined at the point of fabrication. Tuning a JJ is performing some post-manufacture process on the JJ in order to adjust its critical current. Normally, we infer this change in critical current by monitoring the room temperature resistance of the JJ as the two properties are linked. Having feedback from the room temperature resistance is vitally important as it means that not only can you change the properties, but you can target the properties you want to achieve.
The technique works by applying an alternating voltage (one that oscillates from positive to negative values) across a JJ whilst heating it. This causes a rearrangement of the oxygen and aluminium ions inside the insulating aluminium oxide barrier of the JJ.
In our work, we map out how the amplitude of the voltage oscillations, their frequency and the temperature of the junction affects how quickly the junction changes properties. We develop a theoretical model that describes this process and allows us to predict this speed for new parameters. We show that modifications to the barrier are described by a creep process.
We interrogate junctions before and after tuning, looking both at the breakdown voltage of the junctions and their ladder of excited states, following the research published in Nature Physics on the ‘Observation of Josephson harmonics in tunnel junctions’4.
The tuning process increases the average breakdown voltage and also the resistance of the junctions. An increase in the breakdown voltage shows that the weakest points in the barrier have been made stronger, which we cautiously interpret as increasing their thickness. However, the predictions made in the Nature Physics article mentioned previously4 , indicated that making these thin points thicker should have an effect on the ladder of excited states, which we have not observed in this research. Clearly there is still more to understand about these tuning processes and the effects that they have on the materials under consideration.
References
- J. B. Hertzberg, E. J. Zhang, S. Rosenblatt, E. Magesan, J. A. Smolin, J.-B. Yau, V. P. Adiga, M. Sandberg, M. Brink, J. M. Chow, et al., Laser-annealing josephson junctions for yielding scaled-up superconducting quantum processors, npj Quantum Information 7, 129 (2021).
- P. Pappas, M. Field, C. Kopas, J. A. Howard, X. Wang, E. Lachman, J. Oh, L. Zhou, A. Gold, G. M. Stiehl, et al. Alternating-bias assisted annealing of amorphous oxide tunnel junctions Communication Materials 7, 150 (2024).
- Balaji, N. Acharya, R. Armstrong, K. G. Crawford, S. Danilin, T. Dixon, O. W. Kennedy, R. D. Pothuraju, K. Shahbazi, and C. D. Shelly, Electron-beam annealing of Josephson junctions for frequency tuning of quantum processors, arXiv preprint arXiv:2402.17395 (2024).
- Willsch, D. Rieger, P. Winkel, M. Willsch, C. Dickel, J. Krause, Y. Ando, R. Lescanne, Z. Leghtas, N. T. Bronn, et al., Observation of josephson harmonics in tunnel junctions, Nature Physics , 1 (2024).
Read the full preprint on ArXiv
Tuning of Josephson junctions – the effects of depinning physics.
Oscar W. Kennedy, Jared H. Cole, and Connor D. Shelly.
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