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Technologies for tuning superconducting Josephson junctions

David Schuster, Andrew Oriani, Larry Chen

Published Mar 2, 2020• 0 citations
Publisher

Abstract

Technologies for tuning a resistance of tunnel junctions such as Josephson junctions are disclosed. In the illustrative embodiment, a Josephson junction is heated to 85 Celsius, and an electric field is applied to the Josephson junction. The heat and the electric field cause the resistance of the Josephson junction to increase. Monitoring the Josephson junction during the application of the electric field allows for the resistance of the Josephson junction to be adjusted to a particular value.

DOI
10.6082/gah58-myp05

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Technologies for tuning a resistance of tunnel junctions such as Josephson junctions are disclosed. In the illustrative embodiment, a Josephson junction is heated to 85 Celsius, and an electric field is applied to the Josephson junction. The heat and the electric field cause the resistance of the Josephson junction to increase. Monitoring the Josephson junction during the application of the electric field allows for the resistance of the Josephson junction to be adjusted to a particular value.
Josephson junctions are key elements of superconducting quantum technology, serving as the core building blocks of superconducting qubits. We present an experimental study on room-temperature electrical tuning of aluminum junctions, showing that voltage pulses can controllably increase their resistance and adjust the Josephson energy while maintaining qubit quality factors above 1 million. We find that the rate of resistance increase scales exponentially with pulse amplitude during manipulation. Following the manipulation, the resistance increases spontaneously through contributions proportional to both the initial resistance and the preceding manipulation. We show that this spontaneous increase halts at cryogenic temperatures, and resumes again at room temperature. Using our stepwise protocol, we achieve up to a 270% increase in junction resistance, corresponding to a reduction of nearly 2 GHz of the qubit transition frequency. These results establish the achievable range, relaxation behavior, and practical limits of electrical tuning, enabling post-fabrication mitigation of frequency crowding in quantum processors.
Reproducibility of Al/AlO x /Al Josephson junctions is a challenge for scaling up superconducting quantum processors. The frequency uncertainty of the transmon qubits arising from the fabrication process is attributed to deviations in the Josephson junction microstructure and electrical properties. Here, we present a solution for this problem using the post-fabrication Josephson junction thermal annealing process. The developed thermal post-exposure method allows not only to increase the junction resistance by 175%, but also to decrease by 60% with a step of 10% in R n , which opens up new possibilities for tuning the frequency of qubits. The resistance is shown to be strongly temperature dependent, and is weakly dependent on the holding time. The linear dimensions of the electrodes and the sidewalls contribution to the total JJ area also have a significant impact on the final resistance after annealing. Finally, a theoretical model of the structure modification in a tunnel barrier with changes in oxygen concentration gradient is proposed. The proposed thermal annealing approach can be used to form stable and reproducible tunnel barriers and scalable frequency trimming for widely used fixed-frequency transmon qubits.
Gate-tunable Josephson junctions (JJs) are the backbone of superconducting classical and quantum computation. Typically, these systems exploit low-charge-concentration materials and present technological difficulties limiting their scalability. Surprisingly, electric field modulation of a supercurrent in metallic wires and JJs has been recently demonstrated. Here, we report the realization of titanium-based monolithic interferometers which allow tuning both JJs independently via voltage bias applied to capacitively coupled electrodes. Our experiments demonstrate full control of the amplitude of the switching current ( I s ) and of the superconducting phase across the single JJ in a wide range of temperatures. Astoundingly, by gate-biasing a single junction, the maximum achievable total I s is suppressed down to values much lower than the critical current of a single JJ. A theoretical model including gate-induced phase fluctuations on a single junction accounts for our experimental findings. This class of quantum interferometers could represent a breakthrough for several applications such as digital electronics, quantum computing, sensitive magnetometry, and single-photon detection.