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arXiv:2402.17395

Electron-beam annealing of Josephson junctions for frequency tuning of quantum processors

Yashwanth Balaji, Narendra Acharya, Robert Armstrong, Kevin G. Crawford, Sergey Danilin, Dixon, Thomas, Oscar W. Kennedy, Renuka Devi Pothuraju, Kowsar Shahbazi, Connor D. Shelly

Published Feb 27, 2024• 2 citations
arXivHTMLPDF

Abstract

Superconducting qubits are a promising route to achieving large-scale quantum computers. A key challenge in realising large-scale superconducting quantum processors involves mitigating frequency collisions. In this paper, we present an approach to tuning fixed-frequency qubits with the use of an electron beam to locally anneal the Josephson junction. We demonstrate the ability to both increase and decrease the junction barrier resistance. The technique shows an improvement in wafer scale frequency targetting by assessing the frequency collisions in our qubit architecture. Coherence measurements are also done to evaluate the performance before and after tuning. The tuning process utilises a standard electron beam lithography system, ensuring reproducibility and implementation by any group capable of fabricating these Josephson junctions. This technique has the potential to significantly improve the performance of large-scale quantum computing systems, thereby paving the way for the future of quantum computing.

DOI
10.48550/arxiv.2402.17395

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Superconducting qubits are a promising route to achieving large-scale quantum computers. A key challenge in realising large-scale superconducting quantum processors involves mitigating frequency collisions. In this paper, we present an approach to tuning fixed-frequency qubits with the use of an electron beam to locally anneal the Josephson junction. We demonstrate the ability to both increase and decrease the junction barrier resistance. The technique shows an improvement in wafer scale frequency targetting by assessing the frequency collisions in our qubit architecture. Coherence measurements are also done to evaluate the performance before and after tuning. The tuning process utilises a standard electron beam lithography system, ensuring reproducibility and implementation by any group capable of fabricating these Josephson junctions. This technique has the potential to significantly improve the performance of large-scale quantum computing systems, thereby paving the way for the future of quantum computing.
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Variability in Josephson junction area and resistance remains a central challenge in scaling fixed-frequency superconducting qubit architectures, where resistance fluctuations translate into qubit-frequency dispersion and potential frequency collisions. Here, we present a fabrication strategy that combines large-area process control on 200 mm wafers with chip-level electrical fine-tuning to improve junction-resistance repeatability. Within a central 80 × 80 mm 2 field on 200 mm wafers, the Manhattan-style aluminum junction process yielded relative standard deviations of 5.1–5.2% in junction area (RSD A ) and 5.8–7.2% in room-temperature resistance (RSD R ). Alternating-bias assisted annealing (ABAA) further reduced chip-level RSD R to 1.3% in a chip with all 20 junctions tuned and to 1.7% for 16 tuned junctions in a cryogenically characterized 20-qubit chip. In a cross-chip comparison, the coherence distributions of the ABAA-treated chip and the untreated reference chip overlapped. These results show that large-area feed-forward correction and post-fabrication tuning can improve junction-parameter repeatability in fixed-frequency superconducting circuits.
The scalability of superconducting quantum processors is hindered by frequency collisions among qubits, calling for precise and reliable frequency tuning techniques. Here, we develop a laser annealing platform and systematically investigate its effect on superconducting qubits. In contrast to previous reports that consistently observed an increase in the Josephson junction resistance Rn, we find that laser annealing can also reduce Rn by up to 24.9%, corresponding to a significant increase in qubit frequency with a tuning precision of 22.8 MHz. This complementary behavior enriches the phenomenology of laser annealing. Atomic-resolution scanning transmission electron microscopy and electron energy-loss spectroscopy analysis reveal a reduced Al coordination number and an increased oxygen vacancy concentration, which lower the effective barrier height and account for the decrease in Rn. Importantly, laser-annealed qubits maintain high coherence, and storage at −10 °C in high vacuum suppresses long-term resistance drift to below 0.5%. These results demonstrate that laser annealing provides a practical approach for stable and accurate frequency tuning. Beyond this, our findings suggest that the effect of laser annealing is richer than previously recognized and leave open the possibility of developing more versatile, potentially bidirectional frequency control for scalable superconducting quantum processors.
Abstract As superconducting quantum circuits scale to larger sizes, the problem of frequency crowding proves a formidable task. Here we present a solution for this problem in fixed-frequency qubit architectures. By systematically adjusting qubit frequencies post-fabrication, we show a nearly tenfold improvement in the precision of setting qubit frequencies. To assess scalability, we identify the types of “frequency collisions” that will impair a transmon qubit and cross-resonance gate architecture. Using statistical modeling, we compute the probability of evading all such conditions, as a function of qubit frequency precision. We find that, without post-fabrication tuning, the probability of finding a workable lattice quickly approaches 0. However, with the demonstrated precisions it is possible to find collision-free lattices with favorable yield. These techniques and models are currently employed in available quantum systems and will be indispensable as systems continue to scale to larger sizes.
Superconducting quantum processors are one of the leading platforms for realizing scalable fault-tolerant quantum computation (FTQC). The recent demonstration of post-fabrication tuning of Josephson junctions using alternating-bias assisted annealing (ABAA) technique and a reduction in junction loss after ABAA illuminates a promising path towards precision tuning of qubit frequency while maintaining high coherence. Here, we demonstrate precision tuning of the maximum∣0⟩→∣1⟩\vert 0\rangle\rightarrow\vert 1\rangle∣0⟩→∣1⟩transition frequency(f01max⁡)(f_{01}^{\max})(f01max​)of tunable transmon qubits by performing ABAA at room temperature using commercially available test equipment. We characterize the impact of junction relaxation and aging on resistance spread after tuning, and demonstrate a frequency equivalent tuning precision of 7.7 MHz (0.17%) based on targeted resistance tuning on hundreds of qubits, with a resistance tuning range up to 18.5%. Cryogenic measurements on tuned and untuned qubits show evidence of improved coherence after ABAA with no significant impact on tunability. Despite a small global offset, we show an empiricalf01max⁡f_{01}^{\max}f01max​tuning precision of 18.4 MHz by tuning a set of multi-qubit processors targeting their designed Hamiltonians. We experimentally characterize high-fidelity parametric resonance iSWAP gates on two ABAA-tuned 9-qubit processors with fidelity as high as 99.51±0.20%\pm 0.20{\%}±0.20%. On the best-performing device, we measured across the device a median fidelity of 99.22% and an average fidelity of 99.13$\pm$0.12%. Yield modeling analysis predicts high detuning-edge-yield using ABAA beyond the 1000-qubit scale. These results demonstrate the cutting-edge capability of frequency targeting using ABAA and open up a new avenue to systematically improving Hamiltonian targeting and optimization for scaling high-performance superconducting quantum processors.
The reproducibility of qubit parameters is a challenge for scaling up superconducting quantum processors. Signal cross talk imposes constraints on the frequency separation between neighboring qubits. The frequency uncertainty of transmon qubits arising from the fabrication process is attributed to deviations in the Josephson junction area, tunnel barrier thickness, and the qubit shunt capacitor. We decrease the sensitivity to these variations by fabricating larger Josephson junctions and reduce the wafer-level standard deviation in resistance down to 2%. We characterize 32 identical transmon qubits and demonstrate the reproducibility of the qubit frequencies with a 40 MHz standard deviation (i.e., 1%) with qubit quality factors exceeding 2 million. We perform two-level-system (TLS) spectroscopy and observe no significant increase in the number of TLSs causing qubit relaxation. We further show by simulation that for our parametric-gate architecture, and accounting only for errors caused by the uncertainty of the qubit frequency, we can scale up to 100 qubits with an average of only three collisions between quantum-gate transition frequencies, assuming 22%2 cross talk and 99.999.9%99.9 target gate fidelity.