Oral Presentation

Cryogenic Electronics and Superconducting Devices Fabricated from In Situ Grown Heterostructures for Scalable Quantum Computation

Author(s): Lawrence Boyu Young(1,#), Yi-Ting Cheng(1,#), Hsien-Wen Wan(1,#), Wei-Jie Yan(2), Bo-Yuan Chen(3), Kun-Ming Chen(3), Hsiao-Wen Chang(4), You-Sheng Li(1), Ming-Jye Wang(4), Yen-Hsiang Lin(2), Jueinai Kwo(2,*), Minghwei Hong(1,*) 1.National Taiwan University 2.National TsingHua University 3.Taiwan Semiconductor Research Institute, National Institutes of Applied Research 4.Institute of Astronomy and Astrophysics, Academia Sinica # contribute equally *raynien@phys.nthu.edu.tw, mhong@phys.ntu.edu.tw

Presenter: Lawrence Boyu Young (Graduate Institute of Applied Physics, National Taiwan University)

Microwave transceiver technologies have transformed modern society, enabling global communication and exploration of the universe. In recent decades, superconducting quantum computation has emerged as a transformative frontier, promising to revolutionize information processing and address problems intractable for classical computers. These technologies critically depend on microwave receivers operating with ultra-low power consumption and near-quantum-limited noise. The realization of practical quantum computers, involving integration of millions of qubits, demands highly coherent superconducting components as well as ultra-low-power cryogenic electronics with minimal noise for qubit control and readout. We have made advances on both fronts, superconducting devices and cryogenic electronics, where the device components were fabricated using in situ grown heterostructures with pristine interfaces.
For superconducting devices, we have achieved high internal quality factors (Q_i) in resonators fabricated from thin aluminum (Al) films passivated by in situ deposited Al₂O₃. A maximum Q_i of approximately two million near the single-photon limit ranks among the highest reported for Al resonators. Even for 10-nm thick Al resonators, Q_i values exceeding 0.5 million were maintained, well above previously reported results at similar thicknesses. In contrast, devices capped with native AlO_x exhibited markedly lower Q_i due to insufficient protection of superconducting films from air-induced diffusion along grain boundaries. Effective surface passivation is therefore crucial for devices leveraging enhanced kinetic inductance and for Josephson-junction qubits employing ultrathin superconducting electrodes. Moreover, Al₂O₃-passivated resonators demonstrated remarkable aging resistance, retaining Q_i ≈ 5 × 10⁵ after seven months of air exposure, while AlO_x-capped counterparts degraded to ≈ 1 × 10⁵ within two weeks. These findings establish in situ Al₂O₃/Al as a high-coherence, long-term-stability material platform with direct relevance to superconducting qubits and scalable quantum information systems.
In parallel, our cryogenic electronics based on planar bulk (In)GaAs metal–oxide–semiconductor field-effect transistors (MOSFETs) exhibit record-low subthreshold swing (SS) at 4 K. The devices incorporate in situ deposited Al₂O₃/Y₂O₃ or Al₂O₃/HfO₂ dielectrics on freshly grown (In)GaAs(100), achieving interface trap densities (D_it) an order of magnitude lower than those from conventional ex situ processes. We have achieved SS values of 9 mV/dec for GaAs and 2 mV/dec for In_0.53Ga_0.47As, the lowest reported among all planar bulk (In)GaAs MOSFETs, significantly outperforming Si MOSFETs and commercial InGaAs HEMTs. Such ultra-low SS enables near-subthreshold operation with exceptionally low power consumption, high gain-to-power efficiency, and reduced noise, key attributes for cryogenic control and readout circuits in large-scale quantum processors.
The growth of contamination-free interfaces in both MOS and superconducting heterostructures, realized using our uniquely designed ultra-high-vacuum multi-chamber MBE/ALD/analysis system, is central to overcoming the critical materials challenges in these technologies. Together, these advancements on both superconducting quantum devices and cryogenic electronics are paving the way toward scalable, high-performance quantum computing and next-generation microwave systems.