製作及探討臨場沉積氧化鋁/鋁/藍寶石基板之高品質超導共振器

Abstract

量子電腦因具備量子的特性，有能力能在合理的時間內處理完當今最先進的超級電腦都無法解決的複雜問題。在眾多實現量子電腦的物理系統中，超導量子位元被認為是最具潛力的代表，引起了廣泛的關注。然而，建造有實用價值的超導量子電腦，其最大挑戰之一在於提升量子位元的同調時間，這主要是由於材料中過多的缺陷或製程中引入的額外缺陷所致。

在此研究中，我們製作了共振器，以評估材料及製程對共振器品質因子的影響。研究文獻認為，超導體表面的氧化層是導致微波能量損耗的主要來源。鋁是最常用來製作超導量子位元的材料。然而，鋁和氧化鋁的選擇比不高，因此難以在不損傷鋁薄膜的情況下去除表面氧化層。為了減少因氧化層導致的能量損耗，我們使用電子束蒸鍍的方式在鋁薄膜上臨場沉積了一層薄薄的氧化鋁，以抑制原生氧化物的形成。這層刻意沉積的氧化層，其組成原子比例接近化學計量比，預估將具有更低的缺陷密度，因此能減少能量損耗的來源。此外，我們致力於改善每一步製程步驟，以確保所製作的共振器具有最高的品質因子。

綜合這些方法，我們製作的共振器在光子數為4時，內部品質因子穩定超過1×10^6，最高達到2×10^6。我們使用雙能階系統模型分析共振器的內部品質因子與微波功率的變化趨勢，我們得到由雙能階系統所引起的損耗為3.8×10^-7至5.3×10^-7。我們的研究結果展現了使用原位沉積氧化層鈍化超導元件的效果，為實現具有高同調時間的量子位元提供了重要的基礎。

A quantum computer potentially outperforms the most advanced supercomputer in solving complicated problems. Among all the candidates for realizing a quantum computer, superconducting qubits have drawn tremendous attention from global research teams because of their compatibility with commercial microwave equipment for qubit operation and the scalability facilitated by semiconductor microfabrication technologies. However, a main hindrance to building up a useful superconducting quantum computer at scale lies in the limited coherence time of qubits, primarily caused by defects intrinsically existing in device materials or introduced during fabrication. We fabricated resonators to assess the impact of material growth and fabrication processes. The surface oxide of the superconductor has been identified as one major source of microwave energy loss. Aluminum, a widely used material for superconducting qubits, suffers from the difficulty of selectively removing its surface oxide without being damaged. To mitigate the loss originating from the defective surface oxide, we in situ deposited a thin layer of aluminum oxide by e-beam evaporation onto aluminum films to suppress the formation of native oxide. The deposited oxide is near-stoichiometric and presumably hosts fewer defects than native oxide, thereby reducing energy relaxation channels. Additionally, we meticulously optimized each step of the processes, including photoresist patterning, etching, and photoresist removal, to ensure the highest quality of the fabricated resonators. Through these combined efforts, we demonstrated that the internal quality factors of the resonators consistently surpassed 1×10^6 at the lowest photon density of 4, with the highest value of 2×10^6. By analyzing the power dependence of the internal quality factor using a two-level system model, we determined that loss contributed by two-level systems ranges from 3.8×10^-7 to 5.3×10^-7. Our results demonstrate the effectiveness of passivating superconducting devices with in situ deposited oxide, paving the way toward realizing qubits with high coherence time.