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Chinese Science Bulletin (Chinese Version)

超导量子比特中铝超导隧道结的性能

俊杰 常, 国柱 孙, 争鸣 吉, 伟伟 许, 健 陈, 丹丹 沈, 春海 曹, 冉 朱

Published Nov 1, 2011• 1 citation
Publisher

Abstract

超导隧道结约瑟夫森结是超导量子比特的基本元件. 用双层光刻胶构成悬空掩模,用电子束斜蒸发的方法制备Al/Al2O3/Al 隧道结, 用一定的氧气气压氧化铝膜作为势垒层.势垒层质量直接影响铝超导隧道结的各项电学参数. 深入研究了铝超导隧道结的超导临界电流密度Jc和面积归一化电阻Rc(正常态电阻RN与结面积的乘积)与势垒层生长条件的关系,通过改变势垒层生长的氧分压与氧化时间来控制铝超导隧道结的Jc 和Rc, 以使此工艺所制备的铝隧道结漏电流小, 结参数易于控制, 满足制备超导量子比特的要求.

DOI
10.1360/csb2011-56-34-2923

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超导隧道结约瑟夫森结是超导量子比特的基本元件. 用双层光刻胶构成悬空掩模,用电子束斜蒸发的方法制备Al/Al2O3/Al 隧道结, 用一定的氧气气压氧化铝膜作为势垒层.势垒层质量直接影响铝超导隧道结的各项电学参数. 深入研究了铝超导隧道结的超导临界电流密度Jc和面积归一化电阻Rc(正常态电阻RN与结面积的乘积)与势垒层生长条件的关系,通过改变势垒层生长的氧分压与氧化时间来控制铝超导隧道结的Jc 和Rc, 以使此工艺所制备的铝隧道结漏电流小, 结参数易于控制, 满足制备超导量子比特的要求.
摘要 本文在各向异性介质中电磁场二次量子化的基础上,探讨做高匀速运动(超过该介质中的光速)的带电粒子辐射状况,得到了寻常波和非常波辐射强度的表达式,解释了粒子的自旋状态对辐射强度的影响。在粒子的运动方向沿晶体光轴和垂直于晶体光轴的两种情况下,对结果进行了分析。结果表明,量子方法的计算有别于经典结果,当考虑到粒子自旋时,在沿光轴这动的情况下,既辐射寻常波又辐射非常波。 Abstract 作者及机构信息 吕景发 Authors and contacts 参考文献 [1] 施引文献
超导量子比特作为利用约瑟夫森结实现量子位的方案之一,具有易集成、易与外界耦合等优点.用双层光刻胶通过深紫外光曝光制备悬空掩膜结构,Al作为超导材料,采用电子束斜蒸发及静态氧化的方法制备了超导磁通量子比特样品.通过对Al薄膜厚度与平整度关系分析,选取60 nm作为隧道结底层Al厚度,增加蒸发源到样品衬底间的距离,提高了Al薄膜平整度,降低了隧道结漏电流.通过透射电子显微镜分析,观测到Al隧道结三层结构,并得到了势垒层厚度和氧分压的关系.
We have characterized the microwave loss of high-Q parallel plate capacitors fabricated from thin-film Al/Al2O3/Re heterostructures on (0001) Al2O3 substrates. The superconductor-insulator-superconductor trilayers were grown in situ in a hybrid deposition system: the epitaxial Re base and polycrystalline Al counterelectrode layers were grown by sputtering, while the epitaxial Al2O3 layer was grown by pulsed laser deposition. Structural analysis indicates a highly crystalline epitaxial Al2O3 layer and sharp interfaces. The measured intrinsic (low-power, low-temperature) quality factor of the resonators is as high as 3 × 104. These results indicate that low-loss grown Al2O3 is an attractive candidate dielectric for high-fidelity superconducting qubit circuits.
Dissipative two-level systems (TLS) have been a long-standing problem in glassy solids over the last fifty years, and have recently gained new relevance as sources of decoherence in quantum computing. Resonant absorption by TLSs in the dielectric poses a serious limitation to the performance of superconducting qubits; however, the microscopic nature of these systems has yet to be established. Based on first-principles calculations, we propose that hydrogen impurities in Al2O3 are the main source of TLS resonant absorption. Hydrogen is an ubiquitous impurity and can easily incorporate in Al2O3. We find that interstitial H in Al2O3 forms a hydrogen bond (O-H...O). At specific O-O distances, consistent with bond lengths found in amorphous Al2O3 or near Al2O3 surfaces or interfaces, the H atom feels a double well. Tunneling between two symmetric positions gives rise to resonant absorption in the range of 10 GHz, explaining the experimental observations. We also calculate the expected qubit-TLS coupling and find it to lie between 16 and 20 MHz, consistent with experimental measurements.