Tunnel Junction Characterization and Optimization for Quantum Computing Applications

量子计算有望彻底改变现代信息时代。最先进的量子计算平台之一使用超导量子比特。与其他利用俘获离子和光子等基本粒子的量子计算技术相比，超导量子比特是在宏观电磁电路中利用远程多体波函数实现的。这使得超导量子比特可以被设计成实现许多理想的特性，如量子比特之间的可调谐耦合、控制电子和环境浴。就像传统逻辑电路中的晶体管一样，超导量子比特的核心部件是约瑟夫森结。这些装置是使用两个超导电极之间的薄氧化屏障来实现的。电路中的噪声和耗散对该势垒中的缺陷呈指数级敏感。例如，氧化物屏障内部或附近的单原子缺陷可能在位置上波动，导致隧道速率中的低频噪声（1/f噪声）。另一个故障是绝缘体屏障中存在非常小的断裂，通常被称为“引脚孔”，这可能导致从一个超导导线到另一个超导导线的小电流泄漏。在这个项目中，我们与加拿大领先的量子计算技术公司D-Wave Systems合作，计划开发几个关键的表征工具，用于在50 mK工作温度（低于绝对零度的十分之一度）下评估亚微米约瑟夫森结。然后，这些工具将用于优化约瑟夫森结制造工艺，重点是尽量减少1/f噪声，rf损耗和泄漏电流。然后通过测量D-Wave超导量子比特来评估结质量的改进，我们最终期望看到性能的显著提高。

Quantum computing promises to revolutionize the modern information age. One of the most advanced quantum computing platforms uses superconducting qubits. Compared with other quantum computing technologies utilizing fundamental particles such as trapped ions and photons, superconducting qubits are implemented with the long range many-body wavefunction in a macroscopic electromagnetic circuit. This allows superconducting qubits to be engineered to realize a multitude of desirable properties such as tunable coupling between qubits, control electronics, and the environmental bath. Much like the transistor for conventional logical circuits, the core component of superconducting qubits is the Josephson junction. These devices are realized using a thin oxide barrier between two superconducting electrodes. The noise and dissipation in the circuit is exponentially sensitive to defects in this barrier. For example, single atomic defects within or in the vicinity of the oxide barrier can fluctuate in position causing a low frequency noise (1/f noise) in the tunneling rate. Another fault is the existence of very small breaks in the insulator barrier, commonly known as "pin holes", that can cause a small current leakage from one superconducting lead to the other. In this project, working with D-Wave Systems, a leading quantum computing technology company based in Canada, we plan to develop several key characterization tools for assessing sub-micron Josephson junctions at a 50 mK operating temperature (below a tenth of a degree above absolute zero). These tools will then be used to optimize the Josephson junction fabrication process, focusing on minimizing 1/f noise, rf losses, and leakage currents. The improvements in junction quality will then be assessed by measuring D-Wave superconducting qubits where we ultimately expect to see significant gains in performance.