Tunnel Junction Characterization and Optimization for Quantum Computing Applications

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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.