Quantum information processing with tunable and low-loss superconducting circuits

Doctoral thesis, 2020

Today's computer can perform many extraordinary tasks. However, there are problems that not even the most powerful computers in the world are able to solve. To overcome some of the fundamental issues with ordinary computers, it has been suggested to use quantum mechanical systems to store and process information. Quantum computers are highly specialized machines that promise an enormous speed-up for specific problems. Calculations that would take thousands of years could instead be performed in minutes. Currently, prototypes of such computers are being built and researched. This thesis touches on many different topics related to how to build and characterize quantum computers using superconducting circuits.

Information stored in quantum systems is fragile and can easily be lost. In superconducting circuits, one source of loss is defects on the surface of the device. In this thesis, we have developed fabrication processes that minimize the number of defects and yield superconducting qubits with state-of-the-art performance. By careful analysis of the performance, we identified that our circuits are still limited by defects, which means that even better processes are needed in the future.

Nevertheless, we use our low-loss circuits to demonstrate two quantum algorithms. We show that a quantum optimization algorithm can be used to solve logistics problems for airlines trying to optimize their routes and personal assignment. Even if our quantum processor is still too small to outperform a standard computer, our demonstration shows that the algorithm works and that it could be used for real-world applications once we have large enough quantum processors.

Finally, we implement frequency-tunable circuits. By modulating the circuits at specific multiples of a fundamental frequency, we observe splittings of one high-energy light-particle (a photon) into many lower-energy photons. While we mostly investigate some basic properties, such photon-splittings could provide useful resources for quantum computing in the future.