Engineering the coupling of superconducting qubits

Ciani, Alessandro; DiVincenzo, David (Thesis advisor); Hassler, Fabian (Thesis advisor)

Aachen (2019)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019


The way to build a scalable and reliable quantum computer that truly exploits the quantum power faces several challenges. Among the various proposals for building a quantum computer, superconducting qubits have rapidly progressed and hold good promises in the near-term future. In particular, the possibility to design the required interactions is one of the most appealing features of this kind of architecture. This thesis deals with some detailed aspects of this problem focusing on architectures based on superconducting transmon-like qubits. After reviewing the basic tools needed for the study of superconducting circuits and the main kinds of superconducting qubits, we move to the analyisis of a possible scheme for realizing direct parity measurement. Parity measurements, or in general stabilizer measurements, are fundamental tools for realizing quantum error correcting codes, that are believed to be fundamental for dealing with the problem of decoherence that affects any physical implementation of a quantum computer. While these measurements are usually done indirectly with the help of ancilla qubits, the scheme that we analyze performs the measurement directly, and requires the engineering of a precise matching condition. We show how sufficient freedom in the design of the interactions can be achieved with tunable coupling qubits, which are a variant of transmon qubits. In the second part of the thesis, we study instead an alternative model for quantum computation and a possible realization with transmon qubits. The model performs a quantum computation with a time-independent Hamiltonian and it is closely connected to one of the original proposals for a quantum computer due to Feynman. After explaining the basic ideas of the model, we also discuss a new version with a modified dynamics compared to previous proposals, which maps exactly to Feynman’s original idea, and also how to realize a Toffoli gate in the model. We then move to the analysis of an implementation with transmon qubits. We show how it is possible to engineer the Hamiltonian that performs the desired computation in a completely passive way, and with the desired range of parameters, limiting spurious, unwanted terms.