Modeling and suppressing unwanted parasitic interactions in superconducting circuits

Xu, Xuexin; DiVincenzo, David (Thesis advisor); Hassler, Fabian (Thesis advisor)

Aachen : RWTH Aachen University (2021, 2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021


Superconducting qubits are based on collective excitations in Josephson junctions. Their sufficiently high controllability and low-noise interaction with one another make them a viable candidate for implementing large-scale quantum computation. Although there are great improvements in the coherence of these qubits, the progression towards building a fault-tolerant quantum computer is still a major mission. This issue can be reflected in the imperfect gate fidelity. A source of such infidelity is the fundamental parasitic interaction between coupled qubits. This thesis deals with such spurious interaction in two- and three-qubit circuits. The parasitic interaction reflects a bending between computational and noncomputational levels. This bending creates a parasitic ZZ interaction. We first study the possibility of zeroing the ZZ interaction in two qubit combinations: a pair of interacting transmons, as well as a hybrid pair of transmon coupled to capacitively shunted flux qubit (CSFQ). We utilize our theory to accurately simulate experimental results taken by our collaborators from measuring a CSFQ-transmon pair in the absence and presence of cross-resonance (CR) gate. The impressive agreements between our theory and experiment motivated us to study the characteristics of a CR gate that performs with 99.9% fidelity in the absence of static ZZ interaction. Since the CR pulse produces an additional ZZ component on top of the static part, we propose a new strategy for zeroing total ZZ interaction, and name it dynamical ZZ freedom. This freedom can exist in all-transmon circuits and allow to make perfect entanglement. Based on our findings, we propose a new two-qubit gate, namely the parasitic-free (PF) gate. Moreover, we show that we can take advantage of the ZZ interaction and make it stronger in order to perform a controlled-Z gate. Finally, we study the impact of a third qubit on the two-qubit gate performance and discuss several examples to illustrate the properties of two-body ZZ and three-body ZZZ interactions in circuits with more than two qubits.