Characterization of noise in transmon superconducting qubits and fluctuations of their parameters and performance

A functional and operable quantum computer requires high-performing qubits. The performance of a qubit is described by parameters such as coherence and fidelity, for which higher values and stability are sought. In this work, the factors that limit the performance of the qubit or lead to fluctuations in its parameters are investigated. The study is realized for transmon superconducting qubits created using conventional CMOS fabrication methods. For different qubits, long-term measurements of various parameters (e.g., T1, T∗2 ) are performed to study the deviations that occur to them over large timescales. Most of the observed deviations are attributed to TLS defects (Two-Level-System defects). Additionally, a temperature sweep is carried out to study the effect of thermal variations on the qubit. The temperature sweep shows, in particular, the influence of quasiparticles on energy relaxation and thermal photons on dephasing. It also highlights the effect of thermal cycling on TLS defects. This effect of thermal cycling was also observed through QNS (Quantum Noise Spectroscopy), which was implemented using spin locking. Performed QNS at various temperatures shows also the influence of thermal variations on the low-frequency noise.