Study of a two-state fluctuation in a ultra-strongly coupled qubit-resonator system

We report experimental results of two-state-fluctuation (TSF) in a ultra-strongly coupled superconducting flux qubit-resonator system. Rabi model Hamiltonian well known as the representation of the qubit-resonator coupling system, but the spectrum will be reported in this talk has doubly split structure that cannot be described by the Rabi model. This is because the system has the two states of different parameters and the state fluctuate between them. Therefore, doubly split spectrum was observed in the vector network analyzer measurement. It is observed that the value of the splitting also fluctuates randomly with time by repeating the spectrum measurement and the fluctuation shows the Gaussian distribution.
This TSF phenomena could come from a charge offset fluctuation in the superconducting islands in the circuit. The value of the split could be predicted form the circuit parameters. Understanding the mechanism of the fluctuation and its dynamics is important to make noise-resilient superconducting qubits for practical quantum computers.