Quantum dynamics and leakage of the superconducting flux qubit

We study the quantum dynamics of the superconducting flux qubit of Mooij et al, which consists on a SQUID with 3 Josephson junctions. We simulate the corresponding time-dependent Schrodinger equation and also eigenfunctions and eigenvalues are computed. We calculate the dynamical evolution of the qubit device after a pulse in magnetic field is applied from the operational point at half flux quantum. The system is started in the lowest energy level and we calculate the leakage into the non-computational basis after the pulse is applied, computing the change in the average population of the two lowest energy levels. The leakage is analyzed for different pulse intensities. Two different regimes are found for weak and strong pulses. We discuss the relationship of the response to strong pulses with the quantum chaoticity of the spectrum of high energy levels outside the computational space.