Superconducting flux qubit is a promising candidate for quantum computing.
When the superconducting qubit is driven by an oscillating field, it involves
plentiful dynamics of macroscopic quantum states. I have extended the
generalized Floquet approach to investigate multiphoton quantum interference in
the superconducting flux qubit driven by intense ac fields. For resonant and
nearly resonant multiphoton transitions, the generalized Van Vleck nearly
degenerate highorder perturbation theory is extended for the analytic
treatment of the Floquet Hamiltonian, which allows accurate treatment of the ac
Stark shift, power broadening, timedependent and timeaveraged transition
probability. For the plots of the timeaveraged transition probability, the
Bessel functionlike fringe patterns are observed around multiphoton resonance
positions [1], in good agreement with recent experimental results
[Science 310, 1653 (2005)]. Applications of the Floquet theory
to various superconducting qubits lead us to a better understanding of the
results of spectroscopy measurement and the dynamics of strongly driven qubits.

Bessel functionlike fringe patterns around multiphoton resonance positions.
