  Parametric control of a superconducting flux qubit has been achieved by using
two-frequency microwave pulses. We have observed Rabi oscillations stemming
from parametric transitions between the qubit states when the sum of the two
microwave frequencies or the difference between them matches the qubit Larmor
frequency. We have also observed multi-photon Rabi oscillations corresponding
to one- to four-photon resonances by applying single-frequency microwave
pulses. The parametric control demonstrated in this work widens the frequency
range of microwaves for controlling the qubit and offers a high quality testing
ground for exploring nonlinear quantum phenomena.
