  We experimentally demonstrate the coherent oscillations of a tunable
superconducting flux qubit by manipulating its energy potential with a
nanosecond-long pulse of magnetic flux. The occupation probabilities of two
persistent current states oscillate at a frequency ranging from 6 GHz to 21
GHz, tunable via the amplitude of the flux pulse. The demonstrated operation
mode allows to realize quantum gates which take less than 100 ps time and are
thus much faster compared to other superconducting qubits. An other advantage
of this type of qubit is its insensitivity to both thermal and magnetic field
fluctuations.
