Coherent oscillations in a superconducting flux qubit without microwave pulses

We report on observation of coherent oscillations in a superconducting flux qubit by using no microwave excitation but only nanosecond-long dc flux pulses. The investigated circuit is a double-SQUID consisting of a superconducting loop interrupted by a small dc-SQUID, which we control via two bias fluxes φc and φx. The potential energy profile of the qubit has the shape of a double well, where the flux φc controls the height of the barrier between the two minima and the flux φx changes the potential symmetry. The two computational states of the qubit are identified with the two energy minima and physically correspond to clockwise or anticlockwise circulating currents in the double-SQUID main loop. We observed coherent oscillations, in the frequency range between 8 and 20 GHz, induced by fast pulses of the control flux φc modulating the barrier between the two potential wells. The quantum dynamics that leads to this kind of oscillations is composed of a non-adiabatic and adiabatic evolution of the two lowest energy states