  We have observed the coherent exchange of a single energy quantum between a
flux qubit and a superconducting LC circuit acting as a quantum harmonic
oscillator. The exchange of an energy quantum is known as the vacuum Rabi
oscillations: the qubit is oscillating between the excited state and the ground
state and the oscillator between the vacuum state and the first excited state.
We have also obtained evidence of level quantization of the LC circuit by
observing the change in the oscillation frequency when the LC circuit was not
initially in the vacuum state.
