  In the emerging field of quantum computation and quantum information,
superconducting devices are promising candidates for the implementation of
solid-state quantum bits or qubits. Single-qubit operations, direct coupling
between two qubits, and the realization of a quantum gate have been reported.
However, complex manipulation of entangled states - such as the coupling of a
two-level system to a quantum harmonic oscillator, as demonstrated in
ion/atom-trap experiments or cavity quantum electrodynamics - has yet to be
achieved for superconducting devices. Here we demonstrate entanglement between
a superconducting flux qubit (a two-level system) and a superconducting quantum
interference device (SQUID). The latter provides the measurement system for
detecting the quantum states; it is also an effective inductance that, in
parallel with an external shunt capacitance, acts as a harmonic oscillator. We
achieve generation and control of the entangled state by performing microwave
spectroscopy and detecting the resultant Rabi oscillations of the coupled
system.
