Coherent Dynamics of a Flux Qubit Coupled to a Harmonic Oscillator

In the emerging field of quantum computation 1 and quantum information, superconducting devices are promising candidates for the implementation of solid-state quantum bits (qubits). Single-qubit operations 2 , 3 , 4 , 5 , 6 , direct coupling between two qubits 7 , 8 , 9 , 10 and the realization of a quantum gate 11 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 12 , 13 and cavity quantum electrodynamics 14 —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.