Coherent dynamics of a flux qubit coupled to a harmonic oscillator

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Abstract
In the emerging field of quantum computation1 and quantum information, superconducting devices are promising candidates for the implementation of solid-state quantum bits (qubits). Single-qubit operations2,3,4,5,6, direct coupling between two qubits7,8,9,10 and the realization of a quantum gate11 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 experiments12,13 and cavity quantum electrodynamics14—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.