Coherent dynamics of a flux qubit coupled to a harmonic oscillator

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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.

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Figure 1: Qubit–SQUID device and spectroscopy.
Figure 2: Rabi oscillations at the qubit symmetry point Δ = 5.9 GHz.
Figure 3: Generation and control of entangled states.
Figure 4: Oscillator relaxation time.


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We thank A. Blais, G. Burkard, D. DiVincenzo, G. Falci, M. Grifoni, S. Lloyd, S. Miyashita, T. Orlando, R. N. Schouten, L. Vandersyepen and F. K. Wilhelm for discussions. This work was supported by the Dutch Foundation for Fundamental Research on Matter (FOM), the EU Marie Curie and SQUBIT grants, and the US Army Research Office.

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Correspondence to I. Chiorescu or J. E. Mooij.

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The authors declare that they have no competing financial interests.

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Chiorescu, I., Bertet, P., Semba, K. et al. Coherent dynamics of a flux qubit coupled to a harmonic oscillator. Nature 431, 159–162 (2004) doi:10.1038/nature02831

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