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Demonstration of conditional gate operation using superconducting charge qubits

Abstract

Following the demonstration of coherent control of the quantum state of a superconducting charge qubit1, a variety of qubits based on Josephson junctions have been implemented2,3,4,5. Although such solid-state devices are not currently as advanced as microscopic qubits based on nuclear magnetic resonance6 and ion trap7 technologies, the potential scalability of the former systems—together with progress in their coherence times and read-out schemes—makes them strong candidates for the building block of a quantum computer8. Recently, coherent oscillations9 and microwave spectroscopy10 of capacitively coupled superconducting qubits have been reported; the next challenging step towards quantum computation is the realization of logic gates11,12. Here we demonstrate conditional gate operation using a pair of coupled superconducting charge qubits. Using a pulse technique, we prepare different input states and show that their amplitude can be transformed by controlled-NOT (C-NOT) gate operation, although the phase evolution during the gate operation remains to be clarified.

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Figure 1: Pulse operation of the coupled-qubit device.
Figure 2: Magnetic-flux dependence of the current of the control (top) and target (bottom) qubits under the application of pulses shown in Fig. 1c (i).
Figure 3: Pulse-induced current as a function of the Josephson energy of the control qubit.
Figure 4: Truth table of the present C-NOT operation estimated by the numerical calculation (solid blue bars).

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Acknowledgements

We thank B. L. Altshuler, D. V. Averin, S. Ishizaka, F. Nori, T. Tilma, C. Urbina and J. Q. You for discussions.

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Correspondence to T. Yamamoto.

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Yamamoto, T., Pashkin, Y., Astafiev, O. et al. Demonstration of conditional gate operation using superconducting charge qubits. Nature 425, 941–944 (2003). https://doi.org/10.1038/nature02015

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