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Science 4 May 2007:
Vol. 316. no. 5825, pp. 723 - 726
DOI: 10.1126/science.1141324
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Reports

Quantum Coherent Tunable Coupling of Superconducting Qubits

A. O. Niskanen,1,4 K. Harrabi,1 F. Yoshihara,2 Y. Nakamura,1,2,3* S. Lloyd,5 J. S. Tsai1,2,3

To do large-scale quantum information processing, it is necessary to control the interactions between individual qubits while retaining quantum coherence. To this end, superconducting circuits allow for a high degree of flexibility. We report on the time-domain tunable coupling of optimally biased superconducting flux qubits. By modulating the nonlinear inductance of an additional coupling element, we parametrically induced a two-qubit transition that was otherwise forbidden. We observed an on/off coupling ratio of 19 and were able to demonstrate a simple quantum protocol.

1 CREST, Japan Science and Technology Institute, Kawaguchi, Saitama 332-0012, Japan.
2 The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan.
3 NEC Fundamental and Environmental Research Laboratories, Tsukuba, Ibaraki 305-8501, Japan.
4 VTT Technical Research Centre of Finland, Sensors, Post Office Box 1000, 02044 VTT, Finland.
5 Massachusetts Institute of Technology, Cambridge, MA 02139, USA.

* To whom correspondence should be addressed. E-mail: yasunobu{at}ce.jp.nec.com

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Observation of Berry's Phase in a Solid-State Qubit.
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