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Coupling superconducting qubits via a cavity bus


Superconducting circuits are promising candidates for constructing quantum bits (qubits) in a quantum computer; single-qubit operations are now routine1,2, and several examples3,4,5,6,7,8,9 of two-qubit interactions and gates have been demonstrated. These experiments show that two nearby qubits can be readily coupled with local interactions. Performing gate operations between an arbitrary pair of distant qubits is highly desirable for any quantum computer architecture, but has not yet been demonstrated. An efficient way to achieve this goal is to couple the qubits to a ‘quantum bus’, which distributes quantum information among the qubits. Here we show the implementation of such a quantum bus, using microwave photons confined in a transmission line cavity, to couple two superconducting qubits on opposite sides of a chip. The interaction is mediated by the exchange of virtual rather than real photons, avoiding cavity-induced loss. Using fast control of the qubits to switch the coupling effectively on and off, we demonstrate coherent transfer of quantum states between the qubits. The cavity is also used to perform multiplexed control and measurement of the qubit states. This approach can be expanded to more than two qubits, and is an attractive architecture for quantum information processing on a chip.

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Figure 1: Sample and scheme used to couple two qubits to an on-chip microwave cavity.
Figure 2: Cavity transmission and spectroscopy of single and coupled qubits.
Figure 3: Multiplexed control and read-out of uncoupled qubits.
Figure 4: Controllable effective coupling and coherent state transfer via off-resonant Stark shift.


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This work was supported in part by the National Security Agency under the Army Research Office, by the NSF, and by Yale University. J.M.C. acknowledges support from an NSF Graduate Research Fellowship. J.K. and A.A.H. acknowledge support from Yale University via a Quantum Information and Mesoscopic Physics Fellowship. L.F. acknowledges partial support from the CNR-Istituto di Cibernetica, Pozzuoli, Italy. A.B. was supported by NSERC, CIAR and FQRNT.

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Correspondence to J. Majer or R. J. Schoelkopf.

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Majer, J., Chow, J., Gambetta, J. et al. Coupling superconducting qubits via a cavity bus. Nature 449, 443–447 (2007).

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