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Quantum oscillations in two coupled charge qubits


A practical quantum computer1, if built, would consist of a set of coupled two-level quantum systems (qubits). Among the variety of qubits implemented2, solid-state qubits are of particular interest because of their potential suitability for integrated devices. A variety of qubits based on Josephson junctions3,4 have been implemented5,6,7,8; these exploit the coherence of Cooper-pair tunnelling in the superconducting state5,6,7,8,9,10. Despite apparent progress in the implementation of individual solid-state qubits, there have been no experimental reports of multiple qubit gates—a basic requirement for building a real quantum computer. Here we demonstrate a Josephson circuit consisting of two coupled charge qubits. Using a pulse technique, we coherently mix quantum states and observe quantum oscillations, the spectrum of which reflects interaction between the qubits. Our results demonstrate the feasibility of coupling multiple solid-state qubits, and indicate the existence of entangled two-qubit states.

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Figure 1: Two capacitively coupled charge qubits.
Figure 2: Pulse operation of device.
Figure 3: Quantum oscillations in qubits.
Figure 4: EJ1 dependence of the spectrum components obtained by Fourier transform of the oscillations at the co-resonance.

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We thank B. L. Altshuler, X. D. Hu, H. Im, S. Ishizaka, F. Nori, T. Sakamoto and J. Q. You for discussions. D.V.A. was supported by AFORS, and by NSA and ARDA under an ARO contract.

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Correspondence to J. S. Tsai.

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Pashkin, Y., Yamamoto, T., Astafiev, O. et al. Quantum oscillations in two coupled charge qubits. Nature 421, 823–826 (2003).

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