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Coherent control of macroscopic quantum states in a single-Cooper-pair box


A nanometre-scale superconducting electrode connected to a reservoir via a Josephson junction constitutes an artificial two-level electronic system: a single-Cooper-pair box. The two levels consist of charge states (differing by 2e, where e is the electronic charge) that are coupled by tunnelling of Cooper pairs through the junction. Although the two-level system is macroscopic, containing a large number of electrons, the two charge states can be coherently superposed1,2,3,4. The Cooper-pair box has therefore been suggested5,6,7 as a candidate for a quantum bit or ‘qubit’—the basic component of a quantum computer. Here we report the observation of quantum oscillations in a single-Cooper-pair box. By applying a short voltage pulse via a gate electrode, we can control the coherent quantum state evolution: the pulse modifies the energies of the two charge states non-adiabatically, bringing them into resonance. The resulting state—a superposition of the two charge states—is detected by a tunnelling current through a probe junction. Our results demonstrate electrical coherent control of a qubit in a solid-state electronic device.

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Figure 1: Single-Cooper-pair box with a probe junction.
Figure 2: Pulse modulation of quantum states.
Figure 3: Effect of applying pulses as a function of d.c.-induced charge Q 0 and pulse length Δt.
Figure 4: Pulse-induced current as a function of the pulse length Δt.


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We thank W. Hattori, M. Baba and H. Suzuki for experimental help and M. Ueda, Y.Kohno, M. H. Devoret and Y. Ootuka for discussions. This work was supported by the Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology Corporation (JST).

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Nakamura, Y., Pashkin, Y. & Tsai, J. Coherent control of macroscopic quantum states in a single-Cooper-pair box. Nature 398, 786–788 (1999).

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