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Demonstration of two-qubit algorithms with a superconducting quantum processor


Quantum computers, which harness the superposition and entanglement of physical states, could outperform their classical counterparts in solving problems with technological impact—such as factoring large numbers and searching databases1,2. A quantum processor executes algorithms by applying a programmable sequence of gates to an initialized register of qubits, which coherently evolves into a final state containing the result of the computation. Building a quantum processor is challenging because of the need to meet simultaneously requirements that are in conflict: state preparation, long coherence times, universal gate operations and qubit readout. Processors based on a few qubits have been demonstrated using nuclear magnetic resonance3,4,5, cold ion trap6,7 and optical8 systems, but a solid-state realization has remained an outstanding challenge. Here we demonstrate a two-qubit superconducting processor and the implementation of the Grover search and Deutsch–Jozsa quantum algorithms1,2. We use a two-qubit interaction, tunable in strength by two orders of magnitude on nanosecond timescales, which is mediated by a cavity bus in a circuit quantum electrodynamics architecture9,10. This interaction allows the generation of highly entangled states with concurrence up to 94 per cent. Although this processor constitutes an important step in quantum computing with integrated circuits, continuing efforts to increase qubit coherence times, gate performance and register size will be required to fulfil the promise of a scalable technology.

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Figure 1: Two-qubit cQED device, and cavity/qubit characterization.
Figure 2: Origin and characterization of the controlled-phase gate.
Figure 3: Entanglement on demand.
Figure 4: Implementation of Grover's search algorithm.


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We thank V. Manucharyan and E. Boaknin for experimental contributions, and M. H. Devoret, I. L. Chuang and A. Nunnenkamp for discussions. This work was supported by LPS/NSA under ARO contract W911NF-05-1-0365, and by the NSF under grants DMR-0653377 and DMR-0603369. We acknowledge additional support from CIFAR, MRI, MITACS and NSERC (J.M.G.), NSERC, CIFAR and the Alfred P. Sloan Foundation (A.B.), and from CNR-Istituto di Cibernetica, Pozzuoli, Italy (L.F.).

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Correspondence to R. J. Schoelkopf.

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This file contains Supplementary Data, Supplementary References and Figures S1-S3 with Legends. Supplementary Information was corrected on 01 July 2009. (PDF 353 kb)

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DiCarlo, L., Chow, J., Gambetta, J. et al. Demonstration of two-qubit algorithms with a superconducting quantum processor. Nature 460, 240–244 (2009).

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