Standard quantum computation is based on sequences of unitary quantum logic gates that process qubits. The one-way quantum computer proposed by Raussendorf and Briegel is entirely different. It has changed our understanding of the requirements for quantum computation and more generally how we think about quantum physics. This new model requires qubits to be initialized in a highly entangled cluster state. From this point, the quantum computation proceeds by a sequence of single-qubit measurements with classical feedforward of their outcomes. Because of the essential role of measurement, a one-way quantum computer is irreversible. In the one-way quantum computer, the order and choices of measurements determine the algorithm computed. We have experimentally realized four-qubit cluster states encoded into the polarization state of four photons. We characterize the quantum state fully by implementing experimental four-qubit quantum state tomography. Using this cluster state, we demonstrate the feasibility of one-way quantum computing through a universal set of one- and two-qubit operations. Finally, our implementation of Grover's search algorithm demonstrates that one-way quantum computation is ideally suited for such tasks.
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We thank H. J. Briegel, D. Browne and M. Zukowski for theoretical discussions, and C. Först for assistance with graphics. This work was supported by the Austrian Science Foundation (FWF), NSERC, the European Commission under project RAMBOQ, and by the Alexander von Humboldt Foundation.
The authors declare that they have no competing financial interests.
The state fidelities of the output qubits from one-qubit and two-qubit quantum computations. (DOC 189 kb)
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