1. Institute of Experimental Physics, University of Vienna, Boltzmanngasse 5, 1090 Vienna, Austria
2. QOLS, Blackett Laboratory, Imperial College London, London SW7 2BW, UK
3. Department of Physics, Ludwig Maximilians University, D-80799 Munich, Germany
4. Max Planck Institute for Quantum Optics, D-85741 Garching, Germany
5. The Erwin Schrödinger Institute for Mathematical Physics, Boltzmanngasse 9, 1090 Vienna, Austria
6. The School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
7. IQOQI, Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austria
8. Permanent address: Ecole normale supérieure, 45, rue d'Ulm, 75005 Paris, France

Correspondence to: P. Walther¹A. Zeilinger^1,7 Correspondence and requests for materials should be addressed to P.W. (Email: pwalther@quantum.at) or A.Z. (Email: zeilinger-office@quantum.at).

Abstract

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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