High-speed linear optics quantum computing using active feed-forward


As information carriers in quantum computing1, photonic qubits have the advantage of undergoing negligible decoherence. However, the absence of any significant photon–photon interaction is problematic for the realization of non-trivial two-qubit gates. One solution is to introduce an effective nonlinearity by measurements resulting in probabilistic gate operations2,3. In one-way quantum computation4,5,6,7,8, the random quantum measurement error can be overcome by applying a feed-forward technique, such that the future measurement basis depends on earlier measurement results. This technique is crucial for achieving deterministic quantum computation once a cluster state (the highly entangled multiparticle state on which one-way quantum computation is based) is prepared. Here we realize a concatenated scheme of measurement and active feed-forward in a one-way quantum computing experiment. We demonstrate that, for a perfect cluster state and no photon loss, our quantum computation scheme would operate with good fidelity and that our feed-forward components function with very high speed and low error for detected photons. With present technology, the individual computational step (in our case the individual feed-forward cycle) can be operated in less than 150 ns using electro-optical modulators. This is an important result for the future development of one-way quantum computers, whose large-scale implementation will depend on advances in the production and detection of the required highly entangled cluster states.

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Figure 1: Schematic drawing of the experimental set-up.
Figure 2: Active feed-forward of two different single-qubit rotations.
Figure 3: Feed-forward of a two-qubit operation.
Figure 4: Demonstration of Grover’s search algorithm with feed-forward.


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We are grateful to M. Aspelmeyer, Č. Brukner, J. I. Cirac, J. Kofler and K. Resch for discussions as well as to T. Bergmann and G. Mondl for assistance with the electronics. R.P. thanks E.-M. Röttger for assistance in the laboratory. We acknowledge financial support from the Austrian Science Fund (FWF), the European Commission under the Integrated Project Qubit Applications (QAP) funded by the IST directorate and the DTO-funded US Army Research Office.

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Correspondence to Robert Prevedel or Anton Zeilinger.

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Prevedel, R., Walther, P., Tiefenbacher, F. et al. High-speed linear optics quantum computing using active feed-forward. Nature 445, 65–69 (2007). https://doi.org/10.1038/nature05346

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