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Generating, manipulating and measuring entanglement and mixture with a reconfigurable photonic circuit



Entanglement is the quintessential quantum-mechanical phenomenon understood to lie at the heart of future quantum technologies and the subject of fundamental scientific investigations. Mixture, resulting from noise, is often an unwanted result of interaction with an environment, but is also of fundamental interest, and is proposed to play a role in some biological processes. Here, we report an integrated waveguide device that can generate and completely characterize pure two-photon states with any amount of entanglement and arbitrary single-photon states with any amount of mixture. The device consists of a reconfigurable integrated quantum photonic circuit with eight voltage-controlled phase shifters. We demonstrate that, for thousands of randomly chosen configurations, the device performs with high fidelity. We generate maximally and non-maximally entangled states, violate a Bell-type inequality with a continuum of partially entangled states, and demonstrate the generation of arbitrary one-qubit mixed states.

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Figure 1: A two-photon reconfigurable quantum circuit for generating, manipulating and detecting entanglement and mixture.
Figure 2: Classical and quantum interference fringes.
Figure 3: Statistical fidelity of photon coincidence count rates.
Figure 4: Bell states generated and characterized on-chip.
Figure 5: CHSH manifold.
Figure 6: Histogram showing the statistical distribution of quantum-state fidelity between 119 randomly chosen single-qubit target states and the corresponding mixed states generated and characterized on-chip.


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The authors thank N. Brunner, J. G. Rarity and P. Ivanov for helpful contributions. This work was supported by the Engineering and Physical Sciences Research Council (EPSRC), the European Research Council (ERC), Intelligence Advanced Research Projects Activity (IARPA), the Leverhulme Trust, the Centre for Nanoscience and Quantum Information (NSQI), PHORBITECH, the Quantum Information Processing Interdisciplinary Research Collaboration (QIP IRC), and the Quantum Integrated Photonics (QUANTIP) project. J.L.O'B. acknowledges a Royal Society Wolfson Merit Award. M.L. acknowledges the Marie Curie International Incoming Fellowship.

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Correspondence to J. L. O'Brien.

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Shadbolt, P., Verde, M., Peruzzo, A. et al. Generating, manipulating and measuring entanglement and mixture with a reconfigurable photonic circuit. Nature Photon 6, 45–49 (2012).

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