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Heralded generation of entangled photon pairs


Entangled photons are a crucial resource for quantum communication and linear optical quantum computation. Unfortunately, the applicability of many photon-based schemes is limited due to the stochastic character of the photon sources. Therefore, a worldwide effort has focused on overcoming the limitation of probabilistic emission by generating two-photon entangled states conditioned on the detection of auxiliary photons. Here we present the first heralded generation of photon states that are maximally entangled in polarization with linear optics and standard photon detection from spontaneous parametric downconversion1. We use the downconversion state corresponding to the generation of three photon pairs, where the coincident detection of four auxiliary photons unambiguously heralds the successful preparation of the entangled state2. This controlled generation of entangled photon states is a significant step towards the applicability of a linear optics quantum network, in particular for entanglement swapping, quantum teleportation, quantum cryptography and scalable approaches towards photonics-based quantum computing3.

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Figure 1: Set-up for the heralded generation of entangled photon pairs.
Figure 2: Probability of heralded entanglement generation for various beamsplitter transmissions.
Figure 3: Experimentally obtained fidelities for the two-qubit polarization state with respect to the ideal state |φ+〉 for various beamsplitter transmissions.
Figure 4: Effect of higher-order emission on polarization correlations.


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The authors are grateful to R. Prevedel, X. Ma, M. Aspelmeyer, Č. Brukner and T. Pittman for discussions and G. Mondl for assistance with the electronics. This work was supported by the Austrian Science Fund (FWF), the Intelligence Advanced Research Projects Activity (IARPA) under the Army Research Office (ARO), the European Commission under the Integrated Project Qubit Applications (QAP) and Quantum Interfaces, Sensors, and Communication based on Entanglement (Q-ESSENCE) and the IST directorate, the ERC Senior Grant (QIT4QAD) and the Marie-Curie research training network EMALI.

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S.B. and G.C. designed and performed experiments, analysed data and wrote the manuscript. A.Z. supervised the project and edited the manuscript. P.W. designed experiments, analysed data, wrote the manuscript and supervised the project.

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Correspondence to Philip Walther.

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The authors declare no competing financial interests.

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Barz, S., Cronenberg, G., Zeilinger, A. et al. Heralded generation of entangled photon pairs. Nature Photon 4, 553–556 (2010).

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