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arXiv:quant-ph/0607014 · Physical Review A

Entangled mixed-state generation by twin-photon scattering

Graciana Puentes, Andrea Aiello, D. Voigt, J. P. Woerdman

Published Mar 14, 2007• 26 citations
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Abstract

We report experimental results on mixed-state generation by multiple scattering of polarization-entangled photon pairs created from parametric down-conversion. By using a large variety of scattering optical systems we have experimentally obtained entangled mixed states that lie upon and below the Werner curve in the linear entropy-tangle plane. We have also introduced a simple phenomenological model built on the analogy between classical polarization optics and quantum maps. Theoretical predictions from such a model are in full agreement with our experimental findings.

DOI
10.1103/physreva.75.032319

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We report experimental results on mixed-state generation by multiple scattering of polarization-entangled photon pairs created from parametric down-conversion. By using a large variety of scattering optical systems we have experimentally obtained entangled mixed states that lie upon and below the Werner curve in the linear entropy-tangle plane. We have also introduced a simple phenomenological model built on the analogy between classical polarization optics and quantum maps. Theoretical predictions from such a model are in full agreement with our experimental findings.
We experimentally prepare bi-photon mixed states in polarization employing an entangled-classical hybrid photon emitter which can properly model solid-state entangled photon sources with uncorrelated background photons. Polarization-uncorrelated photon pairs in totally mixed (TM) states are embodied with classical thermal radiation, while the polarization-entangled ones in a Bell state are generated by conventional parametric down conversion. The bi-photon states generated from the hybrid photon emitter are characterized in terms of a linear entropy-tangle plane, which reveals the formation of two-qubit Werner states. We also propose a direct way for evaluating the Werner states by means of minimal coincidence counts measurements. This simple method can be widely applicable in examining the bi-photon states from solid-state entangled photon sources, in which the polarization-entangled photon pairs have temporal correlation while the background photons in the TM states do not.
Using correlated photons from parametric down-conversion, we extend the boundaries of experimentally accessible two-qubit Hilbert space. Specifically, we have created and characterized maximally entangled mixed states that lie above the Werner boundary in the linear entropy-tangle plane. In addition, we demonstrate that such states can be efficiently concentrated, simultaneously increasing both the purity and the degree of entanglement. We investigate a previously unsuspected sensitivity imbalance in common state measures, i.e., the tangle, linear entropy, and fidelity.
A parametric source of polarization-entangled photon pairs with striking spatial characteristics is reported. The distribution of the output electromagnetic k\mathbf{k}k modes excited by spontaneous parametric down-conversion and coupled to the output detectors can be very broad. Using these states realized over a full entanglement ring output distribution, the nonlocal properties of the generated entanglement have been tested by standard Bell measurements and by Ou-Mandel interferometry. A ``mode-patchwork'' technique based on the quantum superposition principle is adopted to synthesize in a straightforward and reliable way any kind of mixed state, of large conceptual and technological interest in modern quantum information. Tunable Werner states and maximally entangled mixed states have indeed been created by this technique and investigated by quantum tomography. A study of the entropic and nonlocal properties of these states has been undertaken experimentally and theoretically, by a unifying variational approach.