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. 2019 May 1;8:41. doi: 10.1038/s41377-019-0153-y

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© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 3 — a and b are fourfold coincidences as a function of the idler polarizer angles when the signal polarizer angles were kept at 0° (red) and 45° (black), respectively. The error bar was obtained from the square root of the experimental data. c and d give the real (Re) and imaginary (Im) parts of the ideal density matrix and the measured density matrix of the four-photon quantum states, respectively. The fidelity was 0.78 ± 0.02, which is completely satisfactory for further quantum information processing