Source: http://aoot.osa.org/oe/abstract.cfm?uri=oe-24-24-27059
Timestamp: 2019-04-26 06:10:03+00:00

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High-quality entangled photon pairs generated via spontaneous parametric down-conversion have made great contributions to the modern quantum information science and the fundamental tests of quantum mechanics. However, the quality of the entangled states decreases sharply when moving from biphoton to multiphoton experiments, mainly due to the lack of interactions between photons. Here, for the first time, we generate a four-photon Greenberger-Horne-Zeilinger state with a fidelity of 98%, which is even comparable to the best fidelity of biphoton entangled states. Thus, it enables us to demonstrate an ultrahigh-fidelity entanglement swapping—the key ingredient in various quantum information tasks. Our results push the fidelity of multiphoton entanglement generation to a new level and would be useful in some demanding tasks, e.g., we successfully demonstrate the genuine multipartite nonlocality of the observed state in the nonsignaling scenario by violating a novel Hardy-like inequality, which requires very high state-fidelity.
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Fig. 1 Experimental setup for our “sandwichlike” EPR source. The thicknesses of the LiNbO3 crystals are different in the two paths, which is due to the walk-off effect of the pump beam and the down-converted beams in the BBO crystals. The half-wave plates (HWPs) before the fiber couplers are used for polarization alignment.
Fig. 2 Experimental setup to generate the four-photon GHZ state. The abbreviations of the components are HWP, half-wave plate; QWP, quarter-wave plate; IF, interference filter; PBS, polarizing beam splitter; PAS, polarization analyzing system; APD, avalanche photodiode detector. The two EPR sources generated two pairs of entangled photons 1, 2 and 3, 4. The HWPs after the fiber couplers are used for polarization alignment. The two e-photons are directed to overlap on the central PBS, which can project the input state onto the four-photon GHZ state. One of the fiber couplers is mounted on a translation stage to finely adjust the arriving time of the interfering photons. The tiltable QWP after the PBS is used for tuning the relative phase between the two terms in the GHZ state. Each photon passes though a narrow-band filter for spectral selection and then enters the final PAS. The inset shows the details of the PAS, which consists of one motorized QWP, one motorized HWP, one PBS and two APDs.
Fig. 3 Measurement result of the witness. (a) Fourfold coincidence counts measured in the H/V basis. The data collection time is 10,000 s. The expectation value of A is just equal to the proportion of the two correct terms, which is 0.9897 ± 0.0015. (b) The expectation values of Mk, which yield an average value of 1 4 ∑ k = 0 3 ( − 1 ) k 〈 M k 〉 = 0.9722 ± 0.0041. The data collection time is 2000 s for each measurement settings.
Fig. 4 The real (left) and imaginary (right) parts of the density matrix ρexp, which is reconstructed by the maximum likelihood method from the recorded data. The height of the largest incorrect bar in the two pictures is only 0.03.
Fig. 5 Real (left) and imaginary (right) parts of the reconstructed density matrix of photons 1, 4. (a), (b) The state of photons 2, 3 are projected on |ϕ+⟩, the fidelity of the state of photons 1, 4 corresponding to |ϕ+⟩ is 0.979 ± 0.012. (c), (d) The photons 2 and 3 are projected on |ϕ−⟩, the fidelity of the state of photons 1, 4 corresponding to |ϕ−⟩ is 0.975 ± 0.016.

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