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Two-photon quantum interference and entanglement at 2.1 μm

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Science Advances  27 Mar 2020:
Vol. 6, no. 13, eaay5195
DOI: 10.1126/sciadv.aay5195


  • Fig. 1 Down-conversion efficiency.

    Characterization of the spontaneous down-conversion efficiency in a 50-nm bandwidth centered at the 2090-nm degenerate emission wavelength. (A) The estimated conversion efficiency in the linear regime is of the order of (3.0 ± 0.2) × 10−11 (input power of 10 mW). The conversion efficiency is not constant and grows with the input power as expected in a nonlinear fashion (see the main text). The black dashed line is a fit of the conversion efficiency based on the simple model mentioned in the main text. The light gray dashed curves indicate the 95% prediction bounds. In (B), we show the measured number of photons generated for input powers up to 1 W. The dashed line is based on the fit in (A).

  • Fig. 2 Coincidence measurements at 2.1 μm.

    (A) Coincidence measurement showing the expected peak at zero delay, with accidental peaks at the inverse of the laser repetition rate (~12.5 ns). The bin size considered is ~2.6 ns. The inset shows the experimental setup for this measurement. (B) Measured coincidence-to-accidental ratio (CAR) as a function of the averaged single count rates between detectors 1 and 2. The red curve is a fitted CAR based on the model detailed in the main text.

  • Fig. 3 Two-photon interference.

    (A) Experimental setup for characterizing the two-photon interference. A beam splitter is inserted in front of the coincidence detection together with a tunable delay line allowing the adjustment of the temporal overlap of the down-converted photons at the beam splitter. (B) The observed two-photon interference (HOM dip) where the dots represent the experimental twofold coincidence counts and the solid curve is the fit to the experimental data (see Materials and Methods for details). Errors were estimated assuming Poisson statistics.

  • Fig. 4 Polarization entanglement.

    (A) Experimental setup for entanglement characterization. A tunable polarizer is inserted before each single-photon detection, and coincidences measured for different settings (angles), allowing the demonstration of a violation of the CHSH-Bell inequality (see the main text for details). (B) Measurement settings for the CHSH-Bell test and raw coincidence counts (C) used to determine the Bell parameter S = 2.20 ± 0.09, demonstrating genuine polarization entanglement at 2.1 μm (see the main text for details). The integration time was 30 min for each measurement.

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