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. 2017 Oct 13;8:906. doi: 10.1038/s41467-017-00897-7

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© The Author(s) 2017

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. 2 — Experimental set-up. a Left: Source of heralded single photons. A 1047 nm continuous wave (CW) laser is frequency doubled and subsequently downconverted with two different periodically poled lithium niobate (PPLN) nonlinear crystals. This probabilistically generates photon pairs with central wavelengths at 795 and 1532 nm, which are separated using a dichroic mirror (DM). A Fabry–Perot (FP) cavity spectrally filters the 795 nm photons to 6 GHz in order to match the spectral acceptance bandwidth of our AFC. Wave plates allow adjusting the polarization of the photons to maximize interaction with the AFC. Similarly, the 1532 nm photons are filtered down to 10 GHz using another FP cavity before being detected by a superconducting nanowire single-photon detector (SNSPD)—this detection heralds the presence of the 795 nm photon. Right: AFC preparation. CW laser light at 795 nm, which is frequency and amplitude modulated by an acousto-optic modulator (AOM) and a phase modulator (PM), is used to optically pump the Tm atoms in a cryogenically cooled Tm:LiNbO₃ bulk crystal to create an AFC. Micro electro-mechanical switches (MEMS) allow switching between optical pumping and storage of single photons. An avalanche photodiode (APD) is used to detect the 795 nm photons after interacting with the AFC. The electrical signal from both detectors (APD and SNSPD) is analyzed using a time-to-digital converter (TDC) to register the difference in arrival times of the two detection signals. b Characterization of the single-photon source. Cross-correlation function $g_{ab}^{(2)}$ of the two downconverted photons at 1532 nm (mode a) and 795 nm (mode b) as a function of the pump power. The error bar indicates the standard deviation derived from the Poissonian statistics of the photon detection events. We also show the heralded autocorrelation function of the 795 nm photon $g_{bb}^{(2)}$ for one value of the pump power. c A sample trace of an AFC (solid line) with N = 30 teeth and bandwidth B = 6 GHz and the spectral-density profile of the 795 nm photon (dashed line)