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Calculated dependence of the relative phase on the wavelengths of pump and one of the down-conversion photons for (a) uncompensated and (b) compensated configuration. The evaluation assumes SPDC in the pair of BBO crystals, each thick and cut at ; an overall phase offset is suppressed for clarity. After the optimum compensation using two crystals with thicknesses of 8.20 and , the phase map becomes flat (note the change in the vertical scale), indicating a high purity of the entangled state.
Scheme of the collinear source. A free-running violet laser diode pumps a pair of BBO crystals and collinearly produces via SPDC nondegenerate photons, which are collected into a single mode fiber. The photons are separated into two spatial modes using a wavelength division multiplexer. Two compensation crystals and BBO crystals are used to reverse the time delay effect and spatial lateral displacement effect, respectively, introduced in the down-conversion BBO crystals.
Detected single and coincidence count rates depending on the pump power measured at the position of BBO crystals.
Spectral distribution of down-conversion light with the central wavelengths of and , determined from a Gaussian fit (solid line). The lower peak number of counts at is due to a reduced efficiency of the spectrometer (with resolution ) towards infrared wavelengths. The dashed line shows the numerically simulated spectra obtained under the assumption of narrowband pumping.
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