Fig. 2: Cross-Correlation. | npj Quantum Information

Fig. 2: Cross-Correlation.

From: Passively stable distribution of polarisation entanglement over 192 km of deployed optical fibre

Fig. 2

The cross-correlation function between the time tags from the two detectors shows a peak at a relative delay of \(0.945\) ms, which corresponds exactly to the total loop distance of (192,820.538 \(\pm\) 0.004) m, assuming that the latency of the two detectors is identical. The full-width at half-maximum is \(1\) ns. The three main factors that contribute to the peak width are the chromatic dispersion of the fibre link of \(760\) ps for our signal spectrum, the timing jitter of the detectors and of the time-tagging units — \({\sim}{250}\) ps and the clock accuracy estimated from a separate measurement (using a 1 ms delay in the lab) of about 500 ps. The data shown here was measured over \(100\) s, while the rate of coincident clicks was \(3.8\pm 0.2\) s−1. The orange graph is a fit to a measurement using the same detectors without the long fibre link. It has been normalised to the same height as the measured counts from the link experiment. The FWHM is here \(250\) ps in this case.

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