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(a) The number of 1-photon, accidental 2-photon, and correlated 2-photon absorption events as a function of the pair production rate with and bin-size (dt) of . The dashed line indicates the pair production rate in this experiment. (b) The ratio of correlated 2-photon events versus total (correlated and coincidental) 2-photon events as a function of pair production rate. (c) Count-rates caused by 1 photon events and by correlated 2-photon events as a function of SSPD energy scale in units of the single photon energy. (d) and (e) Single-photon (d) and 2-photon (e) count rates as a function of absorption probability and SSPD energy scale. Line-cuts along the dotted lines give (c).
(a) Experimental setup. Photon pairs are produced in a KTP crystal and separated on a polarizing beam splitter (PBS) after filtering out the pump beam. One of the photons is delayed with respect to the other in a motorized delay line. The photons are collected in single-mode fibers and combined using a fiber beam splitter. The output beam consists of photon pairs with a controlled delay dt and is focused to a spot on the SSPD using a high NA objective. (b) Photon pair source characterization by Hong-Ou-Mandel interference in the fiber beam splitter. The data points correspond to the number of coincidences between two APDs connected to both outputs of the fiber beam splitter (integration time ). If the photons arrive within the coherence time , they bunch and travel as pairs to the same detector, causing the correlations to disappear. At the same time, the number of photon pairs in both arms increases and follows the green dashed line.
(a) SSPD counts (in 120 s) as a function of the delay between two photons forming a pair for several different bias currents. (b) Peak height and background level of the data in (a), indicating that at higher currents, the probability of detecting a single-photon (the background) grows faster than the probability of detecting a pair (the peak height). The solid line is the signal to noise ratio (peak height divided by background) using the right axis. (c) Same as (a), but for different SPDC pump powers. (d) Peak height and background level of the data in (c) and their ratio, showing that both the single- and pair-rates of the SPDC source depend linearly on the pump-power.
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