Is it possible to temporally-resolve photon pairs with a resolution beyond the limit imposed by detector temporal jitter? The answer to this question, thanks to the latest research by Lukens et al., is a definite yes. In this Optics Letters article, the authors show how temporal intensity modulation applied to the signal and idler modes of a spontaneous parametric downconversion (SPDC) photon-pair source may be exploited in order to beat the limit imposed by detector jitter. Specifically, the authors were able to determine for their PPLN-based bi-photon source an upper limit to the bi-photon temporal correlation width of 73.5ps, while the temporal jitter of their avalanche photodiodes is around 350ps, thus yielding an impressive nearly five-fold increase in temporal resolution as compared to direct detection. This is achieved using low duty-cycle pulse sequences, for which the detectors are effectively disabled during a time of 350ps out of each 400ps-duration cycle, i.e. at the cost of a considerable reduction in usable flux. In order to remedy this shortcoming, the authors propose replacing the low duty-cycle pulse sequences, used as modulation functions, with maximal-length sequences (M-sequences) which results in their specific implementation, with M=15, in an upper limit of the bi-photon correlation time of 83.6ps. In other words, the temporal resolution thus obtained is nearly at the same level as that resulting from using the low duty cycle pulse sequences but with a considerably greater usable flux. This research presents a potentially useful technique for significantly enhancing the temporal resolution of quantum optics experiments using currently-available avalanche photodiodes with temporal jitter in the region of hundreds of picosenconds.
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