Variation of the Hong-Ou-Mandel interference dip with crystal length

Two-photon interference, commonly known as Hong-Ou-Mandel (HOM) interference [1] , has attracted a great deal of interest in quantum sensing, including the precision timing measurements [2] and quantum optical coherence tomography [3]. While the dip of the HOM interference is the direct measure of the purity of the single-photon, the width of the HOM dip relates to the coherence time of the interfering photons. The broad consensus for using the HOM interferometer for sensing application remained the HOM dip width as the limiting factor for the precision measurements. As a consequence, the ultra-broadband photon sources have long been hailed as a vital prerequisite for ultra-precise HOM interferometry. Here, we have studied the variation in the width of the HOM dip with the crystal length for the continuous-wave pump laser and measured the HOM dip as low as 8.2 ± 0.2m corresponding to the spectral width of the single-photon source of 79.6±1.9 nm useful for sensitive sensing applications.