Generation of the broadband indistinguishable two-photon state in the telecom band.

The indistinguishable photon-pair sources are valuable in many quantum information applications, such as quantum microscopy, quantum synchronization, and quantum metrology. Based on cascaded sum-frequency generation (SFG) and spontaneous parametric downconversion (SPDC) processes, we propose and demonstrate a scheme for the generation of spatially separated broadband indistinguishable photon pairs in the telecom band by using only one piece of a fiber-pigtailed periodically poled lithium niobate waveguide in a modified Sagnac loop. The measured joint spectral intensity of the generated entangled photon pairs is 7.27 THz (57.6 nm) at the full width at half-maximum (FWHM). The Hong-Ou-Mandel (HOM) interference of the generated broadband photons is measured with bandwidths of 5.35 THz (∼42.8 nm) and 100 GHz (∼0.8 nm), respectively. Visibility of 94.0±1.4 is achieved with the bandwidth of 5.35 THz, demonstrating good indistinguishability of the generated two-photon states, which could benefit the development of quantum microscopy and quantum synchronization.