Keywords: 
• 量子纠缠 /
• 联合频谱 /
• 频率正关联

Abstract: The frequency entangled biphoton source generated via spontaneous parametric down-conversion (SPDC) process has found important applications in the fields of quantum clock synchronization, quantum communication, quantum information processing, etc. As quantum technologies evolve, quantitative characterization of the frequency entanglement becomes necessary and has been implemented by measuring the spectral properties of the biphoton state. However, due to the high dark rate and low quantum efficiency of the InGaAs single-photon detectors, direct measurement of the spectral properties of the biphoton state at optical communication wavelength is hard to implement. In this paper, we report the measurement of the spectral properties of a biphoton state at optical communication wavelength which is generated from periodically poled potassium titanyl phosphate (PPKTP) pumped by an ultra-short pulsed optical source at 787 nm. Based on the coincidence measurement setup together with two infrared spectrometers, the spectra of the signal and idler photons are obtained with their center wavelengths being 1574.4 nm and 1574.9 nm, while their 3-dB bandwidths being 35.3 nm and 37.6 nm respectively. The joint spectrum of the photon pair is observed as well and shows a coincident-frequency entanglement and a joint spectrum bandwidth of 3 nm. According to the ratio of the single-photon spectral bandwidth to the joint spectral bandwidth of the photon pairs, the degree of frequency entanglement is quantified to be 12, denoting a relatively high quality of the entanglement.