Keywords: 
• 啁啾纠缠光子对 /
• 关联时间 /
• 压缩 /
• 二元相位调制

Abstract: Chirped biphotons generated via spontaneous parametric down-conversion in chirped quasi-phase-matched nonlin-ear crystals have ultrabroadband frequency spectra. However, the presence of quadratic frequency phase factor restricts their applications in quantum metrology and quantum lithography due to simultaneously lengthening the correlation times of biphotons. The key point to improve the temporal correlation of chirped biphotons is how to compensate for or remove the quadratic frequency phase factor. Phase compensation methods have been demonstrated to solve this problem in earlier reports. But the compressed efficiencies of these methods are strongly dependent on the length of the utilized dispersive medium and decreased by the higher-order dispersion of the dispersive medium. In this paper, based on the phase transform of a lens for a light field in spatial domain, we theoretically propose a method of the equivalent removal of the quadratic phase by realizing a Fresnel-zone lens-like modulation on the biphotons spectrum in frequency domain, thereby compressing the correlation time of chirped biphotons to the Fourier-transform limited width. By analogy to the idea of Fresnel wave zone plate, this lens-like modulation can be realized by dividing the biphoton spectrum into Fresnel frequency zones and applying only binary spectral phase (0, π) sequentially to these zones. The theoretical results show that the correlation time width of chirped biphotons can be reduced, and the corre-lation signal intensity can be increased compared with the original one, by a factor about 100 and 30, respectively. The physical reason is that these Fresnel frequency zones under binary spectral phase modulation will lead to constructive interference at zero delay and destructive interference elsewhere. This method can significantly enhance biphoton time correlation without biphoton signal loss and avoids the limitations of phase compensation methods. Therefore, we can obtain biphotons with both ultra-broad bandwidth and ultra-short correlation times by using our proposed method. The attainable compression efficiency is constrained by the division resolution of the Fresnel frequency zones and the precision of applied binary phase modulations. It should be noted that a constraint condition about crystal length, chirpparameter and the number of frequency zones is summarized in designing the experimental parameters for the desired compression goal. Since binary spectral phaseπand 0 are easy to obtain and calibrate in practice, we thus believe that our proposed method is feasible to implement experimentally. Moreover, the proposed method can also be generalized to other fields relating to the quadratic phase factor, such as two-photon absorption, second-harmonic generation and chirped pulse compression.