Quantum entanglement has very important applications in quantum information, quantum computation, quantum metrology and quantum imaging, etc.. Traditionally, entangled photon pairs are generated by spontaneous parametric down conversion process (SPDC) in a non-centrosymmetric crystal. However, because of the incompact phase matching condition, the generated photon pairs have very broad bandwidth, typically of the order of several THz.. In recent several years, spontaneous four-wave mixing (SFWM) in cold atoms has become a new way to generate high-brightness, narrow-band non-classical photon pairs. A natural or even sub-natural bandwidth of biphotons, has been demonstrated by this method. 

 The previous theoretical and experimental results show that two-photon wave function by SFWM in cold atoms is determined by both the linear and nonlinear optical responses, and consequently two-photon correlation exists in two characteristic regimes. In the damped Rabi oscillation regime, when the nonlinear optical response is dominant, the two-photon correlation presents a damped Rabi oscillation. In the group delay regime, the two-photon correlation presents a rectangular shape, similar to the conventional type-II SPDC.　

Researchers at Key Laboratory for Quantum Optics, Chinese Academy of Sciences predict the possibility of the interference of narrow-band biphotons generated by spontaneous four-wave mixing with double electromagnetically induced transparency in cold atoms. In an N-type four-level system, an auxiliary optical field can create double transparency windows for the anti-Stokes photons. When the slow light effects in the double transparency windows are very strong, two four-wave mixing channels could exist due to the splitting of phase matching condition. That is to say, they find in the group delay regime, the two-photon correlation can also present a damped Rabi oscillation via an auxiliary optical field.　

This work entitled " Interference of a Narrowband Biphoton with Double Electromagnetically Induced Transparency in an N-Type System " has been published in Chin. Phys. Lett., 2012, 29(4): 044202, http://cpl.iphy.ac.cn/EN/Y2012/V29/I4/044202.

Fig.1. Schematic of the SFWM in a four-level N-type system. (Image by SIOM)

Fig.2 The linear and nonlinear susceptibilities to the anti-Stokes photons in the SFWM. The third-order susceptibility has been zoomed in for a suitable viewing. (Image by SIOM)

Fig. 3. (a) The ideal two-photon correlation with double-EIT in the group delay regime.(b) The numerical simulation. (Image by SIOM)