Generation of single-mode biphotons with the narrowest linewidth to date from a hot atomic vapor

Chia-Yu Hsu¹, Yu-Sheng Wang¹, Jia-Mou Chen^1*, Fu-Chen Huang¹, Yi-Ting Ke¹, Emily Kay Huang¹, Weilun Hung¹, Kai-Lin Chao¹, Shih-Si Hsaio¹, Yi-Hsin Chen², Chih-Sung Chuu¹, Ying-Cheng Chen³, Yong-Fan Chen⁴, Ite A. Yu¹

¹Physics, National Tsing Hua University, Hsinchu, Taiwan
²Physics, National Sun Yat-Sen University, Kaohsiung, Taiwan
³Institute of Atomic and Molecular Sciences,, Academia Sinica, Taipei, Taiwan
⁴Physics, National Cheng Kung University, Tainan, Taiwan

* Presenter:Jia-Mou Chen, email:qbb50914@gmail.com

There are three main methods to generate biphotons: spontaneous parametric down-conversion (SPDC) in nonlinear crystals, cold-atom spontaneous four-wave mixing (SFWM), and hot-atom SFWM. Biphotons generated from the SPDC method usually have higher brightness, but it’s hard to make the spectral linewidth tunable. Biphotons generated from cold-atom SFWM could get a longer correlation time as well as a narrower spectral linewidth, however, because of the time sequence, photons could not generate continuously. Biphotons generated from Hot-atom SFWM overcome these disadvantages; it possesses both higher brightness as compared with photons generated from cold-atom SFWM and a tunable linewidth as compared to with photons generated from SPDC. Nevertheless, biphotons generated from hot-atom SFWM in the previous works have larger linewidths and worse brightness as compared with the other two methods. Our work is the first demonstration that the linewidth and brightness of biphotons generated from hot-atom SFWM can compete with those of cold-atom SFWM and cavity-assisted SPDC

The purpose of our research is to generate biphotons, i.e., heralded single photons, from a hot atomic vapor by using spontaneous four-wave mixing (SFWM) process. We’ve used three laser fields in this experiment: the pump, coupling, and hyperfine optical pumping (HOP) fields. Both pump and coupling fields completely overlapped and propagated in the same direction, which result in a narrow spectral linewidth as well as a high spectral brightness. The HOP field went through the opposite direction and was shaped in a hollowed structure, and it pumped the atoms to the designated hyperfine level before the atom flew into the biphoton-generation region. The purpose of the HOP field is to eliminate the background photons induced by Raman-transition fluorescence.

In our current result, the spectral linewidth can be tuned down to 290 kHz while the maximum two-photon correlation function, [g²]_max is 5.4; it is not only the best result around all hot-atom biphoton results, but is also the narrowest to date among all single-mode biphotons generated from various kinds method. As we enhanced the maximum two-photon correlation function, [g²]_max , by raising the power of coupling field, the biphoton spectral linewidth became large accordingly. The [g²]_max of the 610-kHz biphoton source is 42. By increasing the power of pump field at the 610-kHz biphoton source, we were able to achieve our best generation rate per linewidth result which is 2.3×10⁴ pairs/(s∙MHz), the highest as compared with all the sub-MHz biphoton sources, and maintain the [g²]_max at 6.7.

Keywords: Biphoton generation, Single photon source, Spontaneous four-wave mixing , Electromagnetically induced transparency