New atomic filter with bandwidth close to the atomic natural linewidth

87Rb atoms are irradiated with the circularly polarized coupling light, and in the case of the two-photon resonance, the polarization plane of the linearly polarized signal light will be rotated over an angle when passing through the atomic vapor. An atomic filter with bandwidth of 10 MHz and transmission of 33.2% has been realized based on the quantum interference induced Faraday effect. The center frequency of the filter can be tuned by altering the coupling frequency.

Unltranarrow bandwidth optical filters can be used to efficiently suppress the background light noise while extracting the weak signal. The atomic filtering method has been proved to be one of the most effective ways to realize unltranarrow bandwidth optical filtering, and is able to significantly enhance the sensitivity of the signal detection by the use of absorption, emission and internal energy exchange of atoms. Atomic filters have been applied in the areas of LIDAR, atmospheric remote sensing, laser and quantum communications.

In order to explore new applications for atomic-ensemble-based quantum memories and narrow band photon sources, researchers are working on ultranarrow bandwidth atomic filters with high transmission and large frequency tunability, meanwhile the passband matching the atomic resonant frequency.

The researchers realized an atomic optical filter at ⁸⁷Rb D₂ line with a bandwidth of 25 MHz and a transmission of 18% based on Faraday anomalous dispersion effect and velocity-selective pumping. The filter is composed of two permanent magnets producing the axial magnetic field, two polarization-orthogonal Glan-Tayor prisms with an extinction ratio 10⁵:1, and a frequency stabilized 780 nm diode laser.

The research group, led by Prof. Mingsheng Zhan, from Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, achieved an ultranarrow bandwidth atomic filter working at the D1 line (795nm) of 87Rb atoms based on the quantum interference induced Faraday effect. It is reported in Chinese Optics Letters, Volume 12,Issue 12.

In their research, circular birefringence of the atomic medium was induced by a bunch of circularly polarized coupling light , which led to the polarization rotation of the linearly polarized signal light while passing through the atomic vapor. The frequency of the coupling light was locked to a specific atomic transition, and in the case that the frequency difference of the signal light and the coupling light equals to the separation of 87Rb hyperfine ground state, the signal light with frequency in the narrow EIT window could transmit through the two crossed Glan-Thompson prisms, while the light with frequency outside the EIT window would be rejected. Based on such quantum inference induced Faraday effect, the researchers achieved filter bandwidth of 10 MHz, which is close to the natural linewidth of Rb atoms, and the transmission of 33.2%. The center frequency tuning of the filter was realized by altering the coupling frequency. The dependence of the transmission on frequency detuning and intensity of the coupling light was also studied.

The following work will be focused on further enhancement of the transmission of the atomic filter by selecting different energy levels. Meanwhile, the researchers are trying to suppress the bandwidth through filling the vapor cell with buffer gas to increase the light-atom interaction time.

Atomic filter with ultranarrow bandwith may find applications in the areas of long-distance free space quantum key distributions,quantum internet, laser Doppler velocimetry and laser remote sensing with high sensitivity, allowing them to be operated under conditions of intense sunlight and tiny signal. It may also have potential applications in atomic-based quantum memories and narrow band single photon sources.

新型原子滤光器滤波带宽接近原子自然线宽

圆偏振耦合光作用于87Rb原子，在双光子共振条件下，线偏振信号光通过原子蒸气传输时偏振面将发生旋转。利用量子干涉诱导的法拉第效应，实现了10 MHz的滤光带宽，透过率达到33.2%，通过改变耦合光的频率可实现滤光中心频率的调谐。

超窄带光学滤光可以有效抑制背景光,同时读取微弱的信号光。在激光雷达、大气遥感、激光和量子通信等领域的实践表明，利用吸收、发射及内部能量转换等物理特性的原子滤光是实现超窄带光学滤光的理想方法之一。

原子滤光器能够有效地进行频谱滤波，极大地提高光学信号的探测灵敏度。为了探索超窄带光学滤光在基于原子系综的量子存储器、窄带单光子源等领域的新应用，需要研制滤光中心频率能够与原子的共振频率相匹配的超窄带、高透过率和大调谐范围的原子滤光器。

中科院武汉物理与数学研究所詹明生研究员课题组利用量子干涉诱导的法拉第效应，获得87Rb原子 D1线 (795 nm) 原子滤光的超窄滤光带宽，相关研究结果发表在Chinese Optics Letters 2014年第12期上。

该研究利用一束圆偏振耦合光诱导原子介质的圆双折射效应，导致在原子蒸气中传输的线偏振信号光的偏振面发生旋转。研究中将耦合光锁定在特定原子跃迁上，当信号光和耦合光的频率差等于原子的超精细基态分裂时，频率位于窄的电磁诱导透明“窗口”内的信号光将通过两个正交的格兰-汤姆孙棱镜传输，而其它频段的信号光将被抑制。

利用量子干涉诱导的法拉第效应，研究人员在87Rb原子D1线实现了10 MHz的接近于原子自然线宽的滤光带宽，获得了33.2%的透过率，而且改变耦合激光的频率能够实现滤光通带中心频率的调谐。实验还研究了原子滤光透过率随耦合光频率及光强的变化关系。

在下一步工作中，研究人员将试图通过选择不同的原子能级，实现更高的原子滤光透过率；并利用充入缓冲气体来增加光和原子的作用时间，进一步压窄原子滤光器的通带带宽。