外种子型自由电子激光具有全相干、频谱稳定、极高亮度的优点，可以实现在超小空间和超快时间尺度下对物质结构的研究。具有特殊横向相位模式的光特别是具有螺旋相位的带轨道角动量的涡旋光已经在众多科学领域有了应用，基于自由电子激光原理产生的辐射横向模式基本上为简单的高斯模式，为产生具有横向螺旋相位的相干涡旋X射线，对基于回声谐波放大型(EEHG)自由电子激光产生涡旋光方案进行了深入研究，并且根据上海软X射线自由电子激光装置(SXFEL)的参数，进行了相关方案设计和模拟研究。三维模拟结果表明，外种子型EEHG自由电子激光可以产生峰值功率可达到GW量级的相干涡旋软X射线。

Optical orbital angular momentum (OAM) is a special property of photons and has evoked research onto the light–matter interaction in both classical and quantum regimes. In classical optics, OAM is related to an optical vortex with a helical phase structure. In quantum optics, photons with a twisted or helical phase structure will carry a quantized OAM. To our knowledge, however, so far, no experiment has demonstrated the fundamental property of the OAM at the single-photon level. In this Letter, we have demonstrated the average photon trajectories of twisted photons in a double-slit interference. We have experimentally captured the double-slit interference process of twisted photons by a time-gated intensified charge-coupled device camera, which is trigged by a heralded detection. Our work provides new perspectives for understanding the micro-behaviors of twisted particles and enables new applications in imaging and sensing.