One of the key challenges in scientific researches based on free-electron lasers (FELs) is the characterization of the coherence time of the ultra-fast hard x-ray pulse, which fundamentally influences the interaction process between x-rays and materials. Conventional optical methods, based on autocorrelation, are very difficult to realize due to the lack of mirrors. Here, we experimentally demonstrate a novel method which yields a coherence time of 174.7 attoseconds for the 6.92 keV FEL pulses at the Linac Coherent Light Source. In our experiment, a phase shifter is adopted to control the cross-correlation between x-ray and microbunched electrons. This approach provides critical diagnostics for the temporal coherence of x-ray FELs and is universal for general machine parameters; applicable for wide range of photon energy, radiation brightness, repetition rate and FEL pulse duration.

基于自由电子激光器（FEL）的科学研究的关键挑战之一是表征超快硬X射线脉冲的相干时间，这从根本上影响了X射线与材料之间的相互作用过程。由于缺少反射镜，基于自相关的常规光学方法很难实现。在这里，我们实验证明了一种新颖的方法，对于直线加速器相干光源上的6.92 keV FEL脉冲，其相干时间为174.7阿秒。在我们的实验中，采用移相器来控制X射线和微束电子之间的互相关。这种方法为X射线FEL的时间相干性提供了关键的诊断方法，并且对于一般机器参数是通用的。适用于广泛的光子能量，辐射亮度，