Tavella, F.; Stojanovic, N.; Geloni, G.; Gensch, M.: Few-femtosecond timing at fourth-generation X-ray light sources. Nature Photonics 5 (2011), p. 162-165

The combined qualities of ultra-high intensity and ultra-short pulse duration, available at 4th generation X-ray light sources provide tremendous opportunities for investigation of ultrafast dynamics in complex systems. Such studies require femtosecond synchronization between pump and probe laser pulses. However, the necessary level of stability of the accelerator is currently impossible to achieve. Fortunately, this issue can be managed if the relative timing jitter between light sources is accurately monitored, allowing subsequent data sorting. We present a new, robust, non-invasive approach that provides 4.6 fs rms temporal resolution. This is, to our knowledge, the best resolution ever achieved between an FEL and an external laser. Our method employs coherent terahertz radiation generated at the end of the X-ray undulator by the same electron bunch that emits the X-ray pulse. Therefore, this method can be universally applied at any advanced light source working with ultra short electron bunches and undulators.