Hard X-ray attosecond metrology

Sub-ångström spatial resolution combined with attosecond temporal resolution enables direct observation of electron motion in matter¹. Attosecond hard X-ray pulses generated at X-ray free-electron laser sources provide the requisite spatiotemporal sensitivity, yet their application has been limited by the absence of precise pulse metrology and control. Here, building on a broadband X-ray split-delay line, we establish metrology of attosecond hard X-ray pulses. Field-correlation measurements provide the first time-domain characterization of the pulse coherence time and demonstrate attosecond synchronization between the generated pulse pairs. By exploiting non-sequential two-photon absorption as a nonlinear probe, intensity autocorrelation quantifies the average pulse duration directly in the time domain to be 170 as. The demonstrated experimental tools and techniques enable systematic attosecond time-resolved experiments, including X-ray pump-X-ray probe spectroscopy and diffraction, and open a pathway to probing electronic phenomena with unprecedented spatial-temporal resolution.