Brilliant, ultrashort, and coherent X-ray free-electron laser pulses allow investigations of dynamics at the inherent time and length scale of atoms. However, missing are sequences of phase-locked X-ray pulses, desirable for time-domain correlation spectroscopies and coherent quantum control. In this seminar, I will present a scheme to generate sub-femtosecond, phase-locked X-ray pulse pairs with up to 100 fs delay. This enables time-domain interferometry, such as the X-ray analog of the ubiquitous Fourier transform infrared spectrometer or resonant inelastic X-ray scattering, and, more generally, all of nonlinear and quantum optics requiring coherent copies of beams. S. Reiche, G. Knopp, B. Pedrini, E. Prat, G. Aeppli & S. Gerber, PNAS 119, e2117906119 (2022).