  Bright ultrashort X-ray pulses allow scientists to observe ultrafast motion
of atoms and molecules. Coherent light sources, such as the X-ray free electron
laser (XFEL), enable remarkable discoveries in cell biology, protein
crystallography, chemistry or materials science. However, in contrast to
optical lasers, lack of X-ray mirrors demands XFELs to amplify radiation over a
single pass, requiring tens or hundreds of meters long undulators to produce
bright femtosecond X-ray pulses. Here, we propose a new ultrafast coherent
light source based on laser reflection from a relativistic mirror driven by a
relativistic charged particle beam in micrometer-scale plasma. We show that
reflection of millijoule-level laser pulses from such mirrors can produce
bright, coherent and bandwidth-tunable attosecond X-ray pulses with peak
intensity and spectral brightness comparable to XFELs. In addition, we find
that beam-driven relativistic mirrors are highly robust, with laser-induced
damage threshold exceeding solid-state components by at least two orders of
magnitude. Our results promise a new way for bright coherent attosecond X-ray
pulse generation, suitable for unique applications in fundamental physics,
biology and chemistry.
