  X-ray free-electron lasers have had an enormous impact on x-ray science by
achieving femtosecond pulses with unprecedented intensities. However,
present-day facilities operating by the self-amplified spontaneous emission
(SASE) principle have a number of shortcomings, namely, their radiation has a
chaotic pulse profile and short coherence times. We put forward a scheme for a
neon-based atomic inner-shell x-ray laser (XRL) which produces temporally and
spatially coherent subfemtosecond pulses that are controlled by and
synchronized to an optical laser with femtosecond precision. We envision that
such an XRL will allow for numerous applications such as nuclear quantum optics
and the study of ultrafast quantum dynamics of atoms, molecules, and condensed
matter.
