Broadband high-resolution X-ray frequency combs

Optical frequency combs have had a remarkable impact on precision spectroscopy(1-3). Enabling this technology in the X-ray domain is expected to result in wide-ranging applications, such as stringent tests of astrophysical models and quantum electrodynamics(4), a more sensitive search for the variability of fundamental constants(5), and precision studies of nuclear structure(6). Ultraprecise X-ray atomic clocks may also be envisaged(7). In this work, an X-ray pulse-shaping method is proposed to generate a comb in the absorption spectrum of an ultrashort high-frequency pulse. The method employs an optical-frequency-comb laser, manipulating the system's dipole response to imprint a comb on an excited transition with a high photon energy. The described scheme provides higher comb frequencies and requires lower optical-comb peak intensities than currently explored methods(8-10), preserves the overall width of the optical comb, and may be implemented using currently available X-ray technology(11).