  The remarkable precision of frequency comb (FC) lasers is transferred to the
extreme ultraviolet (XUV, wavelengths shorter than 100 nm), a frequency region
previously not accessable to these devices. A frequency comb at XUV wavelengths
near 51 nm is generated by amplification and coherent upconversion of a pair of
pulses originating from a near-infrared femtosecond FC laser. The phase
coherence of the source in the XUV is demonstrated using Helium atoms as a
ruler and phase detector. Signals in the form of stable Ramsey-like fringes
with high contrast are observed when the FC laser is scanned over P states of
Helium, from which the absolute transition frequency in the XUV can be
extracted. This procedure yields a 4He ionization energy at h 5945204212(6)
MHz, improved by nearly an order of magnitude in accuracy, thus challenging QED
calculations of this two-electron system.
