Researchers at the UVSOR Synchrotron Facility in Japan directly observed how quickly a xenon atom's inner-shell vacancy decays by using pairs of extreme ultraviolet light pulses. The team released photoelectron wave packets from xenon 4d core holes and made them interfere with each other. As they increased the delay between the paired pulses, the contrast of the interference fringes dropped over several femtoseconds. A model using core-hole lifetimes of about 6 femtoseconds from published literature matched the observed rapid fade of the fringe contrast. The findings appear in Physical Review A.