Transient XUV lasing without inversion via He triplet states

We demonstrate transient inversionless laser oscillation in short wavelength regions where population inversion is difficult to achieve. It was shown some time ago that when the excitation of an ensemble of two-level atoms is swept in the direction of lasing, so that atoms are prepared in the excited state just as the radiation from previously excited atoms reaches them, the resulting laser amplifier is ``highly anomalous" and yields superradiant emission without population inversion. We here show that transient lasing without inversion in a three-level system has common features with Dicke superradiance and can yield strong XUV lasing in, for example, He atoms (at 58 nm) or He-like ions such as B$^{3+}$ (at 6.1 nm).