A reliable cw Lyman-$\alpha$ laser source for future cooling of antihydrogen

We demonstrate a reliable continuous-wave (cw) laser source at the 1,$S$–2,$P$ transition in (anti)hydrogen at 121.56,nm (Lyman-$\alpha$) based on four-wave sum-frequency mixing in mercury. A two-photon resonance in the four-wave mixing scheme is essential for a powerful cw Lyman-$\alpha$ source and is well investigated.