Good question, nobody is sure yet how to do it. Indeed any pulsed laser will result in a broad spectrum (spectrum = inverse of laser pulse time e.g. 1 ns pulse

Good question, nobody is sure yet how to do it. Indeed any pulsed laser will result in a broad spectrum (spectrum = inverse of laser pulse time e.g. 1 ns pulse is 1 GHz spectral width). Using frequency combs is a way around this, which are pulsed but the comb teeth are still narrow (approximately 100 Hz). To really go sub Hz one needs to build a continuous wave laser and mostly people are looking towards nonlinear crystals (poled materials, KBBF etc.) to create these using second harmonic generation