Prospects for milli-hertz linewidth lasers using collective emission

Ultra-stable lasers are core components of many precision measurements and clock experiments. The stability of the most stable lasers is limited by thermal noise of the reference cavities to which the frequency of the laser is locked. Overcoming this limitation is a serious challenge. We discuss an alternative route to ultra-stable light that circumvents the need for an improved reference cavity. The idea is to generate light actively from ultra-cold atoms in a lattice clock. We discuss the physical mechanism underlying this light source, experimental requirements, and key characteristics of the generated light such as its intensity, linewidth, and noise properties.