Active Optical Clock
  This letter presents the principles and techniques of active optical clock, a special laser combining the laser physics of one-atom laser, bad-cavity gas laser, super-cavity stabilized laser and optical atomic clock. As an example, a compact version of active optical clock based on thermal Strontium atomic beam shows a quantum-limited linewidth of 0.51 Hz, which is insensitive to laser cavity-length noise, and may surpass the recorded narrowest 6.7 Hz of Hg ion optical clock and 27Hz of very recent optical lattice clock. The estimated 0.1Hz one-second instability and 0.27Hz uncertainty are limited only by the relativistic Doppler effect may be improved to 10mHz by using cold atoms. 
Jingbiao Chen