Institute of Quantum Electronics, School of Electronics Engineering and Computer Science, Peking University, has proposed the concept, principles and techniques of active optical clock. The study is reported in Issue 54 (February, 2009) of Chinese Science Bulletin because of its significant research value.
Up to date, all realize that optical clocks are based on the laser absorption spectroscopy. Thus the available laser with narrowest linewidth limits the linewidth of state-of-the-art optical clocks. However, experimental and theoretical results show that the thermal Johnson noise of cavity mirrors degrades the quantum limitation of Schawlow-Townes linewidth formula of good-cavity laser.
In this work, Prof. Chen proposed the concept, principles and techniques of active optical clock. This is the first extension of Hydrogen maser, which is the most stable atomic microwave clock and related to the Nobel Prize in Physics 1989, from microwave regime to optical regime.
The lasing behavior of active optical clock is a second-order phase transition. On the one hand, the collective emission of radiation from all gain atoms strongly narrows the linewidth of active optical clock described by the modified Schawlow-Townes linewidth formula. On the other hand, in an active optical clock, any shift and noise of the center frequency due to cavity will be reduced to a cavity pulling effect, which can be several orders of magnitude smaller than the general cavity noise.
“Active optical clocks provide several new possibilities of applications: (i) more stable optical clock than any current atomic clocks; (ii) sub-natural linewidth laser spectroscopy; (iii) long coherence time laser with linewidth at mHz level; (iv) Ramsey laser combining stimulated emission process and Ramsey separated oscillatory fields method.” commented Prof. Yiqiu Wang, the co-author of the first book “The principles of quantum frequency standards” in the research field of quantum frequency standards. A series of papers about active optical clock written by Prof. Chen and colleagues have been published in Chin. Sci. Bull., Phys. Rev. A, etc. “It is a novel idea. The active optical clock enriched and expanded the optical clock research.” said one reviewer.