The Schawlow-Townes limit in frequency comb metrology

Since the advent of mode-locked laser frequency combs, metrology has experienced a rapid improvement of the precision in frequency measurements. Best reported microwave-based measurements with cesium fountain clocks reached a fractional uncertainty of 10 -15 in 2005 [1] . In 2010 optical frequency measurements took the lead, and more recent experiments with optical lattice clocks reached 10 -18 values. Frequency transfer experiments even demonstrated sub 10 -20 levels [2] . Fractional uncertainities currently decrease at the pace of one order of magnitude every 4 years. If this Moore’s Law of frequency metrology continues at the same rate, one expects to see 10 -22 uncertainties at the end of this decade. Similar to Moore’s Law, however, the question arises for how long this trend can further continue before some fundamental physical mechanism eventually causes stagnation. And one such fundamental barrier arises due to Schawlow-Townes noise in the mode-locked laser, causing a 1/ f -like drift mechanism between two independent frequency combs.