Ultimate Quantum Noise Limit of Frequency Comb Measurements

Frequency comb measurements have enormously progressed in the last decade, with 10−18 precisions coming into reach [1]. Similar to Moore's law, the precision in frequency metrology measurements has improved by about 3 orders of magnitude in a little bit more than a decade. Along these lines, the question may arise whether there will be continuous improvements for decades to come — or whether, at one point, an ultimate limitation to further improvement arises. Moreover, frequency metrology has proven superior to other methods to evince a drift of fundamental physical constants, e.g., the fine-structure constants, yet no such drift has ever been convincingly proven using either optical [2] or cosmological methods [3]. So the question is: how low can we go before quantum effects impose an ultimate limit for precision frequency metrology?