Improvements in spectroscopic resolution have been the driving force behind many scientific and technological breakthroughs over the past century, including the invention of the laser and the realization of ultracold atoms. Creating and preserving optical phase coherence are one of the two major ingredients (the other being the control of matter) for this scientific adventure. Lasers with state-of-the-art frequency control can now maintain phase coherence over one second, that is, 1015 optical waves can pass by without losing track of a particular cycle. Translating into distance, such a coherent light wave can traverse the circumference of the Earth 10 times and still interfere with the original light. The recent development of optical frequency combs has allowed this unprecedented optical phase coherence to be established across the entire visible and infrared parts of the electromagnetic spectrum, leading to direct visualization and measurement of light ripples. A new generation of atomic clocks using light has been developed, along with new forms of optical interferometry, with an anticipated measurement precision reaching 1 part in 1018. These developments will have impact to a wide range of scientific problems such as the possible time-variation of fundamental constants and gravitational wave detection, as well as to a variety of technological applications including global position systems and deep space navigation.

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