#frequency comb

Time running out for current length of a second | The Independent

Time is running out for the current length of a second after an “optical clock” was sent into space for the first time. Such clocks are up to 1,000 times more accurate than the current international standard, which dates back to 1967 and is based on the natural oscillation of an atom of caesium, rather than the swing of a traditional pendulum. While no one would notice the difference in everyday…

Light can come in many frequencies, only a small fraction of which can be seen by humans. Between the invisible low-frequency radio waves used by cell phones and the high frequencies associated with infrared light lies a fairly wide swath of the electromagnetic spectrum occupied by what are called terahertz, or sometimes submillimeter, waves.  

By remotely "combing" the atmosphere with a custom laser-based instrument, researchers from NIST, in collaboration with researchers from the National Oceanic and Atmospheric Administration (NOAA), have developed a new technique that can accurately measure—over a sizeable distance—amounts of several of the major "greenhouse" gases implicated in climate change.