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Comb technology enables metrologists to relate microwave frequencies to optical frequencies in a single step, without the need for complex frequency chains. Optical combs are actively being investigated in a number of national metrology institutes and at the BIPM, for stability, accuracy and any unsuspected systematic errors. In the meantime they enable frequency measurements that are traceable to the SI second to be made quite quickly on optical and infrared laser sources. There is also the interesting possibility that a combination of super-stable optical frequency sources and comb generators may well enable us to redefine the second in terms of an optical rather than a microwave radiation. Whilst we could still then live with the existing definition of the metre, the future for the next generation of time standards, with applications to navigation and to the ever-growing need for precise time measurement, is an intriguing and exciting one.
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Summary
