抄録
We describe the recent developments in optical-to-microwave frequency comparison, which is the most
fundamental application of an optical frequency comb. We focus on a comparison between 10 MHz and
an optical frequency, which is needed to compare Coordinated Universal Time (UTC) with an optical
clock. We report on the worldwide research situation, the precision limit conditions, and our achievement
of a 10 MHz-to-optical frequency comparison with a 10− 18 precision level at NMIJ/AIST.
