Two Atomic Clocks Ticking as One

How do you set your watch? Perhaps you set it from a radio broadcast, or your computer, or your mobile phone. Each of these devices gets its time in turn from a hierarchy of clocks, typically terminating at a national institute that keeps the reference standards of measurement in your country. If your watch is stable, how well you can rely on it depends on how well you can set it, and thus on propagation delays and “jitter” (phase noise) in the propagation of the reference timing signal itself—whether by radio or over a computer or telecommunication network. In practice, your wristwatch is not stable enough for this to matter, but it is a problem for the best atomic clocks in the national institutes. These are now so stable that they cannot be compared by any of these means, or even by satellite, without noise in the comparison swamping the stability of the clocks. Without new techniques, such as long-range transfer over optical fiber reported on page 441 of this issue by Predehl et al. ( 1 ), we will not be able to take full advantage of these clocks in international timekeeping, and the amazing progress of atomic clocks may grind to a halt.