The basic protocol for quantum time metrology hasn’t changed much since 1955, the year of the atomic clock’s debut: Isolate an atom from external fields, tune coherent radiation to an atomic transition, and count the radiation’s oscillations to measure the ticking of time. (See the article by James Bergquist, Steven Jefferts, and David Wineland in Physics Today, March 2001, page 37.) Unlike pendulums and other mechanical timekeepers, atoms exist in abundant identical copies and don’t wear out with use. Any two clocks set to the same atomic transition should, in theory, keep identical time.
In practice, however, our abilities to isolate atoms and pinpoint their transition frequencies are imperfect. Even in the most careful experiments, environmental influences—stray radiation, interactions between atoms, and the like—inevitably perturb an atom’s internal state. Likewise, atoms’ thermal motion generates Doppler shifts that broaden spectral lines and make exact transition frequencies difficult to locate....
