The precise measurement of time is of such fundamental importance to science and technology that the search for ever greater accuracy continues. The performance of atomic clocks had been improving by a factor of at least 10 per decade for about 50 years. But over the past decade improvements in atomic clock accuracy have dramatically accelerated. Recent advances in laser science—particularly the Nobel Prize–winning development of femtosecond laser frequency combs—and atomic physics have enabled the development of many new types of optical atomic clocks, some based on transitions in single ions in electromagnetic traps and some based on collections of cold neutral atoms held in lattices formed by laser light. Several of these atomic clocks are already stable to within a few hundred femtoseconds per day and continue to rapidly improve.
At this level of performance, formerly negligible effects become important and measurable. For example, the best atomic clocks can now measure changes in gravity over the distance of a stair step, tiny magnetic fields generated by heart and brain activity, and other quantities such as temperature and acceleration. Companies are now manufacturing “chip-scale” atomic clocks the size of a quarter. In addition to keeping time with increasing accuracy, new generations of atomic clocks will be used as exquisite sensors for myriad applications and will become ever smaller and more portable.
Although our ability to measure time will surely improve in the future, nothing will change the fact that it is the one thing of which we will never have enough.