  Atomic frequency standards are used to generate accurate and precise time and
frequency, enabling many communications, synchronization, and navigation
systems in modern life. GPS and other satellite navigation systems, voice and
data telecommunications, and timestamping of financial transactions all rely on
precise time and frequency enabled by atomic frequency standards.
  This review provides a snapshot and outlook of contemporary atomic frequency
standards and the applications they enable. We provide a concise summary of the
performance and physics of operation of current and future atomic frequency
standards. Additionally, examples of emerging frequency standard technologies
and prototype demonstrations are presented, with a focus on technologies
expected to provide commercial or military utility within the next decade.
  We include a comparison of performance vs. size and power for current atomic
frequency standards, and we compare early prototypes of next-generation
frequency standards to current product trends. An empirical relationship
between frequency standard performance and product size is developed and
discussed. Finally, we provide a mapping between applications and frequency
standard technologies.
