to expect that technologies related to caged atoms and clathrates may play a significant role in future high-performance clocks, especially with respect to size and robustness. Techniques of the nanosciences may also become invaluable in devising approaches that allow direct detection of the polarization state of an atom.

Micro- and nanoscale technology also promise to aid the development of a chip-scale atomic clock. If realized, this could profoundly impact the tactical use of the GPS in adverse and jamming environments.

Many realizations of the laser-cooled or laser-excited atomic clocks rely on low-cost and low-power semiconductor lasers. These devices are needed at specific wavelengths corresponding to cesium and rubidium transitions. Issues related to the fabrication, ruggedization, and radiation hardening of semiconductor devices at these specific wavelengths remain for researchers to pursue. Research in other devices, including high-efficiency modulators and switches, passive optical elements, and optical whispering-gallery-mode microresonators also could greatly benefit high-performance PTTI. Such research could advance atomic clocks and local oscillators, including those currently based on the photonics technology, such as the OEO.