better accuracy than current microwave-based devices. Beyond this, research in quantum processes that allow clocks to operate below the fundamental quantum noise level promises the realization of even more performance. Theoretical studies addressing this issue have been pursued, and a potential experimental realization, so-called spin squeezing, has been demonstrated by researchers at NIST.
Device size and power consumption are as important to many PTTI applications as are stability and accuracy. Recent developments in AMO physics may also contribute here. The coherent population trapping (CPT) clock represents a new class of atomic clock having an inherent simplicity that renders it particularly suitable for applications where size and the power consumption bear a premium. While the feasibility of these clocks was first demonstrated in 1984, it is only within the past several years that operating clocks based on this approach have been built in the laboratory. This area of research is still quite young and will require much work before applications might be realized.
Materials science plays a key and often underappreciated role in the development of PTTI devices.1 Materials are crucial to the realization of high-performance clocks, since they help isolate the high-performance elements of the device from environmental effects, reduce the size and weight of devices, and enable operation in adverse environments such as space. Some research has been funded in piezoelectric materials for the development of resonators with performance superior to quartz. Support for this research has been dwindling. Opportunities in other areas of materials research have been overlooked. Below the committee identifies some areas of materials science research that could lead to improvement in PTTI devices and systems.
Many of the designs and processes used in high stability resonators were invented more than 30 years ago. Promising new designs, processes borrowed from semiconductor microfabrication technology, and innovative packaging methods have been proposed but not implemented for lack of resources. Advances in these areas could lead to lower cost, more reliable, more compact packages for high-stability resonators.
Microresonators and thin-film resonators promise to provide miniature (e.g., MMIC-compatible) and high-frequency (above 100 GHz) resonators, filters, and sensors. Both piezoelectric (quartz, aluminum nitride, zinc oxide, and other) and nonpiezoelectric (e.g., silicon) devices show great promise. Silicon microresonator arrays, although not temperature stable, show great promise for on-chip integration as (low-frequency) filters and sensors. Advances in this area have the potential to provide more compact radio frequency front ends (integrated circuit designs rather than discrete bulk devices) by reducing the size of the pre-down-conversion filters and duplexers in receivers and transceivers.