An Assessment of Precision Time and Time Interval Science and Technology (2002)

In typical applications of PTTI, entire systems consisting of the high-performance clocks, local oscillators, and distribution systems provide the needed signal. For this reason, advancing the sciences of PTTI requires research in a variety of fields. Some of the research is already targeted for other applications, and PTTI is a beneficiary of the investments that are being made in them. One such field is high-performance, low-noise electronics, which is required for a variety of applications. Other fields of research are more specific to PTTI and must be supported directly, among them certain areas of AMO physics, materials science, and chemistry.

AMO physics is closely associated with PTTI technology. This area includes atomic physics, quantum metrology, and quantum optics. The most stable and accurate clocks are based on techniques and fundamentals derived from AMO physics. State-of-the-art atomic clocks based on the laser cooling of ions and the laser cooling and trapping of neutral atoms, have one to two orders of magnitude higher accuracy than conventional thermal beam or collisionally cooled atomic clocks. The trapping of atoms with magneto-optic traps (MOTs) or far detuned light fields to allow confinement of neutral atoms is essential for the isolation of atoms from the perturbing collisions that are encountered with cell confinement. This reduces Doppler broadening of atomic transitions by significant amounts, reducing first- and second-order Doppler shifts to nearly negligible values. Laser excitation of atoms is crucial to increasing the signal-to-noise ratio of the observed transition, a parameter that determines the ultimate stability of the clock. Advanced clocks operating with laser excitation are limited only by the fundamental quantum noise limit. AMO physics will undoubtedly continue to advance high-performance PTTI systems. The use of Bose-Einstein condensates (BECs) to realize high-performance clocks holds great promise. The very cold temperatures, the high coherence, and the very high phase space densities associated with BEC are precisely the parameters that relate to the performance of an atomic clock. Development of optical frequency combs that can be used to relate optical and microwave frequencies may enable application of laboratory devices based on optical transitions to possess up to three orders of magnitude

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.

Heterodyned optical atomic clocks have been achieved by the development of microstructured optical fiber combined with advanced mode-locked lasers. Further research to improve the reproducibility of fiber performance and to extend these fibers to infrared wavelengths would improve optical atomic clocks and many other technologies. Currently, there are only a few sources of microstructured fiber, and none are specifically targeting PTTI research.

The feasibility of achieving a 100-fold improvement in stability with the microcomputer-compensated crystal oscillator (MCXO) has been shown. However, to achieve such performance reproducibly and to extend the performance, the frequency versus temperature hysteresis problem for such crystals must first be solved. The noise, power, and size of the MCXO also need to be reduced. These problems have not been solved because the resources directed toward crystal research were inadequate. Improvements in crystal oscillators can decrease the vulnerability of frequency-hopping systems to smart jammers and improve the ability to locate radio emitters in the field.

Several technologies for low-noise oscillators need to be explored. These include (1) surface transverse-wave resonators and dielectric resonator oscillators for ultralow noise floors and (2) novel bulk acoustic wave (BAW) and surface acoustic wave (SAW) resonators for vibration-insensitive oscillators. Fundamental noise studies in piezoelectric devices (for example, of 1/f noise) are also needed. The availability of low-noise resonators and oscillators has the potential to improve missile accuracy and improve surveillance systems, provide more accurate onboard radar systems for missile guidance, improve IFF systems, and enable more effective radar spoofing.

Many key performance parameters of clocks and oscillators are directly impacted by processes related to chemistry. Below, some aspects of chemistry that can potentially lead to higher-performance atomic clocks and local oscillators are identified.

Solid-state chemistry research could further the availability of high-perfection quartz. Evidence indicates that the stability of quartz devices is limited by quartz imperfections, such as dislocations and impurities. The feasibility of growing dislocation-free, ultrahigh-purity quartz has been demonstrated in the United Kingdom, but there is still no method for producing amounts sufficient to meet DOD needs. Developing such a capability could pay off by enabling the production and fielding of low-power, low-cost, high-performance quartz clocks that would make frequency-hopping systems virtually invulnerable to smart jammers and improve the ability to locate radio emitters.

Quartz has been used exclusively in high-stability oscillators but has serious limitations, such as a phase transition at 573°C that prevents the use of high-temperature manufacturing processes for purification and stress relief. During the past decade, new piezoelectric materials were discovered that show

great promise for providing the advantages of quartz without its limitations and giving better performance. Langasite, a lanthanum gallium silicate (La₃Ga₅SiO₁₄) invented in Russia, and lithium tetraborate, invented in the United Kingdom, are examples. These materials have a lower acoustic attenuation and higher piezoelectric coupling. Programs to further develop, characterize (with respect, for example, to linear and nonlinear material constants as functions of angles of cut and temperature), and establish U.S. sources for these and other (e.g., gallium phosphate) materials are needed. New optical materials could also eventually find use as local oscillators. Materials with large electro-optic coefficients, such as ferroelectrics, are used for OEOs. Research is needed to evaluate known materials, with the eventual goal of optimizing material characteristics. For example, ferroelectrics demonstrate piezoelectricity that deleteriously affects vibration sensitivity. Successful demonstration of such new materials could lead to high spectral purity at frequencies above 10 GHz.

Isolation of atoms in atomic clocks is required to obtain the most precise signals. Wall coatings are used to reduce spin relaxation during wall collisions. The wall coatings currently used in hydrogen masers cannot be used for alkali metal (rubidium, cesium)-based atomic clocks owing to long-range electron-transfer (harpooning mechanism) reactions between the alkali atoms and fluorine-containing wall coatings. Even the coatings currently used in hydrogen masers have not been optimized. A research program in self-assembled monolayers (SAMs), for example, would be extremely useful for investigating ways of reducing container wall collision interactions. Reducing collisions could lead to smaller device cavities, higher-performance hydrogen masers, and higher-performance alkali-metal-based clocks and contribute to the realization of a chip-scale atomic clock.

The theory of resonators is highly complex. Finite element model (FEM) calculations of “real” three-dimensional resonators have not been feasible until recently owing to the inordinately long supercomputer calculation times required. With improved supercomputers and algorithms, computer-aided design (CAD) of resonators and oscillators is becoming feasible. Atomistic models are also becoming feasible with the aid of powerful computers. Research programs at universities are needed to realize the required CAD models and simulation capabilities. Improvements in resonator design can lead to devices for improved missile accuracy, better surveillance systems, more accurate onboard radar systems for missile guidance, superior IFF systems, and the ability to effectively conduct radar spoofing.

High-performance atomic clocks could greatly benefit from new approaches for confining atoms and for exciting atomic transitions. Nanosciences are poised to play an important role in reducing the size and power consumption of these devices. MEMS technology already has produced high quality-factor resonators that promise to enable local oscillator operation near the performance level of quartz at a fraction of size, weight, and power consumption.

While the significantly narrow transitions associated with electromagnetically induced transparency in condensed matter systems are currently possible only with cryocooled samples, it is not unreasonable

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.