TECHNICAL ACTIVITIES 1998 - NISTIR 6268

FUTURE DIRECTIONS

• Telecommunications Networks. The Division has had strong interactions with the telecommunications industry on a number of synchronization issues. This industry is going through a period of rapid development and change where measurements and standards can play a positive, organizing role. The Division is in an excellent position to contribute to the development of improved synchronization strategies as well as to support the industry with methods for characterizing the performance of synchronization components and systems.

• Time Series Analysis. Over the last two decades, the Division has been a leader in the development of statistical methods for time series of clock and oscillator data. For such timing systems, the standard deviation does not always yield correct results, particularly where the behavior of systems is observed over long periods (as is done with standards) where assumptions about noise properties are usually incorrect. To take better advantage of time-series-analysis methods requires further developments, especially in the analysis of unequally spaced data. Furthermore, the ideas must be simplified if they are to be widely used. Additional work is also needed on cross-correlation methods and the modified Allan variance, and new measures are needed to more adequately specify the occurrence of large excursions from the mean. Such measures are important in areas like telecommunications where average behavior is less important than short-period, large excursions that lead to lost packets of information.

• Satellite Timing. One of the key limitations to satellite time transfer is the uncertainty in signal propagation time that arises from variations in the index of refraction of the troposphere and in the electron density of the ionosphere. The Division has developed a two-frequency receiver for determining ionospheric delay from GPS satellites and a three-wavelength geodimeter for accurate terrestrial length measurements in air. Expertise developed through this work can be used to improve the accuracy of existing time transfer systems. Furthermore, experience with GPS timing-receiver design and telecommunications satellites can be applied to the development of more accurate time-transfer systems.

• Atomic Physics. In atomic physics, Division programs are centered on strengths in ion trapping, cesium beams, low-noise frequency synthesis, and accurate optical frequency measurements. Example areas of study include:
  • Quantum-limited measurements on atoms. The ability to detect atomic states in individual atoms or ions with nearly 100 % efficiency enables studies of the fundamental limits of quantum noise in measurements. Theoretical studies show that, with the use of certain quantum-mechanically correlated states, the time required to reach a certain measurement precision can be reduced by a factor equal to the number of ions in the sample. Experiments are underway to prepare such states.

  • Quantum computation. The first demonstration of logic gates using quantum bits (superpositions of 0's and 1's) have been performed using trapped atomic ions. Current efforts are devoted to increasing the number of bits in a register to perform calculations and create correlated states.

  • Creation and quantum-state tomography of nonclassical states of motion. Coherent, squeezed, Fock, and "Schrödinger cat" states of motion have been created in the motion of trapped atomic ions. This problem has a formal connection to and provides an interesting complement to studies of cavity-QED.

  • Study of Influence of Radiation Fields on Atoms. Here we compare predictions of quantum and semi-classical theory and study radiation damping and frequency pulling in cavities.

  • High-Resolution Optical Probing of Cold, Trapped Neutral Atoms. Such methods are needed to study trapped neutral atoms and they may be required for future standards applications.

  • Improved Measurements of Atomic Fine-Structure Frequencies. These weak magnetic-dipole transitions can be directly measured using laser-magnetic-resonance methods.

• Molecular Physics. Spectroscopy is being performed with coherent radiation using tunable far-infrared (TuFIR) and laser magnetic resonance (LMR) systems. Both of these provide for highly accurate and sensitive measurements of atomic and molecular spectral frequencies as well as line shapes. Such frequency-based measurements are important not only as wavelength standards, but also in atmospheric physics and astrophysical research. The Division capability can be used to observe and characterize spectra of unusual and rare molecules, molecular ions and free radicals which heretofore have not been observable in the laboratory. The dependence of line shape and width on temperature and pressure, which can also be precisely determined in the laboratory, contributes to remote sensing of important upper atmospheric molecules. Further development of diode lasers will support all of this work and result in unique opportunities for ultrasensitive spectroscopic detection of atoms and molecules providing for monitoring of important species.

• Optical Physics. Studies in optical physics revolve around the need to develop very narrow-linewidth sources of radiation at a variety of frequencies (from the microwave to the visible) and to accurately measure optical frequencies. Laser studies, including studies of fundamental noise processes, involve gaseous lasers, dye lasers, solid-state lasers and diode lasers. Particular emphasis is placed on development of stabilized, narrow-linewidth diode lasers which show great promise for a wide range of standards and precision-measurement applications. There is also need for further development of mechanical isolation of reference cavities, used recently, for example, in demonstrating the world's most stable visible laser. Long-term laser stabilization is best provided by locking to an optical transition in trapped ions or atoms.

  The Division is studying standard approaches to measuring optical frequencies involving frequency multiplication. This requires the development of an understanding of mixers such as Schottky diodes, metal-insulator-metal (MIM) diodes, nonlinear crystals, vacuum-tunnelling diodes and, perhaps, superconducting (Josephson or quasiparticle) junctions. The Division is also studying frequency-division schemes involving diode lasers and nonlinear optics to reach important transitions and spectral regions. Such division should simplify and improve measurement of optical frequencies.

  Laser cooling and Doppler-free spectroscopy are traditionally categorized under optical physics and should be included, since they constitute major components of the Division program.

• Plasma Physics. Stored clouds of beryllium and magnesium ions (which are being studied as prototype frequency standards) constitute very interesting non-neutral plasmas. These plasmas have distribution functions which are closely related to those of neutral plasmas. Under proper laser-cooling conditions, such plasmas become strongly coupled and can be liquid or solid. Important experiments to consider involve study of Bragg scattering, ion diffusion, phase transitions, Coulomb clusters (the classical limit of Wigner crystallization) and multispecies ion plasmas. Such studies are also relevant to the development of frequency standards since a full understanding of the dynamics of ions in these systems provides the basis for determining systematic errors (primarily Doppler shifts) arising from ion motion.

Mission  |  Organization  |  Current Directions  |  Technical Highlights  |  Future Directions

TECHNICAL ACTIVITIES 1998 - Contents

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Online: April 1999