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Future Directions

In responding to its mission, the Division has developed unique capabilities that can be used to address other opportunities. Although the list is far greater than the resources available to pursue them, it is worthwhile to examine them here. Special capabilities of the Division include: (1) low-phase-noise components and systems; (2) satellite-timing receivers and transmitters (3) systems for trapping and cooling ions and for cooling neutral atoms; (4) highly stable lasers and microwave sources for high resolution studies of atoms and molecules; (5) well-characterized atomic beams; and (6) high-resolution systems for imaging atomic particles. The Division also has a strong tradition of accurate frequency measurement across the electromagnetic spectrum and leading-edge talent in statistical analysis of time series of data. Considering these assets, we list the following examples of basic and applied research opportunities.

• Telecommunications Networks. The Division has had strong interactions with the telecommunications industry on a number of synchronization issues. This industry continues to evolve rapidly, and 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.

• Satellite Timing. One of the key limitations to satellite time transfer is the uncertainty in signal propagation time, which arises from variations in the index of refraction of the troposphere, and in the electron density of the ionosphere. The Division has 1) completed preliminary tests on a system for doing time transfer using the phase of the GPS carrier and 2) developed a two-frequency receiver for determining ionospheric delay from GPS satellites. Expertise developed through these diverse projects can be used to improve the performance of time and frequency comparisons made using satellite 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) has been accomplished 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 experiments are needed to guide the development of future neutral-atom frequency standards.

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

• 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 short-term mechanical isolation of reference cavities, used recently, for example, in demonstrating the world's most stable visible laser. Locking to an optical transition in trapped ions or atoms provides the best means for long-term laser stabilization.

  The Division is studying various approaches to measuring optical frequencies involving frequency multiplication and division. Of particular note is the development of custom-fabricated nonlinear mixers using periodically poled lithium niobate (PPLN), which has been used to generate harmonics and sum and difference frequencies in important spectral regions. Combined with diode lasers and other solid-state lasers, these mixers offer unique opportunity for the development of a synthesis chain linking the optical region to the cesium frequency standard.

  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 nonneutral 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 estimating systematic errors (primarily Doppler shifts) arising from ion motion.