pairs in a superconductor. The interactions between pairs are mediated by the so-called Feshbach resonance, in which the relative translational motion of the pairs is strongly coupled to a bound state with a different total spin. Because of the difference in spin, the strength of this interaction can be tuned with an external magnetic field, while the cold atoms are held in an optical dipole trap. The ability to tune the pair interaction has led to a wealth of phenomena. In one experiment, a sudden pulse of magnetic field puts pairs of atoms into a novel quantum superposition that oscillates in time between bound and unbound states.

Another study explores the variation from the BEC regime, where localized pairs could form a Bose condensate to the Bardeen-Cooper-Schrieffer (BCS) regime, where pair formation is mediated by the surrounding condensate atoms and the size of the pair could become very large. This BEC-BCS crossover is important theoretically, and its understanding could impact the theory of High-Tc superconductivity. This work has been enabled by novel techniques developed for extracting information on the pair correlations from noise in absorption images of the cold atomic cloud.

A third line of experimentation seeks to convert the highly interacting pair states into real bound states by removing the appropriate energy from the pairs with an optical Raman process. This has recently succeeded in producing bound diatomic molecules (though still highly excited relative to the molecular ground state) from these interacting cold Fermi pairs.

Techniques for cooling small molecules are also being developed, with the goal of placing the molecules into a single chosen quantum state in order to study the fundamental quantum physics of cold molecules and their chemical interactions in single quantum states. To this end, OH molecules have been cooled with a novel Stark deceleration method and trapped in a magnetic trap.

Work also continues on the development of new systems and techniques in frequency measurement. JILA has long been renowned as a center of excellence for laser frequency stabilization and for the development of methods for precision frequency measurement. One of the most important tools in recent years is the stabilized laser frequency comb, a technique that was brought from conception to fruition at JILA and for which a retired NIST JILA Fellow (now NIST Scientist Emeritus and JILA Fellow Adjoint) shared the Nobel Prize in physics in 2005. The laser frequency comb has produced a revolution in optical frequency measurement and is key to the development of optical atomic clocks. Current work in this division applies the frequency comb technique to a new atomic clock system composed of an optical lattice of laser-cooled strontium atoms. Scientists in this division realized that the energy states of strontium had characteristics that should lead to extremely small systematic frequency uncertainty and thus to potential clock accuracy and repeatability comparable with or better than the best previously identified clock systems. In a tour de force of atom cooling, trapping in an optical lattice and optical clock technology, this novel optical clock features a large number of atoms responding identically, thus providing the precision of a single atom with vastly enhanced signal-to-noise ratio. Fractional frequency instability of 3 × 10−15 has already been achieved, with the expectation that this can be improved by a factor of three. A cooperative effort with the NIST Time and Frequency Division in Boulder has compared the accuracy of this clock with the calcium optical clock at Time and Frequency. This was achieved using the capability for coherent optical phase transfer



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