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|Photo credit: NIST Public Affairs|
|NIST's chip-scale atomic clock includes (from the bottom) a laser, a lens, an optical attenuator to reduce the laser power, a waveplate that changes the polarization of the light, a cell containing a vapor of cesium atoms, and (on top) a photodiode to detect the laser light transmitted through the cell. The tiny gold wires provide electrical connections to the electronics for the clock.|
The latest research in atomic clocks, attosecond laser experiments, and ultracold neutral plasmas were among the featured topics at the 2006 annual meeting of the APS Division of Atomic, Molecular and Optical Physics. The DAMOP meeting took place May 16-20 in Knoxville, Tennessee.
In addition to the technical program, the meeting featured a Wednesday evening public lecture by the University of Nebraska’s Timothy Gay, and banquet keynote address by Patricia Dehmer, associate director of the DOE’s Office of Basic Energy Sciences, on the new “American Competitiveness Initiative.” Conference attendees were also given the opportunity to tour the Spallation Neutron Source and Center for Nanophase materials at Oak Ridge National Laboratory.
Keeping Time with Atoms. Kurt Gibble (Penn State University) reviewed some recent advances in the use of laser-cooled atoms in atomic clocks that have resulted in significant improvements in accuracy. This requires finding creative solutions to the frequency shifts that occur in cold collisions. Those solutions include using adiabatic fast passage to accurately evaluate the cold collision frequency shift, as well as “fountains” based on rubidium atoms.
NIST scientists have built similar fountain atomic clocks using cesium atoms, although the focus of NIST’s John Kitching’s paper at the DAMOP meeting was on recent efforts to develop millimeter-sized devices based on atomic spectroscopy for highly precise timing and sensing applications. Such structures rely on miniature alkali vapor cells–fabricated using standard MEMS techniques–that allow atoms to be confined along with a buffer gas. The atoms in the cell are excited using laser light, as well as magnetic fields generated by microfabricated current loops. Potential applications for such units–which are about the size of a grain of rice and require less than 200 mW of power to operate–include GPS receivers, wireless communication devices, remote monitoring, and explosives detection.
For all the advances in atomic clocks, most commercial atomic frequency standards (AFS) still rely on conventional technology developed in the 1950s. Symmetricom Technology Realization Center has developed specialized laser sources for commercial applications. For example, the Chip-Scale Atomic Clock is smaller and uses less power than other commercial AFS devices, enabling atomic timing accuracy in portable battery-powered applications. The optically pumped cesium beam frequency standard is being developed for deployment onboard the GPS-III satellite constellation.
Combing Optical Frequencies. Femtosecond laser frequency combs (FLFCs) have found widespread use in optical atomic clocks, as well as in optical frequency metrology. Now, these broadband, evenly-spaced arrays of optical frequencies–produced by femtosecond mode-locked lasers–are beginning to play a vital role in other precision measurements, according to NIST’s Scott Didams. These include using optical frequency combs for direct atomic spectroscopy and in more transportable instruments. There are also new possibilities in arbitrary waveform generation, spectroscopic sensing, and secure optical communications, thanks to the development of highly dispersive elements that permit the spatial separation of the frequency comb elements while maintaining high resolution.
Just an Attosecond. Attosecond pulses of light can be generated via the nonlinear interactions between an intense, ultrashort laser pulse and a gas of atoms, via the process of high harmonic generation. According to Joachim Burgdorfer of Vienna University of Technology in Austria, the process has sufficiently advanced to the point where scientists can now generate attosecond electromagnetic pulses of sufficiently short duration to approach the orbital period of a classical atomic electron. This means we may be able to map out the electronic dynamics inside atoms in real time.
At the same session, Gerhard Paulus (Texas A&M University) reported on a novel application of intense few-cycle laser pulses: an attosecond version of the famous double-slit experiment first conducted by Thomas Young in the 19th century. In this instance, the double slit is realized in the time-energy domain (rather than position-momentum), and the “slits” can be opened or closed by changing the temporal evolution of the field of a few-cycle laser pulse.
Interstellar Bio-Building Blocks. Currently, more than 125 different chemical compounds have been detected in the interstellar medium, many of which are unusual, such as metal cyanide species, or organic molecules like acetone or the simple sugar, glycolaldehyde. The common appearance of organic molecules and simple species with a metal center suggests that the building blocks of life may have originated in interstellar space, according to Lucy Ziurys of the University of Arizona. She and her colleagues have developed an effective combination of techniques for studying such elements in the laboratory, with an eye towards evaluating the limits of chemical synthesis in interstellar gas.
Strongly Coupled Plasmas. Ultracold neutral plasmas are produced by photoionization of laser-cooled neutral atoms. Physicists find them of interest for many reasons, most notably the prospect of creating–in a tabletop experiment–a strongly coupled two-component plasma, in which the electrostatic potential energy greatly exceeds the thermal kinetic energy of the particles that comprise the plasma.
Thomas Pattard of the Max Planck Institute for the Physics of Complex Systems in Dresden, Germany, reported on recent work showing that the addition of Rydberg atoms to a plasma allows one to significantly control the electronic temperature in order to achieve both cooling and heating of the plasma electrons. Numerous experiments have also demonstrated a “dipole blockade” effect, in which the disorder-induced heating of the ions is suppressed. Not only is this effect important to the creation of strongly coupled plasmas, it also plays a vital role in certain proposed schemes for quantum information processing.
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