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June 1996 (Volume 5, Number 6)
A joint symposium on Quantum Measurement and Standards, chaired by R. Erdman of Kiethley Instruments, organized by the Instrument and Measurement Science Topical Group and the Precision Measurement and Fundamental Constants Topical Group, focused on increasingly precise measurement techniques. Scientists continue to find ways to improve measurement techniques and devices, in such areas of better atomic clocks, measuring the mass of the kilogram, and redefining the Coulomb — the basic unit of electric charge — in terms of quantum mechanical measurements rather than classical electrical measurements.
Robert Drullinger of the National Institute of Standards and Technology (NIST) reported on ongoing attempts to improve the accuracy of NIST-7, the world's best atomic clock, which already operates with an uncertainty of less than one part in 1014 — a rate that is equivalent to gaining or losing less than a second in over 3 million years. Since the first ammonia-based atomic clock was built in 1948, NIST has improved the performance of their standards by a factor of 10 million. NIST is now working on advanced prototypes that have the potential for an additional factor of improvement of at least 10,000.
According to Drullinger, the highest demands on time and frequency precision measurements are made by users involved with secure communications systems, deep-space navigation systems, and scientific tests of basic concepts in nature. At a lower level, precision measurements are needed for general telecommunications systems, electronic navigation systems for ships, aircraft and land vehicles, and electric power companies that share power across international power grids.
To stay ahead of the demands of industry and science for ever more accurate time and frequency standards, NIST has an active research program aimed at developing the next generation of standards. For example, an atomic fountain standard is currently under development, which operates by launching laser-cooled cesium atoms straight up through a cavity and letting them fall back by gravity. The low velocity of the atoms is expected to reduce the uncertainty in assessing their frequency to a level five to ten times better than the current standard.
Theoretically, the ultimate atomic clock would depend on the behavior of a single stationary, isolated atom, since the fewer the components and their interactions in a timepiece, the greater the accuracy. To this end, a group at NIST's Boulder, Colorado, laboratory has developed a linear ion trap for mercury atoms that have been stripped of one electron. The ions are held in an electromagnetic field and irradiated with laser light in such a way that their motion and temperature are reduced to nearly zero. This eliminates major sources of uncertainty about the resonance frequency, and also allows scientists to observe a given collection of ions for a much longer time. The fundamental uncertainties of this system are thought to be no more than a few parts in 1018.
According to Aaron Gillespie, NIST is also working on a new experiment aiming to re-define the kilogram, the international unit of mass, in terms of fundamental constants in nature. The kilogram is the only international scientific unit still defined in terms of a physical object — a platinum-iridium cylinder stored in Sevres, France.
Using a specially designed balance-wheel apparatus, the NIST experiment yields a value of the kilogram by comparing measurements of mechanical power to measurements of electrical power. It takes advantage of the fact than an electrical watt can be expressed precisely in terms of quantum-mechanical measurements. The apparatus could also be used to monitor possible long-term drift of the kilogram artifact.
Finally, recent advances using quantum dots and single electron transfer suggest that exciting new approaches are on the horizon for developing new quantum current measuring techniques, according to Wiley Kirk of the NanoFAB Center at Texas A&M University.
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