The "Dr. Allen V. Astin" Award & Invited Category

Chip-Scale Atomic Clocks at NIST

We describe recent efforts to develop microfabricated atomic frequency references capable of supporting a wide variety of commercial and military systems such as global positioning and wireless communication. These devices are anticipated to eventually have a volume of 1 cm3, dissipate less than 30 mW of electrical power and maintain a fractional frequency stability better than 10-11 over one hour. Because of the small size and low power requirements, these devices will enable atomic-level timekeeping in portable, battery-powered units.

With fabrication techniques commonly used in microelectromechanical systems (MEMS), we have designed and constructed a series of chip-scale atomic clock physics packages. With volumes below 10 mm3, the physics packages contain a complete integrated assembly for probing the hyperfine frequency of the 87Rb atoms by coherent population trapping [CPT]. This technique allows for a simple and compact device containing a semiconductor laser, optics to shape the laser beam, a vapor cell containing the atoms and a detector. All parts of the package can be microfabricated on whole wafers of glass or silicon allowing a large number of components to be fabricated at the same time and reducing fabrication costs. Our best-performing chip-scale atomic clock at present has a fractional frequency instability of 4.5 x 10-11 at 1 second of integration time. The performance of the chip-scale atomic clock physics package in terms of short- and long-term stability as well as power consumption will be presented.

Applied Category

Stabilization of SPRTs for ITS-90 Calibrations

At the National Institute of Standards and Technology (NIST), the ITS-90 calibration of standard platinum resistance thermometers (SPRTs) is performed in the Platinum Resistance Thermometer Calibration Laboratory over the range from –189 °C to 962 °C. As part of the quality system internal measurement assurance program, the stabilization of the SPRT prior to calibration is used to determine when and if an SPRT is stable enough to be calibrated within the NIST assigned ITS-90 realization uncertainties. Prior to calibration of an SPRT, the SPRT resistance at the triple-point of water [R(TPW)] is determined, the thermometer is annealed a temperature of at least 450 °C for four hours, and then the R(TPW) is again determined. The SPRT must repeat at the R(TPW) after an stabilization cycle to within the equivalent of 0.2 mK within five stabilization cycles to qualify for calibration. The temperature and duration of the annealing depends on the calibration temperature range and the amount of change at the R(TPW) between the “as received” value and the previous NIST calibration value. Additional stability requirements are used during the calibration to qualify the SPRT as NIST-calibrated ITS-90 defining standard. The SPRT must not change by more than the equivalent of 0.3 mK for the R(TPW) measured before and after each other fixed-point cell. The total SPRT R(TPW) change during a calibration must not exceed the equivalent of 0.75 mK. Failure of the SPRT to meet any of the three criteria, results in the rejection of the SPRT for use as a defining standard, and the SPRT is returned without calibration values. The stabilization techniques used and the impact on the overall stability of the SPRT during a calibration are given.

Theoretical Metrology Category

Simplifications From Simulations

Monte Carlo simulation is fast becoming a tool of choice for exploring the probabilistic interpretation of ISO GUM-compliant uncertainty budgets. It is particularly appealing to those uncomfortable with the classical development of statistics by theorems and lemmas. The technique is useful when creating and defending calibration uncertainty statements and when interpreting the results of inter-laboratory comparisons for critical purposes such as ISO 17025 accreditation. We explain the Monte Carlo technique, and demonstrate how to use the Excel spreadsheet environment to build simple yet powerful simulations that can provide insight and understanding to metrologists and clients alike. Topics ranging from high quality random number generation to the physical basis for sampling from a Student distribution are covered, with examples, source code, and real-time demonstrations. Advanced techniques for writing, building and using dynamic link libraries are introduced for routines written in a high level programming language such as C or Fortran. This subject should appeal to potential users of the GUM and its (Draft) Supplement 1 who are seeking rigor and simplicity.

Metrology…Who Benefits and Why Should They Care?

The National Metrology Institutes push the boundaries of metrological capability to ever-greater heights, in turn spurred on by advances in science and technology, the demands of industry and the needs of society. Whilst the measurement community are convinced of the benefits, metrology is often at best an unseen foundation on which other empires are constructed. Many new products and processes, new science and technology, indeed new markets and the legislation that governs them, depend on good metrology. It would therefore seem logical that metrology and measurement are intrinsic elements in planning the processes on which they impact, yet often they not routinely addressed, or at least not in a timely way. The NMI mission includes delivering benefits to the national economy or quality of life for our citizens by working to overcome this inertia. However, in a global economy we increasingly need to rethink our definitions of national impact, as nowadays many drivers and their consequent effects are no longer confined within national boundaries. This paper will reflect on the mechanisms by which metrology impacts our future, the interplay between national and global perspectives, and suggests new thrusts for embedding metrology “upstream” into our economies and lives.