NIST Tech Beat - August 11, 2009

An experimental atomic clock based on ytterbium atoms is about four times more accurate than it was several years ago, giving it a precision comparable to that of the NIST-F1 cesium fountain clock, the nation’s civilian time standard, scientists at the National Institute of Standards and Technology (NIST) report in Physical Review Letters.*

NIST scientists evaluated the clock by measuring the natural frequency of ytterbium, carefully accounting for all possible deviations such as those caused by collisions between the atoms, and by using NIST-F1 as a “ruler” for comparison. The results were good enough to indicate that the ytterbium clock is competitive in some respects with NIST-F1, which has been improving steadily and now keeps time to within 1 second in about 100 million years. (Because the international definition of the second is based on the cesium atom, technically no clock can be more accurate than cesium standards such as NIST-F1.) More importantly, the improved ytterbium clock gives the time standards community more options in the ongoing development and comparisons of next-generation clocks, says NIST physicist Chris Oates, an author of the new paper.

The NIST ytterbium clock is based on about 30,000 heavy metal atoms that are cooled to 15 microkelvins (close to absolute zero) and trapped in a column of several hundred pancake-shaped wells—an “optical lattice”—made of laser light. A laser that “ticks” 518 trillion times per second induces a transition between two energy levels in the atoms. The clock’s enhanced performance was made possible by improvements in the apparatus and a switch to a different form of ytterbium whose nucleus is slightly magnetic due its “spin-one half” angular momentum. This atom is less susceptible to key errors than the “spin-zero” form of ytterbium used previously.

NIST scientists are developing five versions of next-generation atomic clocks, each using a different atom and offering different advantages. The experimental clocks all operate at optical (visible light) frequencies, which are higher than the microwave frequencies used in NIST-F1, and thus can divide time into smaller units, thereby yielding more stable clocks. Additionally, optical clocks could one day lead to time standards up to 100 times more accurate than today’s microwave clocks.

The best optical clocks are currently based on single ions (electrically charged atoms), such as the NIST “logic clock” using an aluminum ion (see “NIST ‘Quantum Logic Clock’ Rivals Mercury Ion as World’s Most Accurate Clock”.) But lattice clocks have the potential for higher stability because they simultaneously average signals from tens of thousands of atoms. Ongoing comparisons of the ytterbium clock to that of the strontium lattice clock located nearby at JILA, a joint institute of NIST and the University of Colorado at Boulder, (see “Collaboration Helps Make JILA Strontium Atomic Clock ‘Best in Class’”) should help enable worldwide tests of optical clock performance with extremely high precision. At this point it is far from clear which atom and clock design will be selected by research groups around the world as a future time and frequency standard.

Advances in atomic clock performance support development of technologies such as high data rate telecommunications and the Global Positioning System (GPS). Optical clocks are already providing record measurements of possible changes in the fundamental “constants” of nature, a line of inquiry that has huge implications for cosmology and tests of the laws of physics, such as Einstein’s theories of special and general relativity. Next-generation clocks might lead to new types of gravity sensors for exploring underground natural resources and fundamental studies of the Earth. Other possible applications may include ultra-precise autonomous navigation, such as landing planes by GPS.

* N. D. Lemke, A. D. Ludlow, Z.W. Barber, T. M. Fortier, S.A. Diddams, Y. Jiang, S. R. Jefferts, T. P. Heavner, T. E. Parker and C.W. Oates. Spin-1/2 optical lattice clock. Physical Review Letters. Published online Aug. 3, 2009.

Researchers at the National Institute of Standards and Technology (NIST) have developed a new calibration technique that will improve the reliability and stability of one of NIST’s most versatile technologies, the microhotplate. The novel NIST device is being developed as the foundation for miniature yet highly accurate gas sensors that can detect chemical and biological agents, industrial leaks and even signs of extraterrestrial life from aboard a planetary probe.

The tiny microhotplates—no wider than a human hair—are programmed to cycle through a range of temperatures. They can be coated with metal oxide films tailored to detect specific gas species. Airborne chemicals attach to the surface of the detector depending on the type of film and the temperature of the surface, changing the flow of electricity through the device, which serves as the “signature” for identifying both the type and concentration of the gas in the ambient air.

Accurate microhotplate temperature measurements are crucial for the discrimination and quantification of gas species, while reliable, long-term operation demands that the microhotplate’s temperature sensors be either highly stable or able to sense when they’ve drifted, a functionality known as a “built-in self test” (BIST). As demonstrated for the first time in a paper in an upcoming issue of IEEE Electron Device Letters,* the new calibration method satisfies both requirements.

A portion of the polysilicon heater making up the microhotplate originally served as the device’s temperature sensor. However, this sensor would slowly drift over time from its initial calibration. Within three months, the temperature readings were off by as much as 25 degrees Celsius at high temperatures.

The NIST engineers overcame this shortcoming by using data from two additional temperature sensors—a highly stable, thin-film platinum/rhodium thermocouple integrated in the microhotplate structure for one sensor and the thermal efficiency of the structure itself for the other. Comparing the temperatures reported by these two sensors provides the microhotplate with its internal monitoring system. As long as the absolute value of the difference between the reported temperatures remains below a specified threshold value, the average of the two readings is considered reliable. Should the difference exceed the threshold, the system reports an error.

The original polysilicon sensor still provides the microhotplate’s initial temperature measurement, which is used to calibrate the other two sensors. With the complete “check and balance” system in place, temperature measurements are accurate to within 1.5 degrees Celsius.

Having successfully demonstrated the new temperature calibration system for their microhotplate, the NIST researchers are working on additional advancements for the technology. Next in line is the development of a built-in system for sensing contamination of the metal oxide films critical to the microhotplate’s use in gas detection.

* M. Afridi, C. Montgomery, E. Cooper-Balis, S. Semancik, K.G. Kreider and J. Geist. Analog BIST functionality for microhotplate temperature sensors. IEEE Electron Devices, Volume 30, No. 9 (September 2009).

As a result of a major inter-laboratory study, the standards body ASTM International has been able to update its guidelines for a commonly used technique for measuring the size of nanoparticles in solutions. The study, which was organized principally by researchers from the National Institute of Standards and Technology (NIST) and the Nanotechnology Characterization Laboratory of the National Cancer Institute, enabled updated guidelines that now include statistically evaluated data on the measurement precisions achieved by a wide variety of laboratories applying the ASTM guide.

Data from the inter-laboratory comparison gathered from 26 different laboratories will provide a valuable benchmark for labs measuring the sizes and size distribution of nanoparticles suspended in fluids—one of the key measurements in nanotechnology research, especially for biological applications, according to materials researcher Vince Hackley, who led the NIST portion of the study.

Size is an important characteristic of nanoparticles in a variety of potential uses, but particularly in biotech applications where they are being studied for possible use in cancer therapies. The size of a nanoparticle can significantly affect how cells respond to it. (See, for example “Study: Cells Selectively Absorb Short Nanotubes,” NIST Tech Beat, March 30, 2007.)

One widely used method for rapidly measuring the size profile of nanoparticles in, say, a buffer solution, is photon correlation spectroscopy (PCS), sometimes called “dynamic light scattering.” The technique is powerful but tricky. The basic idea is to pass a laser beam through the solution and then to measure how rapidly the scattered light is fluctuating—faster moving particles cause the light scattering to change more rapidly than slower moving particles. If you know that, plus several basic parameters such as the viscosity and temperature of the fluid, says Hackley, and you can control a number of potential sources of error, then you can calculate meaningful size values for the particles.

ASTM standard E2490 is a guide for doing just that. The goal of the ASTM-sponsored study was to evaluate just how well a typical lab could expect to measure particle size following the guide. “The study really assesses, in a sense, how well people can apply these techniques given a fairly well-defined protocol and a well-defined material,” explains Hackley. Having a “well-defined material” was a key factor, and one thing that made the experiment possible was the release this past year of NIST’s first nanoparticle reference standards for the biomedical research community—NIST-certified solutions of gold nanoparticles of three different diameters, a project also supported by NCL. (See “NIST Reference Materials Are ’Gold Standard’ for Bio-Nanotech Research, ” NIST Tech Beat, Jan. 8, 2008.)

The inter-laboratory study required participating labs to measure particle size distribution in five samples—the three NIST reference materials and two solutions of dendrimers, a class of organic molecules that can be synthesized within a very narrow size range. The labs used not only PCS, but also electron and atomic force microscopy. The results were factored into precision and bias tables that are now a part of the ASTM standard.

Physicists at the National Institute of Standards and Technology (NIST) have demonstrated sustained, reliable information processing operations on electrically charged atoms (ions). The new work, described in the Aug. 6, 2009, issue of Science Express,* overcomes significant hurdles in scaling up ion-trapping technology from small demonstrations to larger quantum processors.

In the new demonstration, NIST researchers repeatedly performed a combined sequence of five quantum logic operations and 10 transport operations while reliably maintaining the 0s and 1s of the binary data stored in the ions, which serve as quantum bits (qubits) for a hypothetical quantum computer, and retaining the ability to subsequently manipulate this information. Previously, scientists at NIST and elsewhere have been unable to coax any qubit technology into performing a complete set of quantum logic operations while transporting information without disturbances degrading the later processes.

The NIST group performed some of the earliest experiments on quantum information processing and has previously demonstrated many basic components needed for computing with trapped ions. The new research combines previous advances with two crucial solutions to previously chronic vulnerabilities: cooling of ions after transport so their fragile quantum properties can be used for subsequent logic operations and storing data values in special states of ions that are resistant to unwanted alterations by stray magnetic fields.

The NIST experiments described in Science Express, stored the qubits in two beryllium ions held in a trap with six distinct zones. Electric fields are used to move the ions from one zone to another in the trap, and ultraviolet laser pulses of specific frequencies and duration are used to manipulate the ions’ energy states. The scientists demonstrated repeated rounds of a sequence of logic operations (four single-qubit operations and a two-qubit operation) on the ions and found that operational error rates did not increase as they progressed through the series, despite transporting qubits across macroscopic distances (960 micrometers, or almost a millimeter) while carrying out the operations.

The NIST researchers applied two key innovations to quantum-information processing. First, they used two partner magnesium ions as “refrigerants” for cooling the beryllium ions after transporting them, thereby allowing logic operations to continue without any additional errors due to heating incurred during transport. The strong electric forces between the ions enabled the laser-cooled magnesium to cool down the beryllium ions, and thereby remove heat associated with their motion, without disturbing the stored quantum information. The new experiment is the first to apply this “sympathetic cooling” in preparation for successful two-qubit logic operations.

The other significant innovation was the use of three different pairs of energy states within the beryllium ions to hold information during different processing steps. This allowed information to be held in ion states that were not altered by magnetic field fluctuations during ion storage and transport, eliminating another source of processing errors. Information was transferred to different energy levels in the beryllium ions for performing logic operations or reading out their data values.

* J.P. Home, D. Hanneke, J.D. Jost, J.M. Amini, D. Leibfried and D.J. Wineland. Complete methods set for scalable ion trap quantum information processing. Science Express. Posted online Aug. 6, 2009.

The National Institute of Standards and Technology (NIST) has released its final version of a publication that represents a major step toward building a unified information security framework for the entire federal government.

“This final publication represents a solidification of the partnership between the Department of Defense, the Intelligence Community, and NIST and their efforts to bring common security solutions to the federal government and its support contractors,” says Ron Ross, of NIST’s computer security division. “The aim is to provide greater protection for federal information systems against cyber attacks.”

Comments received from the public since June did not result in any major changes in the final publication, according to Ross.

Historically, information systems at civilian agencies have operated under different security controls than military and intelligence information systems. When complete, the unified framework will result in the defense, intelligence and civil communities using a common strategy to protect critical federal information systems and associated infrastructure.

The National Institute of Standards and Technology (NIST) and the American National Standards Institute (ANSI) have published a report on the inaugural meeting of the Nuclear Energy Standards Coordination Collaborative (NESCC), a new ANSI Standards Panel, co-chaired by NIST and ANSI, to address the current and future standards needs of the nuclear energy industry.

Based on applications and communications received by the Nuclear Regulatory Commission (NRC), more than 30 new nuclear power reactors are expected to be under construction by 2020. In addition, more than 100 existing reactors must be re-licensed—including an assessment of the integrity of the reactor core after 50 years of operation. Because it has been more than 25 years since a new nuclear power reactor has been constructed in the United States, material science and construction techniques have changed substantially, and many of the construction codes and techniques and documentary standards of the past century need to be updated. New and advanced nuclear reactor designs, for example, require standards in areas such as fire protection, seismic requirements, and graphite-core support structures.

The NESCC was formed to address these needs. In addition to NIST and ANSI, the panel is supported by the NRC, the Department of Energy (DOE) and more than 30 private sector standards development organizations, professional societies and industry associations.

The new report explains the rationale for the collaboration; a review of the draft group charter; a discussion of the NESCC’s organizational structure; and identification and discussion of potential standards topics for technical task groups to address. NESCC is accepting proposals throughout the summer for task groups; interested individuals may contact NIST’s Ambler Thompson (ambler@nist.gov; (301) 925-2333). The chosen task groups will meet during the next NESCC meeting, scheduled for the week of Dec. 7, 2009.

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Editor: Michael Baum

Date created: August 11, 2009
Date updated: August 11, 2009
Contact: inquiries@nist.gov