The Quantum Electrical Metrology Division at the National Institute of Standards and Technology (NIST), in cooperation with the National Research Center (NRC), offers awards for postdoctoral research for American citizens in the fields described below. The Division conducts research in fundamental electrical metrology, mixed signal testing strategies, power and energy metrology, electrical discharge-physics and metrology to support digital video and video display characterization to provide, through both experimental and theoretical work, the necessary basis for solving the measurement-related requirements of the electronics and electrical-equipment industries.
NIST affords great freedom and an opportunity for both interdisciplinary research and research in well-defined disciplines. These technical activities of NIST are conducted in its laboratories, which are based in Gaithersburg, a large complex of modern laboratory buildings in a Maryland suburb of metropolitan Washington, DC. Although applications for NIST Research Associateships are accepted throughout the year, they are evaluated by the panels only during February. Additional information can be found at the NRC Research Associateships Program website: http://nationalacademies.org (click on Careers and Fellowships and then Graduate and Postdoctoral Fellowships, Associateship Programs).
The following Fellowship opportunities are for the Fundamental Electrical Metrology Group and the Applied Electrical Metrology Group, both in Gaithersburg, Maryland. For opportunities in the Quantum Devices Group in Boulder, Colorado, please use the link from our main page to find those opportunities.
As part of a metrology program to meet the need for improved signal acquisition and processing systems, NIST researchers are developing theoretical models, experimental methods, and standards for waveform metrology of conducted signals. This theoretical and experimental research is applied to the development of standards for high-speed A/D converters and waveform recorders operating at signal frequencies up to 5 GHz. Theoretical aspects of the work include Fourier analysis, deconvolution techniques, and time-domain analysis, while the experimental part of the program includes work in precision pulse generation, static and dynamic testing, and programming for hardware control in assembly and higher level languages.
Theoretical and experimental research is being conducted in synthesizing precision alternating current (ac) waveforms for use in ac voltage and arbitrary waveform standards operating nominally below 100 MHz. Theoretical work includes the use of Walsh and triangular functions as the basis for improved waveform fidelity, while experimental work involves high-speed, high-accuracy digital-to-analog conversion; precision, high-speed switching; assembly and higher level language programming for hardware control; and wideband, fast-settling amplifiers.
New strategies are needed to evaluate the performance of complex electronic circuits, devices, and instruments using the fewest possible tests. The testing strategies program in progress includes theoretical studies in modeling of nonlinear systems, optimization techniques using linear matrix methods, statistical and random processes, and neural networks. In addition, experimental work addresses strategies for component and instrument testing, fault diagnosis, functional testing, and calibration. Desktop computers, workstations, and supercomputers are available for computer simulation and analysis.
Theoretical and experimental research opportunities exist to study different problems related to picosecond electrical pulse metrology. These problems generally fall into one of three categories: fast pulse generation, with a goal of creating better pulse generator standards; fast pulse transmission, with goals of characterizing and understanding various transmission line structures from a time domain perspective; and fast pulse measurement, with goals of developing faster, more accurate and more robust pulse measurement systems. Most of these problems also offer opportunities to study signal processing as it relates to discrete, time domain measurement systems. Research facilities include several automated, fast electrical pulse sampling systems, and fast-pulse laser systems useful for electro-optic sampling, photoconductive switch pulse generation, and other experiments.
This research area provides U. S. industry with the link between the dc and corresponding ac electrical standards by improving and maintaining the U. S. national standards of ac-dc difference, which are used to provide calibrations and measurement services for thermal voltage converters and current shunts. We are developing a new capability for calibrating high current (100 A) shunts and high voltage (500 V to 1000 V) thermal converters. More reliable semiconductor thin-film converter devices are also being developed that can be readily fabricated and have state-of-the-art ac-dc differences. A new national primary standard is also being developed to support measurement uncertainties at the 0.1-microvolt per volt level.
The physics of electrical discharges in gases, liquids, and at gas-solid interfaces is investigated using measurement systems and theoretical models. Unique measurement systems allow observation of the stochastic behavior and memory effects associated with such pulsating electrical discharges as Trichel-pulse corona, partial discharge, and dielectric barrier discharges. Theoretical models are used to study stochastic properties of partial discharge, electron-avalanche growth and transitions from avalanche to streamer or glow discharge in nonuniform fields. Other research includes (1) studying the electron avalanche-to-streamer transition in insulating gases such as SF6-N2 and O2-N2 mixtures using intensified gated charge coupled device camera, (2) making fast measurements of correlations between electrical and optical emission characteristics of transient discharges, and (3) determining correlations between acoustical and electrical detections of partial discharges in liquids.
Experimental and theoretical research is under way to investigate chemical processes in radio-frequency (rf) glow discharges, corona discharges, and dc Townsend discharges that are relevant to processing of semiconductor materials or gas decomposition in gaseous dielectrics.
Experiments are performed to measure the mass and kinetic energy of ions emanating from rf and dc discharges using a quadruple mass spectrometer equipped with an energy analyzer. This diagnostic has been applied to rf and dc discharges in low-pressure gases such as argon, oxygen, SF6, CF4, and their mixtures in "standard reference" discharge cells. Comparisons are made to complementary optical and electrical diagnostic measurements, including optical absorption, optical emission, Langmuir probe, LIF, and electrical measurements. Related research includes (1) measurements on ion transport and ion-molecule chemistry using a uniform-field drift tube mass spectrometer system; and (2) measurements of corona discharge-induced oxidation and decomposition rates for SF6 and other gaseous dielectrics using a gas chromatograph mass spectrometer, and (3) development of chemical kinetics models for glow-type corona discharges in electronegative gases.
We use novel methods combining laser, electron, and ion technologies to study fundamental processes underpinning the behavior of gas discharges and their applications. Many of the processes studied involve neutral species in their ground and excited states, positive and negative ions, slow electrons, and photons. Special emphasis is placed on electron attachment to neutral molecules, excited molecules, and radicals; electron impact ionization; electron detachment processes; and reaction mechanisms involving neutral and charged species in a variety of environments. Current work focuses on electron interactions with excited molecules and radicals, and on the measurement of ion transport parameters in plasma processing gases. Both are important in plasma processing technologies. A related project is underway to develop a data base of critically analyzed electron-collision cross sections for industrial gases such as CF4, CHF3, and Cl2.
Because of their immunity to electromagnetic interference, optical sensors based on the electro-optic Pockels´ and Kerr effects and on the magneto-optic Faraday effect are advantageous for measuring high voltages and currents. These sensors are now being employed by the electric power industry in diagnostics, metering, and protection applications. They have been used for diagnostics in large pulse-power machines such as radiation simulators and electromagnetic launchers. Our research goals are to develop and evaluate the performance of electrical measurement systems and to develop techniques to ensure their long-term reliability. We are evaluating the response of sensors to steady-state signals and submicrosecond pulses, and assessing measurement uncertainties in well-characterized systems, such as high voltage dividers, Rogowski coils, and derivative (E-dot and B-dot) sensors. For ac measurements, new circuit designs are being developed for active high-voltage dividers with improved stability and reduced measurement uncertainties. In addition, experiments and mathematical models are used to characterize the dependence of the electrical and optical properties of optical sensor materials on environmental parameters (e.g., temperature, pressure, and radiation). Finally, numerical techniques are developed and applied to identify the sources and magnitudes of measurement errors and to compensate for them.
The Division is engaged in research on methods to improve accuracies of fundamental physical constants and to develop better and more accurate techniques for measuring and maintaining basic electric units. Research includes developing nuclear magnetic resonance-based current and voltage standards and measurements of the proton gyromagnetic ratio, absolute ampere, absolute volt, absolute farad and ohm, quantized-Hall resistance, and fine-structure constant. We are particularly interested in refining our current techniques and/or initiating new experiments to increase knowledge of these quantities or other constants of comparable importance, especially those involving the electrical units.
Our goal is to accurately define the electrical Watt as determined from Josephson Volt and Quantum Hall Ohm in terms of their SI definitions, which are related to the Kilogram, Second, and Meter. This experiment uses an ampere balance and has the potential to electronically monitor the Kilogram, which is the last artifact standard and may not be a true constant, and also to determine Planck´s constant and the mass of the electron. To perform this difficult and timely experiment, scientists are needed with experience in precision measurements of force and mass (balance design), velocity and index refraction (interferometer design), and voltage and current (magnets and moving coils) to 0.01 ppm uncertainty. A good understanding of classical electromagnetics, mechanics, and optics is necessary, and experience with electromagnetic interference protection and vibration isolation would be useful.
In nanoscale electronic circuits, we can observe Coulomb blockade or single electron tunneling (SET) effects. For metrological applications, the basic device is the single electron pump, which allows control of electrical flow in units of 1 e. This device enables accurate measurements of electrical current or charge. The Electricity Division studies such effects and their implications for precision metrology of the electrical units. We are pursuing two goals, both in close collaboration with our Boulder location. The first involves using the electron pump to charge up a cryogenic capacitor. Then, by comparison to the Calculable Capacitor and Josephson Volt experiments, we will make metrological measurements of the electrical charge, e, or the fine structure constant, a. Our second goal is to investigate ways to increase the value of the current, for use as a direct current standard. Our current approach is to use Si-based SET pumps, which hold the potential to be parallelized.
The use of quantum coherence for computing has gathered a lot of attention in the past few years, since it was shown that some computing algorithms can be vastly sped up using quantum computers (QC). One type of device envisioned for QC is that based on single-electron tunneling (SET) devices in the superconducting state, where the quantum computer "qubit" is the presence or absence of one extra pair of electrons. We are studying such an application in collaboration with a number of groups, including the University of Maryland and SUNY Stonybrook. One of our interests is in the problem of the charge offset and noise, which limit any possible use in QC. In addition, we are interested in the use of Si-based SET devices for detection of single spins, which form the basis for another possible QC qubit.
NIST is developing cryogenic current comparator (CCC) systems using high-temperature superconductor (HTS) magnetic shields, current carrying sheaths, and SQUIDS. The goal is to develop HTS CCC systems achieving resistance ratio uncertainties at 77 K of about 1 part in 109-two orders of magnitude better than room-temperature current comparators. The CCC systems have the potential to become the industrial standard for high-accuracy resistance ratio comparisons.
Research focuses on designing and developing complex HTS structures for use as shields and sheaths for HTS CCC systems. The sheaths must have excellent superconducting properties to support SQUID-based measurements of small dc surface currents (Is) with decay and rms noise of less than 10-6 Is per second. We are investigating thick-film and bulk HTS high-density superconducting phase materials with transition temperatures above 77 K. An external HTS hollow shield is required to provide a region of low ambient magnetic field (shielding factor = 1000) around the SQUID and current carrying sheaths.
Facilities include a wide range of ultra-high-precision electrical measuring apparatus for testing cryogenic instruments and quantized Hall resistance standard.
The physics of Josephson junctions, driven at microwave or millimeter wave frequencies, has important applications to ultra-high-precision voltage measurements. Among the behaviors observed but not well understood are resonance patterns within series-array Josephson junctions at frequencies between 70 GHz to 95 GHz, variable stability in time of quantized voltage steps in these devices, and the generation of Shapiro voltage steps at fractional values in both series arrays and high-temperature superconducting single junctions. Related applications in voltage measurement include the characterization of noise in electronic instrumentation, especially Zener-diode based references, at submicrovolt levels for normal measurement frequencies (>10 µHz), and nonlinear noise for much lower frequencies (>1 µHz).
Research facilities include three Josephson array voltage calibration stations, wideband frequency sources up to 40 GHz, phase-locking millimeter wave sources (70GHz to 95 GHz), a high-resolution spectrum analyzer, power meters, an assortment of high-precision voltage and frequency measurement and reference instrumentation, and various waveguide-equipped probes and magnetically shielded Dewars for cryogenic measurements.
Research focuses on developing improved reference standards of resistance and more accurate measurement systems for comparing resistance standards with the quantum-Hall effect. This research will involve investigation of new resistance alloys at room and cryogenic temperatures, alternating current/direct current (ac/dc) characteristics of resistors, and designs for constructing ruggedized standards. The following measurement systems are also being developed: (1) cryogenic current comparator resistance bridges, (2) SQUID-based nanovoltmeters, (3) ac resistance bridges, and (4) automated resistance bridges.
Research facilities include a resistor fabrication/heat-treatment laboratory equipped with a 1,000 o C programmable process furnace, cryogenic equipment consisting of dc and radio-frequency SQUID instrumentation, and a 16 T quantum-Hall system with a 3He refrigerator.
The Electricity Division is involved in a continuing research program on the quantum Hall effect, with emphasis placed on using it to maintain the U.S.-legal unit of resistance and to determine the fine structure constant to the highest possible accuracy. Any experiments that would further the understanding of the quantum Hall effect or explore its limitations would be of interest. Such experiments could include temperature and current dependence, current localization distributions (edge and bulk effects), voltage quantization (breakdown effect), and ac operation to quantized Hall resistance measurements that lead to ac impedance standards. Theoretical studies are also needed in all of these areas.
The apparatus consists of a two 16 T persistent-current superconducting magnets, a top-loading He-3 refrigerator, a variable temperature insert, and an automated quantized Hall resistance measurement systems with 0.008 ppm uncertainty parts-per-billion uncertainties.
In support of this research, a clean-room sample preparation facility has been installed that is equipped with a micrometer photo-mask aligner, wire bonder, annealing oven, and probe test station as required for the definition, mounting, ohmic contracting, and room-temperature testing of semiconductor samples for quantum Hall experiments.
National Standards Laboratories around the world use quantized Hall resistors to maintain their resistance standards. However, too little is known about the physical principles affecting their operation and the mechanisms responsible for their degradation. Furthermore, there are no reliable or repeatable techniques for fabricating devices of standards quality. Research in this area spans the fields of chemistry, materials, and physics. This program centers on (1) furthering the understanding of the physical principles that influence device performance under typical operating conditions; (2) developing techniques to fabricate reliable, standards-quality quantized Hall resistors; and (3) extending the range of operation of these devices to higher temperatures, lower magnetic fields, and higher currents. Sample preparation facilities include a clean room with equipment for optical photolithography, a wire bonder, an alloying furnace, and thin-film deposition. Research facilities include a cryostat with a superconducting magnet capable of testing samples at temperatures as low as 1.1 K in fields up to 16 T and an automated digital voltmeter-based measurement system capable of uncertainties as low as 0.05 ppm.
This work ties the U. S. legal system of units for capacitance and resistance to the SI system of units. This is done through the calculable capacitor experiment that can also be used to determine a value for the fine structure constant by comparison with the quantized Hall resistance. This determination of the fine structure constant currently has an uncertainty of 2 in 108, which is the lowest in the world for this method. Work is in progress to further reduce the uncertainty of this determination. Other research in this area involves the development of precision ac transformer bridges for capacitance and resistance measurements with uncertainties of a few parts in 109. The techniques for building these bridges have been developed at NIST for scaling of resistance and impedance measurements of the highest levels of accuracy. Current efforts include extending these bridge measurements from 1592 Hz, which is currently used, to frequencies between 100 Hz to 2000 Hz. These ac bridges will also be used in the ac quantum Hall experiment and the single electron tunneling experiment.
NIST's flat panel display laboratory serves the display industry by developing and quantifying good electronic display metrology for industrial use. With the explosion of the information age, the Internet, and e-commerce, the use of flat panel displays has become a growing need for U. S. industries. Good display measurement methods are needed because of the fierce competition between technologies, allowing consumers to compare features of displays accurately and fairly. NIST is doing research in (1) equipment on improving measurements made on displays; (2) development of display metrology with various standards organizations; (3) development of display metrology assessment methods and equipment to provide guidance for the implementation of good measurement methods in the display industry; and (4) display reflectance characterization, measurements, and modeling using the bi-directional reflectance distribution function.