NIST Backs Quantum-level Temperature Measurement

Release time:2017-07-04
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source:EE Times
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Forget Kelvin, Fahrenheit, and Celsius, not to mention centigrade. The quantum-level accuracy of a temperature measurement unit called the SI (Système International d'unités, or International System of Units), the newest addition to the metric system, will obsolete the old Newtonian mechanics K, F, and C by 2019, according to the National Institute of Standards and Technology (NIST, Boulder, Colo.), which has developed a solid-state apparatus for measuring temperature in SI using the Boltzmann constant.

The shift to the SI standard should be especially helpful to electronics engineers seeking to measure hot spots on a die, increasing the accuracy of their measurements as well as speeding up the process. The Kelvin, for instance, can only be measured down to a parts per million with expensive traditional instruments, whereas the SI is hoped to spur inexpensive electronic instruments that can measure down to parts per billion.

Temperature is a tricky unit to measure, since it depends on the material used and becomes less accurate as you get further away from its “central” degrees unit: 273.15 K = 32˚F = .01˚C. Today that central unit is standardized as the point at which water freezes, but that metric is fraught with inaccuracies, stemming from impurities in the water and the inevitable presence of an undetermined amount of “heavy water” (H2O with a neutron attached) in a given sample. Today, engineers must take into account the type of material as well as the temperature in order to compensate for the inaccuracies of using traditional instruments and units to measure heat.

The quantum voltage noise source from NIST uses the voltage noise from electrons in a resistor to measure temperature more accurately. Source: Dan Schmidt/NIST
The quantum voltage noise source from NIST uses the voltage noise from electrons in a resistor to measure temperature more accurately.
Source: Dan Schmidt/NIST

The SI unit sidesteps these inaccuracies by using quantum measurements of the motion of electrons (Johnson noise) in the sample, thus standardizing on an easily obtainable quantum measurement of the Boltzmann constant (the average kinetic energy of particles). Every laboratory worldwide aims to begin using SI for quantum-accurate temperature measurements within the next two years.

NIST’s inexpensive apparatus depends on a quantum voltage noise source (QVNS) that basically measures the voltage noise from electrons rattling around in a resistor. The apparatus provides a rock-solid reference measurement of the Boltzmann constant which can then be used to determine the SI temperature of any substance.

The SI unit is the result of a worldwide effort. NIST is just supplying the prototype of a simple QVNS reference measurement of the Boltzmann constant, as it has done for other inexpensive methodologies, materials, and laboratory instruments that it has developed over the years for accurately calibrating virtually every unit of measurement used worldwide.

The die of the NIST apparatus supplies a super-accurate Boltzmann constant, which makes temperature readings using the SI unit of measurement easy to perform. Source: NIST
The die of the NIST apparatus supplies a super-accurate Boltzmann constant, which makes temperature readings using the SI unit of measurement easy to perform.
Source: NIST

Representatives from around the world will vote on whether to redefine the temperature standard as the SI unit in November 2018 at the General Conference on Weights and Measures in Versailles, France. There is little doubt that the SI will replace the Kelvin as the international standard of temperature, but NIST is not waiting for the vote and is making its apparatus details available now.

NIST physicist Samuel Benz holds the new, solid-state instrument (left) and an older, mechanical instrument (right) used to measure the Boltzmann constant.
NIST physicist Samuel Benz holds the new, solid-state instrument (left) and an older, mechanical instrument (right) used to measure the Boltzmann constant.

“After the Boltzmann constant is defined, a successful Johnson noise thermometry [JNT] system will allow sub-50-parts-per-million measurements of temperature over a range of 100 K to 1,000 K using a rack-mountable system. Since this system is based on the definition of the Boltzmann constant and quantum phenomena, it would be self-calibrating. The other requirement for a successful JNT system would be disseminating it to other users outside of NIST,” institute researcher Nathan Flowers-Jacobs told EE Times in an exclusive interview.

“Shrinking the Johnson noise thermometry system, removing the need for liquid cryogens, and making it user-friendly are going to be a multiyear program. Therefore, we hope that it will begin to be used outside of NIST within five years,” Flowers-Jacobs said.

NIST scientist Horst Rogalla was leader of the Johnson noise thermometry project. The National Institute of Metrology in China also contributed to the work.

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The National Institute for Science & Technology (NIST) has requested drastic cuts in funding to its measurement activities for fiscal 2019. The budget includes a nearly 28% ($176 million to $127 million) cut to the agency's Fundamental Measurement, Quantum Science and Measurement Dissemination activities. The FY2019 budget includes the shutting down of time and frequency standard broadcasts from Ft. Collins, Colorado (WWV) and Kaua'i, Hawaii (WWVH). The two station broadcast time announcements, standard time intervals, standard frequencies, UT1 time corrections, a BCD time code, geophysical alerts, marine storm warnings, and Global Positioning System (GPS) status reports. WWVB, also from Colorado, does not appear to be affected. The station's reference signal is used by consumer devices such as clocks. WWV is the longest active broadcast station in the world, with its 100th anniversary coming on Oct. 1, 2019. WWV radio station in Ft. Collins, Colorado is slated to shut down in its 100thyear because of budget cuts. Source: NISTThe NIST stations broadcast time and frequency information over carriers at 2.5 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz. WWVH broadcasts on the same frequencies except for 20 MHz. Equipment uses these finely calibrated signals to assure accurate time and frequency. While many engineers have built electronic equipment — some for hobbyist kits — that relied on these signals, do we need them anymore given that we have time available online and through GPS? For example, your cell phone's time can be set by the network and you can use NIST's time and date page or Internet Time Service. The only advantage I can see is the radio-based time and frequency services aren’t dependent on the internet. All you need is a receiver. But then, it's so easy to add a GPS receiver to your equipment or design one yourself. The closing of WWV and WWVH will trim $6.3 million from the NIST budget. Another $3.5 million cut will come from the agency's "Lab to Market" activities, whose goal, according to NIST, is to accelerate technology transfer from federal laboratories to industry. NIST has not provided information on what percentage of the total budget that amount represents. EE Times contacted NIST for that information. NIST replied by saying "Ask your Congressman." That's just what I'll do. NIST did not provide information as to cuts to calibration services. Calibration services are paid for by those who request them. 
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