This past month NIST presented an atomic standard for temperature.
Today, at an American Physical Society meeting in Baltimore, Md., the organization presented a system that precisely measures the vibrations of silicon-nitride molecules in response to changes in temperature, ranging from cryogenic to room temperature. The effort may lead to high-accuracy temperature readings in extremely sensitive lab applications and industry procedures.
The method employs a beam of silicon nitride with small reflective cavities. The scientists directed a laser through a thin crystal beam, and measure the changes in reflected wavelength, which they could directly relate to the picometer-scale thermal vibrations.
But what makes the method so dependable is the scientists' ability to calibrate the thermal vibrations to fluctuations in electron spin, which remain constant independent of temperature. Since both vibrations are measured using the same laser, any measurements that indicate changes in quantum vibrations would also indicate an error in the readings for thermal vibration.
“At top is an electron micrograph of the silicon nitride beam. The bottom shows how the beam deforms as it vibrates (length scale greatly exaggerated) with the red regions showing the most deformation, and the blue regions not moving at all. Over one vibrational period the center of beam goes from being stretched out as shown, to being compressed inward, and then back.” (from the NIST article)This is fundamental science, and that usually means some years before commercialization can happen. The summary abstract on the NIST website goes into more details on how it all works.
The NIST team’s approach arose from their efforts to observe the vibrations of a small transparent beam of silicon nitride using laser light. Thermal energy—often expressed as temperature—makes all objects vibrate; the warmer the object, the more pronounced the vibrations, though they are still on the order of just a picometer (trillionths of a meter) in size for the beam at room temperature. To observe these tiny perturbations, the team carved a small reflective cavity into the beam. When they shone a laser through the crystal, the light reflecting from the cavity experienced slight shifts in color or frequency due to the beam’s temperature-induced vibrations, making the light’s color change noticeably in time with the movement.Unlike the articles on the atomic clocks, this piece doesn't show any accuracy or precision numbers for this new way of measuring temperature. NIST hints that their goal is to use this as an integrated atomic calibration system for temperature measurement. They would pair this sensor with on-chip photonic thermometers that are also under development at NIST which are more sensitive but require frequent calibration. The photonic thermometers would offer the high temperature sensitivity demanded by some applications, such as pharmaceutical manufacturing and other high performance industrial applications, while this new quantum thermometer would act as an integrated temperature standard, able to keep the photonic thermometer calibrated over long periods of time.
But these were not the only vibrations the team members could see. The team also spotted the much more subtle vibrations that all objects possess due to a quantum-mechanical property called zero-point motion: Even at its lowest possible energy, the beam vibrates ever so slightly due to the inherent uncertainty at the heart of quantum mechanics. This motion is independent of temperature, and has a well-known amplitude fundamentally dictated by quantum mechanics. By comparing the relative size of the thermal vibration to the quantum motion, the absolute temperature can be determined.
“Our technique allowed us to tease the quantum signals out from under the much larger thermal noise,” says the team’s Tom Purdy, a physicist at NIST’s Physical Measurement Laboratory and at the Joint Quantum Institute. “Now we can directly connect temperature to the quantum mechanical fluctuations of a particle. It sets the stage for a new approach to primary thermometry.”