The beginning of hurricane season in the northern Atlantic coincides this year with the publication of new research from the National Institute of Standards and Technology concerning improvements to the metrology of the unit of pressure: the pascal (Pa). Scientists are working to relate the pascal to quantum properties. Making the pascal a quantum-based standard means that measurement devices do not need to be calibrated and that the standard, because it depends on invariant physical constants, is intrinsically correct.

The quantum-based metrology initiatives published in the Journal of Vacuum Science & Technology A consist of a group of devices whose capabilities are intended to span the range of pressures from deep vacuum to several atmospheres. The two most mature are the Fixed-Length Optical Cavity and the Cold Atom Vacuum Standard.

The Fixed-Length Optical Cavity is an interferometric device, measuring 1 Pa to approximately 100 kPa. This device consists of a laser wavelength-locked to a Fabry-Perot cavity and measures the index of refraction, which can be converted to pressure. Because a gas’s refractivity is a function of the polarizability of its particles, the refractivity is a fundamental quantum standard.

The Cold Atom Vacuum Standard employs a cold atom trap and is anticipated that it will measure pressures from approximately 10−6 Pa into the 10−10 Pa range. Previous research results show a relationship between the lifetime of a cold trap and its vacuum level. This device measures the collision cross sections between sensor atoms and background molecules, a fundamental quantum property, and uses that relationship to determine the number of background particles, a value which can then be converted to pressure.

Source: “Quantum based vacuum metrology at the National Institute of Standards and Technology,” by Julia Scherschligt, James Fedchack, Zeeshan Ahmed, Daniel S. Barker, Kevin Douglass, Stephen Eckel, Edward Hanson, J. H. Hendricks, Nikolai Klimov, Thomas Purdy, Jacob Ricker, Robinjeet Singh, and Jack Stone, Journal of Vacuum Science & Technology A (2018). The article can be accessed at