High-accuracy dimensional measurements are critically important to scores of processes in industry, medicine, and science. The quality of those measurements depends on the availability of absolute length scales to calibrate instruments such as interferometers that are used in the most exacting applications. Atomic vapor-cell technology can fill that need by providing a quantum-based standard of length derived directly from the SI.
Because the speed of light (the wavelength times the frequency) is a universal constant with a specific, defined value, if the frequency is known exactly, so is the wavelength. The frequency of many optical transitions in atoms have been measured, resulting in a set of known wavelengths that can be used as length references. NIST’s chip-scale length standard works by using a tunable laser to find the precise frequency at which an atomic transition takes place, and then locking the laser onto that frequency. The frequency is uniquely determined by the laws of quantum mechanics. So, when it is realized, the laser light has an exactly known wavelength -- an SI-traceable length standard -- that can serve as a reference for interferometers used for distance measurements with uncertainties in the range of parts in 1011.
Such a scale could also be important for systems that transfer high-density, two-way information in light traveling through optical fibers. A process called wavelength-division multiplexing encodes a few or many dozens of multiple signals in simultaneous streams of different wavelengths. For that technique to work effectively, the wavelengths of the component streams must be carefully differentiated using a standard to avoid mixing.
There are several alternative designs for a vapor-cell length standard. At present, NIST scientists regard two as most promising. The first employs vapors of acetylene or hydrogen cyanide, both of which molecules have vibrational resonances in useful wavelength ranges around 1.5 micrometers. The other would contain a rubidium vapor, and would transfer information via photonic channels made of silicon nitride -- a material that is fully transparent to light of the wavelengths of interest.
Eventually, the researchers believe that the vapor-cell length standards could function with cells as small as 100 cubic micrometers. They would be highly manufacturable, with hundreds of units on a single silicon wafer, thanks to existing microfabrication technology and to the rapidly expanding capabilities in manufacture of photonic circuits.