The Korea Research Institute of Standards and Science (KRISS) has re-measured the frequency of a key transition of the strontium atom - for the first time since 1938. To do this, the team used a photonic diffraction chip: a plate with a microstructure that guides and reflects light. The uncertainty of the new value is more than 50 times smaller than before. The work was published in the journal Metrologia.
Strontium is used in precision quantum technologies - optical lattice clocks and quantum sensors. To work with the atoms, they are slowed and held by lasers with a precisely tuned frequency. This requires knowing the exact resonant frequency of light corresponding to the transition in the atom.
The previous value was obtained in 1938 by comparing the solar spectrum with a strontium arc discharge. Its uncertainty is too large for modern lasers, so researchers selected the frequency by observing the atoms' response - like turning the knob of an analog radio receiver to catch a station.
With the development of light control and the miniaturization of quantum devices, an accurate absolute frequency value became necessary. The team measured the absolute frequency of the strontium optical transition at a wavelength of 461 nanometers (nm - one billionth of a meter). After a year of precise measurements, the uncertainty was reduced to about 5 megahertz (MHz - one million oscillations per second), which is more than 50 times better than the previous result.
The measurement was tied to South Korea's national time and frequency standard, which KRISS maintains. Because this standard is linked to the International System of Units (SI), the results can be reliably compared with the same reference anywhere in the world, regardless of laboratory or equipment.
The key device is a photonic diffraction chip created by KRISS together with the Korea Advanced Institute of Science and Technology (KAIST) and the National NanoFab Center. A thin grating on the surface of the chip, installed in a vacuum, deflects and reflects light in specified directions. This reduces errors from mechanical vibrations compared with conventional setups that require aligning several external mirrors. In addition, a single chip can create laser beams in several directions, simplifying the optical setup.
Having reduced hardware errors, the researchers independently determined the transition frequency by two methods with different principles to verify reliability. One analyzes the atoms' response to light as a function of laser frequency; the other calculates the frequency from the velocity distribution of an atomic beam slowed by lasers. The values agreed within the uncertainties, and the final result was calculated taking into account the uncertainty of each result according to international rules.
The new value can serve as a reference for precise cooling, trapping and control of strontium atoms in optical lattice clocks, quantum sensors and neutral-atom quantum computers. Together with the optical frequency synthesis technology under development, this will free quantum devices from reliance on bulky external atomic frequency standards. Such technologies are expected to accelerate the miniaturization and commercialization of quantum devices based on cold atoms.
"The main value of the work is not simply re-measuring a single frequency, but that we have provided in advance a reference point for the next generation of quantum technologies," said Dr. Jae Hoon Lee, a principal research scientist in KRISS's atomic quantum sensor group. He added that the institute, as the national metrology body, will continue to create standards for new areas of measurement in quantum science.