After 88 Years, KRISS redetermines a key strontium frequency with 50 times better precision

The Korea Research Institute of Standards and Science (KRISS, President Dr. Ho Seong Lee) has re-measured a key transition frequency of the strontium (Sr) atom for the first time in 88 years since 1938, using a photonic grating chip, and has established a new absolute frequency value with a measurement uncertainty reduced by more than 50 times. By securing an accurate reference value needed for next-generation quantum technologies ahead of time, this achievement is expected to greatly contribute to the precise control and miniaturization of quantum devices, including atomic clocks and neutral-atom quantum computers.

Strontium is an atom widely used in precision quantum technologies such as optical lattice clocks and quantum sensors. To use strontium in these technologies, fast-moving atoms must be slowed down and trapped with precisely tuned lasers, and for this, it is essential to know exactly the resonant frequency of the laser light for the atomic transition.

However, the previous frequency value was obtained in 1938 by comparing the solar spectrum with the arc light of strontium, and its uncertainty was too large to be used directly for the precise laser frequency settings of today. As a result, researchers have had to observe the atoms' response directly and adjust the laser frequency as needed, much like turning the dial of an analog radio to tune in to a station.

As frequency control and measurement technology for light has advanced and the miniaturization of quantum devices has been pursued, the need for an accurate absolute frequency value has grown even greater. In response, the KRISS research team measured the absolute frequency of the 461-nanometer (nm) optical transition of strontium atoms. After a year long campaign of precision measurements, the uncertainty was reduced to about 5 megahertz (MHz), an improvement of more than 50 times over the previous value. Notably, this measurement established traceability to the national time and frequency standard of Korea Standard Time, which KRISS establishes and maintains. Because this standard is linked to the International System of Units (SI), it can be reliably compared and used against the same reference anywhere in the world, regardless of differences in laboratory or equipment environments.

The key device that enhanced measurement stability is the “photonic grating chip,” jointly developed by KRISS with the Korea Advanced Institute of Science and Technology (KAIST) and the National NanoFab Center (NNFC). The fine grating structure on the surface of the chip, installed inside a vacuum, diffracts and reflects light in set directions, reducing measurement errors caused by mechanical vibrations compared with conventional methods that require aligning multiple external mirrors. In addition, a single chip can generate laser beams in multiple directions, simplifying the optical setup.

Having reduced measurement errors on the equipment side, the researchers then determined the transition frequency independently using two methods based on different principles, verifying the reliability of the results. One method analyzes how the atoms respond to light depending on the laser frequency, while the other deduces the transition frequency from the velocity distribution of an atomic beam slowed by lasers. The values obtained from the two methods agreed within their margins of error, and the final value was calculated by reflecting the uncertainty of each result in accordance with international guidelines.

The newly established absolute frequency value can be used as a reference for precisely cooling, trapping, and controlling atoms in strontium-based optical lattice clocks, quantum sensors, and neutral-atom quantum computers. Combined with optical frequency synthesis technology that is currently being developed, it frees next-generation quantum devices from being tethered to bulky, external atomic frequency references. These breakthrough technologies are expected to accelerate the miniaturization and commercialization of cold-atom-based quantum devices.

Dr. Jae Hoon Lee, Principal Research Scientist in the Atomic Quantum Sensing Group at KRISS, said, “I believe the main significance of this research is not simply that we re-measured a single frequency value, but that we proactively delivered an accurate measurement reference point for next-generation quantum technologies.” He added, “As a national metrology institute (NMI), KRISS will continue to pioneer standards for the new areas of measurement needed in quantum science and technology.”

This research was carried out with support from the Ultra-precision Quantum Measurement Sensor Specialized Research Laboratory program, funded by the Defense Acquisition Program Administration (DAPA) and the Korea Research Institute for Defense Technology Planning and Advancement (KRIT) (KRIT-CT-23-001). The results were published in July in Metrologia (IF 3.2), a leading international journal in the field of measurement standards.