KRISS remeasures absolute frequency of strontium atom for first time since 1938

KRISS researchers inspect a strontium transition frequency measurement device. [KRISS]
KRISS researchers inspect a strontium transition frequency measurement device. [KRISS]

South Korean researchers have remeasured a key frequency of the strontium atom that had been in use for 88 years, reducing the measurement uncertainty by more than a factor of 50.

The Korea Research Institute of Standards and Science (KRISS) announced Tuesday that a research team led by Senior Researcher Lee Jae-hoon had precisely measured the absolute frequency of strontium atoms using a photonic grating chip and established a new reference value.

Strontium is a key element used in optical lattice atomic clocks, quantum sensors and neutral-atom quantum computers. Harnessing it for quantum technology requires cooling fast-moving atoms with lasers and then controlling them with precision — a process that demands an accurate knowledge of the exact light frequency to which the atoms respond most strongly.

The previously accepted frequency value dated to 1938, when it was derived by comparing solar spectra with strontium arc light, leaving it with large uncertainty. Researchers had to manually adjust laser frequencies by observing atomic responses rather than applying a reliable reference value.

The team spent a year precisely measuring the transition frequency of the 461-nanometer blue light used to cool and control strontium atoms, establishing a new absolute frequency value of 650.503815(5) THz. The measurement uncertainty was reduced to approximately 5 megahertz — an improvement of more than 50 times over the previous figure.

The photonic grating chip used to measure the strontium transition frequency. [KRISS]
The photonic grating chip used to measure the strontium transition frequency. [KRISS]

At the heart of the achievement is a photonic grating chip jointly developed by KRISS, KAIST and the National NanoFab Center.

The chip, installed inside a vacuum chamber, features a microscale grating structure on its surface designed to diffract and reflect light in a consistent direction. Compared with conventional setups that require aligning multiple external mirrors, the chip reduces measurement error caused by mechanical vibration and simplifies the optical apparatus by delivering laser beams in multiple directions from a single device.

To verify the reliability of the measurements, the team applied two independent methods: one analyzing the light response of atoms at varying laser frequencies, and another back-calculating the frequency from the velocity of an atom beam decelerated by a laser. The two results agreed within the margin of error.

The team also used Korea's national time and frequency standard — which maintains Korea Standard Time — to obtain measurements traceable to the International System of Units (SI), meaning the values can be compared and applied on the same basis anywhere in the world regardless of differences in laboratory or equipment conditions.

The established absolute frequency value is expected to be used in cooling and controlling atoms in strontium-based optical lattice atomic clocks, quantum sensors and neutral-atom quantum computers. Combined with optical frequency synthesis technology, it could reduce the need to manually search for laser frequencies, contributing to the miniaturization and commercialization of quantum devices.

"This research goes beyond remeasuring a single frequency value — it proactively establishes an accurate measurement reference point for next-generation quantum technology," Lee said. "We will continue to pioneer new measurement standards needed in quantum science and technology."

The findings were published in the July issue of Metrologia, an international journal in the field of measurement standards.


nbgkoo@heraldcorp.com