- Collaborative research by Prof. Kim Keun-su (Yonsei Univ.) and Prof. Yang Bohm-jung (Seoul National Univ.) published in Science

– Expected to contribute to quantum computing, quantum sensing, and other quantum technologies

A new method has been developed in Korea to measure—and express numerically—the “quantum distance” of particles in the microscopic world*, at scales as small as one millionth the width of a human hair, smaller than an atom.

*Microscopic world refers to scales of around one‑millionth the thickness of a human hair, where the behavior of particles is described by quantum mechanics.

The Ministry of Science and ICT (Minister Yoo Sang‑im, hereinafter “MSIT”) announced that Professor Kim’s team at Yonsei University and Professor Yang’s team at Seoul National University have succeeded in the world’s first direct measurement of the quantum distance of electrons in a solid.

“Quantum distance” is a physical quantity that numerically describes the quantum‑mechanical similarity between microscopic particles—how much two quantum states resemble each other. It ranges from 0 for identical states to 1 for completely distinct (“orthogonal”) states. This measure is considered essential for evaluating accuracy in quantum computing and tracking state changes in quantum sensing.

Recent findings indicate that quantum distance in solids is intimately linked not only to basic material properties but also to complex phenomena such as superconductivity.

Globally, researchers have attempted to measure this quantity, but only indirect methods have been used—until now.

This groundbreaking direct measurement of quantum distance was made possible by the close collaboration between Prof. Yang’s theoretical group and Prof. Kim’s experimental team, both of which brought 5–10 years of accumulated expertise.

Prof. Yang’s group, in collaboration with MIT, previously published approximate quantum‑distance measurements in Nature Physics, highlighting their theoretical strength. Prof. Kim’s experimental group had earlier reported results on black phosphorus using angle‑resolved photoemission spectroscopy (ARPES)* in Nature in 2024.

*ARPES (Angle‑Resolved Photoemission Spectroscopy): A technique that analyzes the energy and momentum of electrons ejected from a material when illuminated by light.

The joint team focused on black phosphorus, known for its relatively simple structure. Prof. Yang’s theoretical group determined that quantum distance in black phosphorus is governed by phase difference*.

*Phase difference refers to the shift between the peaks of two waves of the same wavelength. Since quantum particles exhibit wave behavior, phase differences in electron waves are significant.

Using polarized light in ARPES, Prof. Kim’s team observed that signal intensity varied with the electron phase difference, enabling them to precisely measure the quantum distance.

Professors Kim and Yang emphasized the significance, stating:

“Just as precise measurements are vital for safe architectural construction, accurate quantum-distance measurement is essential for reliable quantum technologies. This research lays a foundational tool for quantum computing, sensing, and broader quantum applications.”

Supported by MSIT’s Basic Research Program, this study was published in Science on June 6 (Korean time; June 5, 2 p.m. EDT) under the title “Direct measurement of the quantum metric tensor in solids.”