Scientists achieve first quantum distance measurement in a solid material

An international team of researchers led by Yonsei University has achieved the first direct measurement of the quantum metric tensor in a solid material, marking a breakthrough in quantum physics that could advance quantum computing technologies. The findings, published in the journal Science on June 5, 2025, represent a significant step forward in understanding quantum geometric properties of matter.

Quantum Distance Breakthrough in Black Phosphorus

The research team, led by Keun Su Kim from Yonsei University's Department of Physics and Bohm-Jung Yang from Seoul National University, successfully measured quantum distance—a fundamental property that quantifies the quantum mechanical similarity between two quantum states. Using black phosphorus as their test material, the researchers employed angle-resolved photoemission spectroscopy (ARPES) combined with synchrotron radiation from the Advanced Light Source in the United States.

According to the Korea JoongAng Daily, "quantum distance refers to how similar or different the quantum states of two electrons are from each other," with values ranging from zero for completely opposite states to one for identical states. This property had previously existed only in theoretical science until this direct measurement was achieved.

Technical Achievement and Methodology

The experimental breakthrough involved extracting the momentum space distribution of the pseudospin texture from the valence band through polarization-dependent ARPES measurements. Black phosphorus was chosen for its structural simplicity, making it an ideal material for studying quantum distance of electrons. The collaboration between Yonsei University's experimental group and Seoul National University's theoretical team enabled this first successful measurement of complete quantum metric tensors of Bloch electrons in solids.

Implications for Quantum Technology

Kim emphasized the fundamental importance of measuring quantum distance for understanding anomalous quantum phenomena in solids, including superconductors, and for advancing quantum science and technologies. "As an example, a precise measure of quantum distances would help develop fault-tolerant quantum computation technologies," he explained.

The research is expected to lead to better semiconductor technologies, higher transition-temperature superconductors, and superior quantum computers compared to conventional systems. The breakthrough provides insights into quantum geometric responses across a wide class of crystalline systems and could pave the way for quantum technology-led advances.

"Just as precise distance measurement is essential for safely constructing buildings, accurately measuring quantum distance is crucial for ensuring the reliable operation of quantum technologies," Kim said during a press briefing announcing the results.