How Did Scientists Finally Measure Quantum Distance in Solids?

A global research team has achieved what quantum physicists only dreamed of for decades: directly measuring the quantum distance, an essential property defining the similarity between quantum states in a real solid.

This feat was accomplished using black phosphorus, crystallizing theory into experiment.

The quantum distance metric promises to power the next wave of progress in quantum science and technology. The experiment, revealed in June 2025 by Yonsei University's team, lays essential groundwork for safer, more reliable quantum devices.

Decoding Quantum Distance

Quantum distance measures how alike two quantum states are. Think of it as a scale: on one end, values near zero mean the states are entirely different; on the other, values closer to one signal nearly identical quantum conditions. Previously, this concept only appeared in theoretical models, lacking real-world confirmation until now.

Did you know?
Quantum distance was a purely theoretical concept until its direct measurement in 2025; now, the gap between theory and experiment is closing, opening doors to quantum technology advances.

The Breakthrough Approach: Why Black Phosphorus?

Researchers selected black phosphorus, a crystalline solid with a simple structure, optimizing conditions for extracting intricate quantum details. Its electronic properties provided an ideal playground, allowing subtle features of quantum geometry to become observable through advanced measurement methods.

Experimental Technique: Inside the Lab

The team employed angle-resolved photoemission spectroscopy (ARPES), a high-precision method that reads electrons’ behavior in solids. By using polarization-dependent ARPES and synchrotron radiation from the Advanced Light Source in the U.S., they took pictures of how electrons move and their pseudospin, which are important for figuring out quantum distance.

ALSO READ | NASA’s Bold Plan: Moon Nuclear Power by 2030 Amid Global Rivalry

Collaboration Makes the Difference

It took tight coordination between experimenters at Yonsei University and theorists at Seoul National University. By mapping the momentum-space distribution of the pseudospin texture from the material’s valence band, they managed to piece together the quantum metric tensor physics’ version of a metric tape for electrons.

Why This Matters: Technology and Beyond

Quantum distance data could help decode and control exotic phases in materials, optimize semiconductors, and make possible more robust quantum computers.

As team leader Keun Su Kim noted, measuring quantum distance is as critical to quantum tech as reliable surveying is to building construction; without it, progress remains foundationally shaky.

Implications: Where Could This Lead?

The direct measurement offers a blueprint for similar research in other crystalline systems. Scientists expect this to trigger a new round of advancements in solid-state quantum physics, possibly leading to higher-temperature superconductors or fail-proof quantum bits, the building blocks for the computers of tomorrow.

Looking Forward: Quantum Future Takes Shape

This landmark achievement demonstrates how persistent curiosity, technical innovation, and collaborative synergy can turn theoretical science into working reality. The direct measurement of quantum metrics paves the way for new discoveries and applications in both physics and technology.