Century-Old Quantum Debate Settled: USTC Experiment Verifies Bohr’s Principle

A research team from the University of Science and Technology of China (USTC) has conducted a groundbreaking experiment that provides definitive evidence in one of the most famous debates in the history of physics: the Einstein-Bohr debate on the nature of quantum mechanics. By physically realizing a thought experiment proposed by Einstein in 1927 using a single atom and a single photon, the team’s results strongly support Bohr’s principle of complementarity.

The debate originated at the 1927 Solvay Conference. Einstein, skeptical of quantum mechanics' inherent uncertainty, conceived an experiment aiming to simultaneously demonstrate both the particle and wave nature of light. Bohr countered, using the uncertainty principle, that any attempt to measure the “which-path” information of a photon would inevitably disturb the system and erase its wave-like interference pattern. This theoretical impasse lasted for nearly a century due to immense technological challenges.

The USTC team, led by Professors Pan Jianwei, Lu Chaoyang, and Chen Mingcheng, overcame these challenges. They trapped and cooled a single rubidium atom to its motional ground state, reducing its momentum uncertainty to the level of a single photon's momentum. This atom acted as the idealized “movable slit” in Einstein's proposal. The experiments clearly demonstrated a trade-off: when the atom’s state could potentially reveal the photon’s path, the interference pattern diminished; when the path information was not accessible, a clear interference pattern emerged.

This work marks a significant advance in testing the foundations of quantum mechanics and opens new avenues for quantum information science. The findings were published in the prestigious journal Physical Review Letters on December 2, 2025. 1.https://journals.aps.org/prl/abstract...

2.https://physics.aps.org/articles/v18/...