Two independent experiments published in Physical Review Letters confirm Niels Bohr’s complementarity principle: detecting which path a photon takes destroys the interference pattern that reveals its wave nature. One team at MIT used single atoms as slit analogues and weak light to show a quantitative trade-off between path information and interference visibility. A second team at USTC trapped a rubidium atom with optical tweezers and reached the same conclusion. Together, the studies settle a century-old debate and open new avenues to study decoherence and entanglement.
Two New Quantum Experiments Settle the Einstein–Bohr Debate — Bohr Was Right
Two New Quantum Experiments Proved Einstein Wrongkoto_feja - Getty Images
Two independent experiments, published in Physical Review Letters, have provided clear experimental support for Niels Bohr’s century-old complementarity principle: obtaining which-path information about a photon destroys the interference pattern that reveals its wave-like behavior. The results recreate, in practical form, the thought experiment Albert Einstein proposed in the 1920s and show that Bohr’s objection was correct.
Background: The 1927 Debate
In the late 1920s, as quantum mechanics emerged, a key controversy arose between Albert Einstein and Niels Bohr over the nature of quantum objects. Bohr argued that wave and particle properties are complementary — you cannot observe both aspects simultaneously with arbitrary precision. Einstein proposed clever thought experiments (including variants of a double-slit set-up with a recoiling slit or spring) intended to show that, in principle, both particle-like path information and wave-like interference could be known at once. Bohr countered that quantum uncertainty and the measurement process would prevent this.
The New Experiments
Two separate teams have now built controlled, modern realizations of the core idea and reached the same conclusion in favor of Bohr.
MIT (Wolfgang Ketterle and colleagues)
The MIT group implemented an "idealized" double-slit analogue using individual atoms as the scatterers (the slit analogues) and very weak light so that each atom scattered at most one photon. By tuning how much path information could be extracted from the system, they observed a quantitative inverse relationship: as which-path information increased, the visibility of the interference pattern decreased.
University of Science and Technology of China (USTC)
The USTC team trapped a single rubidium atom with optical tweezers and used lasers and electromagnetic control to prepare and probe its quantum state, scattering light in two distinct directions. Their measurements also showed that collecting path information reduces interference visibility, consistent with Bohr's complementarity principle.
“Seeing quantum mechanics ‘in action’ at this fundamental level is simply breathtaking,” Chao-Yang Lu of the USTC team told New Scientist. “Bohr’s counterargument was brilliant. But the thought experiment remained theoretical for almost a century.”
Why This Matters
Beyond settling a historic conceptual dispute, these experiments provide tunable, high-precision platforms for studying foundational quantum processes. They make it possible to probe how information extraction (measurement) drives decoherence and how that process relates to entanglement and the quantum-to-classical transition. Such capabilities could lead to clearer, quantitative understanding of how and when quantum systems lose coherence and begin to behave classically.
Bottom line: With modern experimental control, physicists have demonstrated that measuring a photon’s path erases its interference — a direct, empirical vindication of Bohr’s complementarity principle and a practical toolkit for exploring deeper questions in quantum foundations.
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