In 2025, quantum mechanics celebrates its 100th anniversary, marking the groundbreaking contributions of Werner Heisenberg, Erwin Schrödinger, and Niels Bohr in 1925. This discipline, which revolutionized our understanding of the physical world, is celebrated through a remarkable experiment conducted at the Massachusetts Institute of Technology (MIT). Published in Physical Review Letters (Fedoseev et al., 2025), this minimalist double-slit experiment employs single atoms as slits, providing compelling evidence for Bohr’s complementarity principle while refuting Albert Einstein’s hypothesis that a photon’s path through a double slit could be measured without disrupting its interference pattern.
Historical Context
The double-slit experiment has long been a cornerstone of quantum mechanics, vividly demonstrating the wave-particle duality of light and matter. In 1927, it sparked a famous debate between Einstein and Bohr at the Solvay Conference. Einstein posited that a photon passes through one of the two slits, imparting measurable momentum to the slit without affecting the characteristic interference pattern. In contrast, Bohr’s complementarity principle asserted that measuring a quantum object’s particle properties inherently disturbs its wave-like behavior, as these properties are mutually exclusive. The MIT experiment revisits this historic debate, leveraging cutting-edge technology to address the question with unprecedented precision.
Experimental Setup
The experimental approach is a technical triumph. The researchers suspended approximately 10,000 ultracold atoms in an optical lattice created by precisely tuned laser beams. These atoms were arranged to serve as the smallest conceivable slits, spaced sufficiently apart to avoid mutual interference. A weak laser beam, composed of single photons, was directed through these atomic slits. The scattering of light by the atoms was meticulously analyzed to investigate the quantum mechanical interactions between photons and atoms. To eliminate potential disturbances from the optical lattice, some measurements were conducted within an extremely brief 1-microsecond window during which the atoms floated freely before beginning to fall due to gravity. This innovative approach ensured minimal external influence, creating a near-ideal environment for studying quantum phenomena.
Results and Interpretation
The experiment’s results are unequivocal: Precisely determining a photon’s path through the double slit disrupts its wave-like behavior. The more accurately the researchers measured the photon-atom interactions, the more the coherence of the light decreased, leading to a significant weakening of the interference pattern. This effect persisted even when the optical lattice was deactivated, ruling out potential disturbances from the experimental setup. These observations confirm Bohr’s complementarity principle, which states that a quantum object’s particle and wave properties cannot be observed simultaneously with arbitrary precision. Concurrently, the results refute Einstein’s 1927 hypothesis that momentum transfer could be measured without destroying the interference pattern.
Significance for Quantum Mechanics
This experiment not only validates a fundamental principle of quantum mechanics but also showcases the extraordinary precision of modern experimental techniques. Using single atoms as slits minimizes external influences, enabling near-perfect control of experimental conditions. This provides an ideal platform for investigating quantum phenomena and could inspire future experiments on topics such as quantum entanglement, decoherence, or even quantum gravity. The ability to test quantum mechanical principles at the atomic scale with such precision highlights the advancements in experimental physics since the early days of quantum mechanics.
Outlook and Centennial
Coinciding with the 100th anniversary of quantum mechanics, this experiment celebrates the seminal contributions of Heisenberg, Schrödinger, and Bohr in 1925. It underscores the enduring relevance of early debates about the nature of quantum objects and demonstrates how modern technologies can address historical questions with unparalleled clarity. Future research could leverage this minimalist approach to explore additional quantum phenomena, such as the dynamics of quantum systems in controlled environments or the development of novel quantum technologies. This experiment represents not only a scientific milestone but also a symbolic one, bridging the past, present, and future of quantum mechanics.
Conclusion
The minimalist double-slit experiment, utilizing single atoms as slits, provides compelling evidence for Niels Bohr’s complementarity principle and refutes Einstein’s assumption of measurable momentum transfer without interference pattern disruption. This work deepens our understanding of the foundations of quantum mechanics and demonstrates the power of precise experimental designs in modern physics. As quantum mechanics marks its centennial, this experiment stands as a testament to the enduring strength of this discipline, which continues to shape our understanding of the universe.
Reference: Fedoseev, V., Lee, Y. K., Ketterle, W., et al. (2025). Minimalist Double-Slit Experiment with Single Atoms. Physical Review Letters, 134(2). doi:10.1103/zwhd-1k2t