Einstein’s Slit with a Single Atom
In 1927, Niels Bohr introduced complementarity to quantum mechanics. A pair of complementary properties cannot be observed or measured simultaneously, he asserted. Finding the concept unpalatable, Albert Einstein promptly devised a thought experiment to expose what he believed was complementarity’s contradictory character. Now Jian-Wei Pan of the University of Science and Technology of China (USTC) and his collaborators have performed Einstein’s experiment in their lab [1]. Their realization is not the first, nor was its vindication of complementarity needed. However, the USTC setup has the potential to explore other, less established aspects of quantum mechanics.
In Einstein’s thought experiment, particles pass through a horizontally oriented double slit. Interference fringes recorded by a detector reveal the particles’ wave-like nature. But before the particles reach the double slit, Einstein had them pass through a single slit held between two momentum-sensitive springs. Particles destined for, say, the upper slit would impart detectable downward momentum on the single slit, revealing their particle-like nature. They would also, as wave-like objects, still form fringes. And because particle-likeness and wave-likeness are complementary, the fringes’ existence would violate complementarity. Bohr resolved the apparent paradox by pointing out that a precise measurement of a particle’s momentum would, thanks to the uncertainty principle, blur the particle’s position and wash out the fringes.
In the USTC lab experiment, the particles were single photons, and the single slit was a lone rubidium atom trapped in an optical tweezer. By tuning the photons’ momentum uncertainty, Pan and his collaborators could make the fringes more or less blurry, in line with theory. Whereas complementarity rests on firm physical foundations, the researchers anticipate using their apparatus to explore the more open question of how decoherence and entanglement influence each other.
–Charles Day
Charles Day is a Senior Editor for Physics Magazine.
References
- Y.-C. Zhang et al., “Tunable Einstein-Bohr recoiling-slit Gedankenexperiment at the quantum limit,” Phys. Rev. Lett. 135, 230202 (2025).