Probing quantum mechanics with nanoparticle matter-wave interferometry

The quantum superposition principle is a fundamental concept of physics and the basis of modern quantum science and technology. Yet, its validity for increasingly complex systems remains an open experimental question, as its key features are not observed on the macroscopic scales of everyday life. This raises the question of how quantum properties persist or change with growing size and complexity. Matter-wave interferometry provides a stringent model test by coherently delocalizing individual massive particles over distances far exceeding their physical size.

Here, we report a novel experimental platform that extends matter-wave interference to a qualitatively new class of matter - large metal clusters - and present the first observation of quantum interference of sodium nanoparticles containing more than 7,000 atoms, with masses exceeding 170 kDa. The particles propagate in a Schrödinger-cat state reaching a macroscopicity of µ = 15.5, surpassing previous experiments by an order of magnitude and providing the most stringent bounds to date on generic macrorealistic modifications of quantum mechanics.