Quantum Superposition Reaches Virus Scale
University of Vienna scientists achieved quantum superposition in clusters of 5,000–10,000 sodium atoms (8 nm wide, ~170k–200k atomic mass units) using laser-grating interferometry. This pushes wave-particle duality into virus-sized objects — the largest scale ever demonstrated. The breakthrough opens pathways toward testing quantum effects in biological systems and gravity interactions.
The experiment, led by Markus Arndt and Stefan Gerlich at the University of Vienna, marks a significant leap in matter-wave interferometry. Lead author and doctoral student Sebastian Pedalino successfully demonstrated the wave nature of metallic nanoparticles, which are more massive and complex than the molecules previously tested. This achievement was the culmination of years of work, pushing the boundaries of quantum mechanics to a scale previously thought to be in the classical realm. To achieve this, the team had to meticulously isolate the sodium clusters in an ultra-high vacuum to prevent any environmental interaction, a phenomenon known as decoherence, which would instantly destroy the quantum state. For two years, the researchers only observed flat lines, indicating the particles were behaving classically, before they finally detected the tell-tale interference pattern that confirmed the quantum superposition. This work builds on a long history of experiments that have progressively tested the quantum nature of larger and larger objects. Early experiments in the 1990s demonstrated interference with atoms, which was later extended to buckyballs (molecules of 60 carbon atoms) and then to complex organic molecules with masses up to 10,000 atomic mass units. This latest result with sodium clusters is an order of magnitude more massive than any previous interference experiment. The size of these sodium clusters, around 8 nanometers in diameter, is comparable to that of modern transistors and even small viruses, firmly pushing quantum phenomena into the macroscopic world. While other experiments have placed heavier objects, like a 16-microgram crystal, into a superposition, those were over infinitesimally small distances. This experiment combines a large mass with a significant spatial separation of the wave function. By demonstrating that quantum mechanics still holds true at this scale, the experiment places stricter limits on alternative theories that propose a breakdown of quantum principles above a certain size. This opens the door to using such massive quantum states as highly sensitive detectors for forces and fields, and to explore the fuzzy boundary between the quantum and classical worlds. Looking ahead, researchers aim to achieve superposition with even larger objects, including biological entities. A major goal is to create superpositions of objects massive enough that their gravitational interactions come into play. This could provide a long-sought experimental testbed for theories of quantum gravity, exploring whether gravity itself behaves as a quantum force.
