In a recent groundbreaking quantum experiment, scientists have demonstrated the unbelievable phenomenon of a tiny metallic “cluster” composed of thousands of atoms being able to “exist in multiple places simultaneously” — that is, in a spatial superposition of center of mass wavefunctions.
The research team at the University of Vienna utilized advanced laser technology to observe quantum interference phenomena on nanoscale particles made of sodium, with sizes far larger than particles typically exhibiting such quantum behavior in the past.
This achievement pushes the boundaries of quantum mechanics research, revealing that even objects surprisingly “larger” in size still adhere to the seemingly counterintuitive and peculiar rules of the quantum world — showing that even tiny metallic “clusters” can exist in quantum states simultaneously distributed in multiple locations.
Published in the journal Nature in January, this new study from the University of Vienna and the University of Duisburg-Essen demonstrates that metallic nanoclusters composed of thousands of sodium atoms display quantum behavior, despite their size and mass being significantly larger than particles usually used in such experiments.
This breakthrough represents one of the most robust verifications of quantum mechanics to date, advancing the testing range to scales approaching the macroscopic world.
Prior to this, the mass record for matter wave interference experiments was maintained at approximately 2.8 million Daltons (Dalton, abbreviated Da, equivalent to atomic mass unit amu) created by large complex molecules. However, this study directly escalates the scale almost 6 times, achieving quantum delocalization for the first time in the new material category of “metallic nanoclusters.”
The significance of this experiment lies in striking a balance between “high mass” and “long coherence time,” representing one of the most stringent tests at the boundary of “wave-particle duality.”
Quantum physics describes a microcosm where matter can exhibit characteristics of both particles and waves. Scientists have repeatedly confirmed through interference and double-slit experiments that electrons, atoms, and small molecules possess these unique quantum properties. Nevertheless, in everyday life, ordinary objects like rocks, dust, or glass beads appear to follow predictable classical physics laws — staying at a single location and moving along defined paths.
Led by Markus Arndt and Stefan Gerlich of the University of Vienna, the research team has now extended this quantum effect to significantly larger metal nanoclusters.
The sodium nanoclusters used in the experiment have a diameter of about 8 nanometers, akin to modern transistor components. Each cluster’s mass exceeds 170,000 Daltons, heavier than most proteins.
Even at such scale, these particles still exhibit measurable quantum interference phenomena.
The first author of the paper, doctoral student Sebastian Pedalino, states: “Intuitively, one would expect such large metallic clusters to move like classical particles. However, they still generate interference phenomena, demonstrating that quantum mechanics remains valid at this scale without needing to introduce alternative theoretical models.”
To conduct this experiment, researchers created ultra-cold sodium clusters composed of 5,000 to 10,000 sodium atoms. These particles then passed sequentially through three transient diffraction gratings generated by ultrafast UV laser pulses.
The first laser pulse prepared the spatial coherence of the particles: precisely positioning each cluster’s location to about 10 nanometers. The second pulse acted as a diffraction grating, splitting each cluster’s wavefunction into a superposition of multiple paths, allowing the particles to enter a quantum superposition, enabling them to simultaneously traverse through multiple paths within the experimental setup. When these wave paths overlap at the end of the experiment, they form detectable striped interference patterns, aligning completely with the predictions of quantum theory.
The results indicate that these particles do not stay fixed at a single position during their flight but rather have their quantum states distributed within a space range tens of times larger than the particles themselves. This demonstrates that even at such a large mass, wave properties of matter still persist.
Physicists refer to this state as “Schrödinger cat states,” named after the famous thought experiment proposed by Austrian physicist Erwin Schrödinger: before observation, a cat exists simultaneously in both “alive” and “dead” states.
In this study, researchers describe these metal clusters as being “both here and not here at the same time” — exhibiting a “Coherent Superposition.” This does not mean objects are duplicating like in science fiction movies, but rather their wavefunctions extend in space, maintaining phase correlations among multiple paths.
The theoretical foundation of such near-field interferometry was gradually established over the past twenty years by Klaus Hornberger, one of the co-authors of this new study from the University of Duisburg-Essen. He, along with Stefan Nimmrichter, formulated the concept of “macroscopicity log-scale” to compare the ability of different experiments to test quantum mechanics limits in even the tiniest deviations.
The team achieved a macroscopicity value of μ=15.5 in this new experiment, surpassing similar experiments globally by approximately an order of magnitude (about ten times).
To attain a similar level of quantum mechanics testing precision with electrons, scientists would have to maintain the electrons’ quantum superposition for nearly 100 million years; the Vienna team accomplished the same testing standard in just about one-hundredth of a second using metallic nanoclusters.
Apart from validating the fundamental principles of quantum mechanics, this research aids scientists in further understanding why quantum effects dominate the microcosm while objects in our daily lives exhibit normal behavior conforming to classical physics laws.
The research team plans to test larger particle sizes and more varied materials in future studies, pushing such quantum experiments forward by several orders of magnitude. With upgrades to experimental facilities and improvements in instrument performance, researchers anticipate conducting measurements with higher sensitivity.
The interferometer used by the University of Vienna team also functions as an extremely high-precision force sensor, capable of detecting weak forces as small as 10⁻²⁶ newtons (N). Researchers suggest that future iterations could further enhance sensitivity, opening possibilities for highly precise measurements of isolated nanoclusters’ electrical, magnetic, and optical properties.
Ultimately, these technologies have the potential to drive further advancements in nanotechnology and high-precision sensing technologies. ◇