Physicists Place Thousands of Atoms in Two Places at Once in Record Quantum Test

To do this, the team cooled the atoms to very low temperatures and used special laser beams. The lasers acted like grids that the atoms passed through. Behind the grids, the scientists saw a wave-like pattern. The pattern is proof that the atoms moved through two paths at once.

The result helps test the rules of quantum physics. These rules say that very small things can be in more than one place at the same time. The new experiment shows that this rule still works for objects much bigger than before.

Level 3 — Intermediate

Researchers at the University of Vienna and the University of Duisburg-Essen have placed metallic nanoparticles built from thousands of sodium atoms into a coherent quantum superposition, producing one of the strongest demonstrations to date that quantum mechanics applies to objects far heavier than the single atoms and small molecules used in earlier work. The results, posted to a preprint server this month and submitted to a leading physics journal, push the macroscopic quantum frontier roughly an order of magnitude past the previous record.

The team prepared ultracold clusters containing between 5,000 and 10,000 sodium atoms, then sent them through a sequence of three diffraction gratings written into ultraviolet laser beams. When a detector behind the gratings recorded a regular wave-like fringe pattern, the experimenters could conclude that each cluster had passed through both possible paths in superposition — the matter-wave analog of light interfering with itself in a double-slit setup.

What sets the result apart is the so-called μ parameter, a quantitative measure of how strongly a system exhibits non-classical behavior. The Vienna/Duisburg team reached μ = 15.5, more than ten times higher than the best previous experiments with much smaller objects. Crucially, the nanoparticles studied had effective masses above 170,000 atomic mass units, putting them well beyond the molecular scale that has anchored quantum tests for the past two decades.

Why does it matter? Demonstrating quantum behavior in larger and heavier systems sharpens long-standing tests of whether quantum mechanics survives at sizes approaching everyday objects, or whether some new physics intervenes. If the rules continue to hold as masses climb, they will need to be reconciled with a world that, at human scales, looks emphatically classical. The Vienna group has already outlined follow-up experiments designed to extend the test to particles ten times heavier still.

superposition: a quantum state in which a system is in multiple possibilities at once

diffraction grating: a regularly patterned structure that splits light or matter waves into directions

ultraviolet: light with a wavelength shorter than visible violet

matter wave: the wave-like behavior of particles such as atoms and molecules

fringe pattern: alternating bright and dark bands produced by interfering waves

non-classical: behavior that ordinary, large-scale physics cannot explain

atomic mass unit: a tiny unit of mass roughly equal to a single proton

macroscopic: large enough to see with the naked eye

Level 4 — Advanced

A collaboration between the Aspelmeyer-Arndt group at the University of Vienna and the Schöllkopf cluster-beam laboratory at the University of Duisburg-Essen has reported the highest-mass demonstration of quantum superposition achieved to date, sending metallic clusters of between five and ten thousand sodium atoms through a three-grating Talbot-Lau interferometer and recording fringe patterns consistent with each particle traversing both arms of the interferometer simultaneously. The work, submitted to Nature Physics and posted as a preprint earlier this month, pushes the so-called macroscopic-quantum frontier roughly an order of magnitude beyond every prior experiment.

The interferometer geometry is conventional in outline but ferocious in detail. Ultracold sodium clusters were produced by supersonic expansion and a buffer-gas cooling stage, then propagated through three phase-only ultraviolet gratings before encountering a position-sensitive detector. The recorded fringe contrast yielded a measured quantum signature value of μ = 15.5 — roughly an order of magnitude larger than the best previous benchmarks set with porphyrin oligomers and gold nanoclusters in the 25,000-atomic-mass-unit range. Crucially, the cluster masses in this experiment exceeded 170,000 atomic mass units, placing them solidly in the regime where standard collapse models had been argued to suppress observable interference.

The theoretical stakes are sharp. So-called continuous spontaneous localization (CSL) models, proposed in part to explain the apparent classicality of the macroscopic world, predict that quantum coherence should be progressively quenched as system mass grows. The Vienna/Duisburg result tightens the constraints on those models considerably; if extended to particles around an order of magnitude heavier still — a goal the collaboration has formally outlined — much of the parameter space invoked by CSL phenomenology will be excluded outright, forcing either a refinement of the model or an admission that the underlying ontology is fully quantum at all observed scales.

Practical consequences flow as much from the techniques as from the physics. Talbot-Lau interferometry at these mass scales requires exquisitely controlled molecular beams, ultraviolet gratings that minimize photon-recoil decoherence, and ambient vibration suppression below the level of routine laboratory floors. The Vienna team has signaled that the same toolkit will be repurposed for inertial sensing applications, where a sufficiently massive matter-wave interferometer could outperform classical accelerometers and gravimeters by orders of magnitude — a long-rumored pathway for next-generation navigation systems and for tests of weak-equivalence-principle violations at unprecedented sensitivity.

Talbot-Lau interferometer: a kind of matter-wave interferometer that uses near-field self-imaging of gratings

supersonic expansion: a cooling technique in which a gas is forced through a tiny nozzle into vacuum

phase-only grating: a grating that imprints a phase shift on a passing wave without absorbing it

porphyrin oligomer: a heavy molecule built from linked porphyrin rings, used in past quantum tests

continuous spontaneous localization: a class of theoretical models in which quantum coherence is suppressed as mass grows

ontology: the philosophical study of what kinds of things actually exist

decoherence: the loss of quantum behavior due to interaction with an environment

weak equivalence principle: the rule that gravitational and inertial mass are the same for every body

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Physicists Place Thousands of Atoms in Two Places at Once in Record Quantum Test

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