In a sealed box at nearly absolute zero, seven atoms tunnel as a single object through a barrier they should not be able to cross—and in that impossible journey, they create a phantom duplicate of themselves. Researchers at Southern University of Science and Technology in Shenzhen have just demonstrated this feat, producing what physicists call a Schrödinger cat state using ultracold atoms trapped in optical lattices, and in doing so, they've pushed quantum mechanics into strange new territory.
The work matters because it challenges a fundamental assumption about how the quantum world works. Quantum tunneling—the phenomenon where tiny particles pass through solid energy barriers like ghosts through walls—has always been the domain of the impossibly small. Electrons do it naturally. Protons almost never do. The heavier the object, the less likely it can tunnel, and the exponential penalty for mass has kept this bizarre quantum trick firmly in the subatomic realm. But what if you could make a cluster of atoms heavy enough to be called "massive" by quantum standards, and still make it tunnel? What would that even mean?
Bing Yang, who led the team, articulated the driving question behind the research: can quantum tunneling be pushed to much larger, potentially macroscopic objects? To answer it, his group turned to ultracold atoms—particles cooled to temperatures near absolute zero, a process that amplifies their quantum behavior by dramatically increasing their de Broglie wavelength. These atoms were trapped in optical lattices, artificial cages made from laser beams arranged in a superlattice pattern of double-well units.
The key innovation was engineering weak binding interactions between the atoms, much weaker than the tunneling barrier itself. This prevents the quantum wave packet from collapsing as more atoms join the cluster. The team then exploited what they call high-order tunneling processes—up to seventh-order in their experiment—tuning them to achieve tunneling strengths comparable to single atoms. Seven atoms, bound together loosely, can now tunnel as effectively as one. It is a scalable trick, which means it could theoretically work for even larger clusters.
When those seven atoms tunnel through the barrier, they create what is known in quantum mechanics as a spatial superposition: the cluster exists in two places at once until measured. This is the Schrödinger cat state—named after physicist Erwin Schrödinger's famous thought experiment in which a cat in a sealed box is simultaneously alive and dead. Here, the cluster is simultaneously here and there, at two positions at once, as long as no one looks.
The implications ripple outward. The research, published in Nature Physics, opens pathways toward more sensitive quantum sensors and measurement tools, devices that exploit superposition to detect changes in their environment with exquisite precision. More fundamentally, it creates a laboratory for exploring how quantum mechanics and gravity might interact—two of physics's greatest frameworks that remain stubbornly incompatible.
What Yang and his colleagues have shown is that the boundary between the quantum realm and the everyday world is far more permeable than we thought. Seven atoms are not a macroscopic cat, but in the mathematics of quantum mechanics, they might as well be. They've proven that even relatively massive objects can inhabit two places at once, and that opens a door to understanding how the microscopic and the visible somehow merge into our familiar, singular reality.