Austrian physicists measured the Leggett-Garg inequality using neutron interferometry, leading to a clear conclusion: no classical theory can accurately describe reality.
This study provided new evidence that a particle can exist in a superposition of two states associated with different spatial locations, even at distances of a few centimeters.
Can quantum objects be explained by classical theory, or do particles indeed exist in multiple places at once, as quantum physics suggests?
In 1985, scientists Anthony Leggett and Anupam Garg answered this question by proposing a theoretical test: if the world can be described without invoking superposition and other quantum phenomena, it must satisfy a specific inequality. If the world is truly quantum in nature, this inequality will be violated.
The Leggett-Garg inequality is conceptually similar to the more commonly known Bell inequality, which describes the relationship between the behavior of two entangled objects. However, the Leggett-Garg inequality concerns the temporal evolution of the state of a single object.
Scientists hypothesized that an object could be measured at different times, with each measurement yielding one of two possible results. By statistically analyzing how these results correlate over time, it is possible to test whether classical realism holds up.
If classical realism is correct, the correlation level cannot exceed a certain limit (where K is less than 1). However, if quantum mechanics determines the behavior of objects, K will be greater than 1.
Previous experiments have already shown violations of the Leggett-Garg inequality in several quantum systems, including photonic qubits, nuclear spins and superconducting qubits.
However, the research conducted by physicists from the Vienna University of Technology introduces remarkable progress, as reported by Physics world. By using a neutron interferometer, they were able to measure the positions of neutrons in three different areas: in front of, inside and behind the interferometer.
“The results we obtained cannot be explained within the framework of macrorealistic theories – only by quantum theory,” Sponar said. According to the measurements, the neutron must exist in a coherent superposition of states, a fundamental property of quantum mechanics.
This research shows that particles can occur in multiple places at the same time, which cannot be explained by classical physics. It confirms that the strange behavior predicted by quantum physics is real and cannot be described by any traditional theory. This is a big step in understanding how the quantum world works.
