SoylentNews

hubie writes:

A single neutron moves along two paths simultaneously, in clearly quantifiable proportions:

The double-slit experiment is the most famous and probably the most important experiment in quantum physics: individual particles are shot at a wall with two openings, behind which a detector measures where the particles arrive. [...]

"In the classical double-slit experiment, an interference pattern is created behind the double slit," explains Stephan Sponar from the Atomic Institute at TU Wien. "The particles move as a wave through both openings at the same time, and the two partial waves then interfere with each other. In some places they reinforce each other, in other places they cancel each other out."

[...] Of course, this wave distribution cannot be seen by looking at a single particle. Only when the experiment is repeated many times does the wave pattern become increasingly recognisable point by point and particle by particle.

They set up a double-slit experiment using neutrons as a source, but they also manipulated the spin of the neutron. If you flip the spin on only one of the two paths, you can tell which path the neutron took; however, when you do this, the double-slit interference pattern disappears due to quantum complementarity. Instead of flipping the neutron spin, they only change the spin a little bit, which preserves the interference pattern, but they only gain a little bit of information about which path the neutron took, so they still have to use many neutrons to build up the interference pattern.

They were able to show that if you reverse the applied rotation to the beam after it had recombined, you can determine through which path the neutron went for each individual neutron. If it had taken only the path on which the spin has been rotated, the full angle of rotation would be necessary to rotate it back. If it had taken only the other path, no reverse rotation would be necessary at all.

Through detailed calculations, the team was able to show: Here, one does not merely detect an average value over the totality of all measured neutrons, but the statement applies to each individual neutron. It takes many neutrons to determine the optimal angle of rotation, but as soon as this is set, the path presence determined from it applies to every single neutron detected.

"Our measurement results support classical quantum theory," says Stephan Sponar. "The novelty is that one does not have to resort to unsatisfactory statistical arguments: When measuring a single particle, our experiment shows that it must have taken two paths at the same time and quantifies the respective proportions unambiguously." This rules out alternative interpretations of quantum mechanics that attempt to explain the double-slit experiment with localised particles.

A more technical presentation of the paper can be found here.

Journal Reference:
H. Lemmel et al., Quantifying the presence of a neutron in the paths of an interferometer, Phys. Rev. Research 4, 023075 (2022).
DOI: 10.1103/PhysRevResearch.4.023075

---

Original Submission