energy

With information from the Vienna University of Technology – 11/18/2022

**Two simultaneous tracks**

The double slit experiment is the best-known and perhaps the most important experiment in quantum physics: individual particles are thrown against a wall with two slits, behind which a detector measures where the particles have arrived.

The result is that each individual particle passes through the left and right slits, which indicates that the particles do not move along a specific path, as is the case with classical objects, but along multiple paths simultaneously – the explanation for this is that Particles can also behave like waves.ðŸ‡§ðŸ‡·

However, this can only be proven by repeatedly performing the experiment and evaluating the results of many particle snapshots and their respective discoveriesâ€”that is, an explanation based on statistics.

Now, a team from Austria and Japan has managed to set up a variant of this experiment that can correct this imbalance: a single neutron is measured at a specific location, and because of a very sophisticated measurement setup, this single measurement actually proves that the particle is moving along a path. Along two different paths in the same time.

It is even possible to determine the ratio in which the neutron is distributed between the two paths. Thus, the phenomenon of quantum superposition has been proven without resorting to statistical arguments.

**Double slit experiment**

“In a classic double-slit experiment, an interference pattern was created behind the double-slit,” explains Professor Stefan Spoonar, from the Vienna University of Technology. Separate the particle from its propertiesðŸ‡§ðŸ‡· “The particles move like a wave through the two slits at the same time, and then the two partial waves interfere with each other. In some places they reinforce each other, in other places they cancel each other out.”

The result is a detector full of bands, which physicists call “margins.”

The probability of detecting a particle behind the double slit at a very specific location depends on this interference pattern: where the quantum wave is amplified, the probability of detecting the particle is high; Where the quantum wave cancels out, the probability is low. Thus, 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 more and more distinguishable point by point and particle by particle.

“Then, the behavior of individual particles is interpreted based on results that only become visible through the statistical investigation of many particles,” said Holger Hoffmann of Hiroshima University in Japan, who developed the theory behind the new experiment. “Of course, this is not entirely satisfactory. Then we study how the phenomenon of two-way interference can be demonstrated on the basis of single-particle detection.”

Configure the new experience.

[Imagem:Â Laurent Thion/ILL]

**Spinning neutron**

The new experiment is made possible with the help of neutrons: neutrons are fired towards a crystal that splits the particle/wave into two partial waves, very similar to what happens in the classic experiment with the double slit. The two partial neutron waves travel along different paths and eventually recombine. They overlap each other and are then measured.

But there’s something new, with another property of the neutron being explored: its spin, the particle’s angular momentum. Spin can be affected by magnetic fields, which cause the angular momentum of the neutron to point in a different direction. If the neutron’s spin is only taken in one of the two paths, it is possible to determine which path it took later. However, the interference pattern also vanishes as a result of Integration in Quantum MechanicsðŸ‡§ðŸ‡·

“So we rotated the role of the neutron a bit,” explained team member Hartmut Lemmel. So the interference pattern remains, because you can only get very little information about the trajectory. In order to still get accurate information about the trajectory, this Poor average This was repeated several times in conventional experiments. However, you only get a statistical statement about the full set of neutrons and can say very little about each individual neutron. “

Scheme of synchronous two-path neutron detection.

[Imagem:Â Hartmut Lemmel et al. – 10.1103/PhysRevResearch.4.023075]

**reverse rotation**

The situation is different if, after the merger of the two partial waves of neutrons, another magnetic field is used to spin back up. By trial and error, one can determine the angle of rotation needed to rotate the rotation from its superimposed state to its original orientation.

The strength of this spin is a measure of how strong the neutron is in each path. If it had just taken the way the spin was rotated, it would have taken the full angle of rotation to spin it again. If he had just taken the other way, a reverse turn would not have been necessary.

In the experiment the team conducted, using a special asymmetric beam splitter, it was found that neutrons were present in one-thirds in one path and two-thirds in the other.

Through detailed calculations, the team was able to show that the experiment doesn’t just detect an average value over the sum of all measured neutrons â€” the judgment applies to each individual neutron. It takes many neutrons to determine the optimal spin angle, but once it’s set, being in the exact path from it applies to every neutron detected.

“Our measurements support classical quantum theory,” Spoonar said. “The novelty is such that no one needs to resort to unsatisfactory statistical arguments: when a single particle is measured, our experience shows that it must have traveled two trajectories at the same time and unambiguously determine the proportions involved.”

But, as always in science, this huge improvement in one of physics’ most famous experiments has raised a new question: Why do neutrons choose a trajectory in the ratio of one-third to two-thirds (33/66) and half-and-half (50)? / 50)?

**index:**

Article: *Measurement of neutron presence in the tracks of the interferometer*

Authors: Hartmut Limmel, Nils Gerets, Armin Danner, Holger F. Hoffmann, Stefan Spoonar

Journal: Physical Review Research

Volume: 4, 023075

DOI: 10.1103/PhysRevResearch.4.023075

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