Neutrons In The Double-Slit Experiment Really Do Individually Take Both Paths

An important principle of quantum mechanics has been confirmed via a variation of a thought experiment suggested by Einstein, made possible by technological advances. The researchers provide evidence for quantum superposition using individual particles, rather than statistical techniques.

A team of scientists has performed the double-slit experiment using neutrons, adding spin measurement equipment to investigate the path each neutron takes with the rigor previous generations of physicists only imagined. In the journal Physical Review Research the authors report a result consistent with the neutron dividing itself, with part going through each slit.

Dr Stephan Sponar from the Atomic Institute at TU Wien and co-authors used a standard beam splitter so neutrons could travel along two possible paths. They applied a magnetic field on one path only, and then measured the effect on each neutron’s spin.

“The results show that individual particles experience a specific fraction of the magnetic field applied in one of the paths, indicating that a fraction or even a multiple of the particle was present in the path before the interference of the two paths was registered,” the paper claims. “The obtained path presence […] is not a statistical average but applies to every individual neutron.”

Schematic of the experiment; a standard two-slit experiment but with neutrons not protons and a magnetic field on one path to alter the neutron spin. Image Credit: Lemmel et al./Physical Review Research

The work confirms a claim physicists have been making for almost a century, but through a method many considered impossible.

An introduction to quantum physics courses usually involves the two-slit experiment, where light is shone on two narrow gaps in a slide before hitting a screen behind. In the world we are familiar with, water passing through two slits like this creates an interference pattern as the two waves interact. Meanwhile solid objects, such as baseballs, would pass through one slit or the other and not interfere with each other afterward.

Light, or subatomic particles, combine the two. “In the classical double-slit experiment, an interference pattern is created behind the double slit,” said Sponar in a statement. “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.”

This is a demonstration of how, at the level of the very small, things can be both particles and waves.

Physicists have demonstrated this effect for decades, reducing the light emitted from a source to such a low level that only a single photon reaches the slide at a time. When this happens, the photon interferes with itself just as if there were multiple photons, some passing through one slit and some through the other, proving its dual nature. The photon passing through both slits at once is an example of quantum superposition, an object being in two places simultaneously.

However, just as students are deciding this quantum stuff isn’t as hard to grasp as they were told, they are hit with a curveball. Measure the photon’s passage and superposition will be lost, (at least if the measurement is reliable). The act of observation changes the outcome. To avoid this and reveal superposition in action, it has been necessary to use statistical analysis of where on the screen multiple photons land.

Here, the team replaced the photon with a neutron. Yet they claim to have measured the neutron without the measurement destroying superposition. The authors’ advanced equipment was able to determine how much each neutron’s spin was altered by the magnetic field without distorting the results.

“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,” Sponar said. The experiment, if confirmed, would end rearguard attempts to explain the results of previous double-slit experiments without resorting to superposition.