An important principle of quantum mechanics has been confirmed via a variation of a thought experiment suggested by Einstein, made possible by advances in technology. Researchers provide evidence for quantum superposition using individual particles rather than statistical techniques.
A team of scientists performed the double-slit experiment using neutrons, adding spin-measuring equipment to study the path each neutron takes with the rigor that previous generations of physicists could not have imagined. In the journal Physical Review Research, the authors report a consistent result with the neutron splitting, with some passing through each slit.
Dr. Stephan Sponar of the Atomic Institute of TU Wien and his co-authors used a standard beam splitter so that neutrons could travel along two possible paths. They applied a magnetic field to a single path, then measured the effect on the spin of each neutron.
“The results show that individual particles experience a specific fraction of the applied magnetic field 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”, paper claims. “The path presence obtained […] is not a statistical average but applies to each individual neutron.
The work backs up a claim that physicists have been making for nearly a century, but through a method considered by many to be impossible.
An introduction to quantum physics lessons usually involves the two-slit experiment, where light is thrown across two narrow spaces in a slide before hitting a screen behind it. In the world we know, 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 either slot and not interfere with each other afterwards.
Light, or subatomic particles, combine the two. “In the classic double-slit experiment, an interference pattern is created behind the double-slit,” Sponar said in a statement. “The particles move like a wave through the two apertures at the same time, and the two partial waves then interfere with each other. In some places they reinforce each other, in others they cancel each other out. .”
It’s 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 by a source to such a low level that only one photon hits the blade at a time. When this happens, the photon interferes with itself as if there were multiple photons, some going through one slit and some through the other, proving its dual nature. The photon crossing both slits at once is an example of quantum superposition, an object being in two places simultaneously.
However, just when the students decide that this quantum trick isn’t as hard to figure out as they’ve been told, they’re hit by a curveball. Measure the passage of the photon and the superimposition will be lost, (at least if the measurement is reliable). The act of observation changes the result. To avoid this and reveal the layering in action, it was necessary to use a 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 the superposition. The authors’ state-of-the-art equipment was able to determine how much the spin of each neutron was modified by the magnetic field without falsifying 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 put an end to rearguard attempts to explain the results of previous double-slit experiments without resorting to superposition.