One Particle on Two Paths: Quantum Physics is Right

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.

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