Quantum neutron cheshire cat

Quantum states can be super imposed or twisted in entirely different spaces and places. That’s just the nature of this sort of mechanics at the super small.


The spin states of neutrons can be separated like a cheshier cat. image: TU Vienna / Leon Filter

This sounds much better than Schrödingers cat at least, pretty cool if you think about it, less live and dead and more cheesy grin like.

The press release from Vienna University of Technology describes the situation like this:

Neutrons are not electrically charged, but they carry a magnetic moment. They have a magnetic direction, the neutron spin, which can be influenced by external magnetic fields.

  • First, a neutron beam is split into two parts in a neutron interferometer. 
  • Then the spins of the two beams are shifted into different directions: The upper neutron beam has a spin parallel to the neutrons' trajectory, the spin of the lower beam points into the opposite direction.
  •  After the two beams have been recombined, only those neutrons are chosen, which have a spin parallel to their direction of motion. 
  • All the others are just ignored. "This is called postselection", says Hermann Geppert. "The beam contains neutrons of both spin directions, but we only analyse part of the neutrons."

These neutrons, which are found to have a spin parallel to its direction of motion, must clearly have travelled along the upper path - only there, the neutrons have this spin state. This can be shown in the experiment. 

If the lower beam is sent through a filter which absorbs some of the neutrons, then the number of the neutrons with spin parallel to their trajectory stays the same. If the upper beam is sent through a filter, than the number of these neutrons is reduced.

But as it turns out...

the spin can be slightly changed using a magnetic field. When the two beams are recombined appropriately, they can amplify or cancel each other. 

This is kind of like the grin staying in position. 

What’s it good for?

Mostly for doing really high precision measurements. Especially ones that are sensitive to being disturbed or perturbed. If you know one value changes then you might be able to compensate for the other value. 

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