Quantum split: Particle this way, properties that way - physics-math - 23 July 2014 - New Scientist

Quantum split: Particle this way, properties that way - physics-math - 23 July 2014 - New Scientist: In Grenoble, the Vienna team used a feeble magnetic field and a weakly interacting neutron absorber to make the weak measurements. They found that when they put the absorber in one path of the interferometer (say left), there was a discernible effect at the output. But when they put it in the right path, it had no such effect. The neutrons were travelling in one path only.

Next, the experimenters introduced a weak magnetic field near each arm of the interferometer, to interact with the spin of the neutrons. When they did this in the left path, there was no change in the interferometer's output. If they introduced the magnetic field in the right path, though, there was a change: the magnetic field had interacted with the spin. In other words, they had confirmed that the spin had chosen the path not taken by the parent neutron...