Physicists Claim First Observation of a Quantum Cheshire Cat 148
KentuckyFC writes "Last year, a group of theoretical physicists suggested a bizarre experiment based on a quantum phenomenon known as weak measurement. Unlike ordinary measurements that always change the state of a quantum object, a weak measurement extracts such a small amount of information that it leaves the quantum state intact. For example, a weak measurement can detect the presence of a photon by the deflection it causes when it bounces off a mirror. However, this does not change the photon's quantum state. The new idea was to make two weak measurements on a quantum system that is in a superposition of states, the goal being to separate the location of this quantum system from its properties, like a Cheshire cat. Now a group of experimentalists say they've observed a quantum Cheshire cat for the first time in an experiment involving neutrons. They passed a beam of neutrons through a magnetic field to align their spins and then sent them through an interferometer in which the neutrons pass down both arms of the experiment at the same time. They then used weak measurements to locate the neutrons in one arm while measuring their magnetic properties in the other. Voila! A quantum Cheshire cat."
Why a Cheshire Cat? (Score:4, Insightful)
Re:"MEOW" (Score:3, Insightful)
and the photon that leaves the cat isn't really the same photon that reflects off the mirror.
Re:The most insightfull part of TFA (Score:4, Insightful)
Personally i dont even understand why those guys are thinking they are measuring the properties of the same neutron.
(Most insightful part of comment highlighted.) Because they're scientists with more knowledge of physics than you or me?
I don't understand why you'd automatically assume they haven't measured the same neutron. When someone with more physics degrees than me makes a new claim about physics, I tend to default to the understanding that I'm not entirely qualified to go jabbering on the internet that they've probably just got it wrong - certainly without giving any reason beyond "I don't get it so it can't be right."
Perhaps they have got it wrong; time will tell. I think it's safe to assume that at the very least they remembered to rule out the obvious alternative explanations before publishing.
Re:The most insightfull part of TFA (Score:4, Insightful)
Incorrect, quoting wikipedia:
Furthermore, versions of the experiment that include particle detectors at the slits find that each photon of light passes through one slit (as would a classical particle), but not through both slits (as would a wave).
That doesn't mean that in versions without particle detectors the photons don't go through both slits.
Any photons which are detected are forced to have gone through one slit or the other. If the detectors are 100% efficient, all the photons will be absorbed so there'll be no interference pattern to detect. If the detectors aren't 100% efficient (or not present) any undetected photons will go on to produce the interference pattern - meaning they must have gone through both slits (since the experiment produces the same result when photons are emitted one at a time).
The experiment might have been interesting if the scientists had shot single neutrons instead of stream of multiple neutrons.
It still is interesting, because (as I understand it) they detected the presence of neutrons only in one arm and their spins only in the other.