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Physicists Claim First Observation of a Quantum Cheshire Cat
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."
Did they kill the cat, by looking?
Is it just me or does that sound a lot like a Heisenberg Compensator ?
Beam me up!
Why the acid trip Alice in wonderland analogy? Does it convey additional information about what they're doing, or is it just obfuscating what they're doing. I vote obfuscation, but it might just over my head right now. Stupid, grinning cat with no head.
"a weak measurement extracts such a small amount of information that it leaves the quantum state intact."
That's not correct description -- the quantum state is changed, albeit less than with projective measurement. The paper itself calls it in the abstract "minimal disturbing" measurement, not the "non-disturbing" measurement.
and the photon that leaves the cat isn't really the same photon that reflects off the mirror.
At issue is whether the result is really paradoxical or simply an ordinary consequence of the way the experiment is set up. For example, perhaps the experiment measures the properties of different neutrons in each of these places.
Personally i dont even understand why those guys are thinking they are measuring the properties of the same neutron.
Here's a more familiar example of a weak measurement. QM says you can't measure the magnetic moment of a single particle along two perpendicular axes at the same time. And yet, you can easily measure the magnetic moment of a bar magnet along two perpendicular axes at the same time. How is that possible? The bar magnet's moment is just the sum of the ones from all the particles that make it up. So by measuring the total magnetic moment, aren't you measuring the moments of all the individual particles, and hence violating the uncertainty principle?
The answer is no. When you measure the total moment of a macroscopic magnet, you only need to interact very very weakly with any individual particle, so the experiment only has a tiny effect on the state of each one. The more particles you sum over, the less information you need about each one, so the less restrictive the uncertainty principle becomes.
But the mathematical details of the explanation are curious. Weak measurements were originally proposed based on time reversible interpretations of QM, in which the future can affect the past and it's basically arbitrary which direction you call "forward in time". It was later shown that other interpretations also predicted them - of course they must, since the interpretations are mathematically equivalent. But the explanations are very different. Other interpretations explain them through an incredibly complicated series of cancellations, whereas in time reversible QM the explanation is straightforward, almost obvious. So is this evidence that time reversible QM is correct? At the moment, that question is more philosophy than science, but it's interesting to think about.
"I'm too busy to research this and form an educated opinion, but I do have time to tell everyone my uninformed opinion."
wrong cat buddy
-The wise argue that there are few absolutes, the fool argues that there are no probabilities.
It's a lot simpler. They had one neutron in two places and measured different properties of it in the different places.
The new thing is that it's a bit more "real" that it's the same particle in two places than has been done before. I'd guess it's theoretically impossible to measure the same thing in two places, but I really don't know that much about quantum mechanics.
Holy crap! I forgot to feed my cat! Being a physicist, I now have no idea whether he's alive or dead.
The splitter is functionally equivalent to taking a measurement, you may not be extracting the information, but it is an interaction, and it's the interaction part of "taking a measurement" that changes it, not the information extraction/conversion into another form.
We know the splitter changes it, because its velocity changes... it leaves along a different vector.
The revolution will not be televised... but it will have a page on Wikipedia
If quantum suicide is right, then if the LHC is producing black holes, you'd better stay on Earth: On earth, you'd always be in the branch where no black holes are produced, while on Mars you'd see Earth disappear in a black hole and know that the very moment something goes wrong on Mars, you've got no hope to get any help from Earth.
Well, thinking more about it, maybe your best bet is to make your decision based on a quantum random experiment.
The Tao of math: The numbers you can count are not the real numbers.