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."

Comments

I need to know...

[3seas]

Did they kill the cat, by looking?

Re:I need to know...

[Em Adespoton]

Did they kill the cat, by looking?

No... you did, by being curious.

Chesire Cat

[BorgDrone]

Is it just me or does that sound a lot like a Heisenberg Compensator ?

Beam me up!

Why a Cheshire Cat?

[medv4380]

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.

Re:Why a Cheshire Cat?

[retroworks]

Per the article:

"The paradox arises when the team carried out two weak measurements. The first found the presence of neutrons in one arm while the second noted their magnetic properties in the other arm. “The neutrons behave as if particle and magnetic property are spatially separated while travelling through the interferometer,” they say. In other words, they observed a quantum Cheshire cat."

Per the peer review: "Twas brillig, and the slithy toves. Did gyre and gimble in the wabe: All mimsy were the borogoves, And the mome raths outgrabe."

Re:Why a Cheshire Cat?

[Em Adespoton]

Its because the CC's attributes can be in one place while its body is somewhere else... after all, it can be between the state of abnormal and nothing: the last thing to fade is the smile (not the teeth and lips, but the smile) and it can interact without in fact being there.

Re:Why a Cheshire Cat?

[kaoshin]

Quantum Cheshire Cat

The original poster didn't read even the abstract

[Nightlight3]

"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.

Re:"MEOW"

[phrostie]

and the photon that leaves the cat isn't really the same photon that reflects off the mirror.

The most insightfull part of TFA

[mrwolf007]

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.

Re:The most insightfull part of TFA

[wonkey_monkey]

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

[wonkey_monkey]

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.

Weak measurements

[SoftwareArtist]

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.

Schrodinger called

[s.petry]

wrong cat buddy