The Quantum Cheshire Cat: Can neutrons be located at a different place than their own spin?

Jul 29, 2014
The basic idea of the Quantum Cheshire Cat: In an interferometer, an object is separated from one if its properties -- like a cat, moving on a different path than its own grin. Credit: TU Vienna / Leon Filter

The Cheshire Cat featured in Lewis Caroll's novel "Alice in Wonderland" is a remarkable creature: it disappears, leaving its grin behind. Can an object be separated from its properties? It is possible in the quantum world. In an experiment, neutrons travel along a different path than one of their properties – their magnetic moment. This "Quantum Cheshire Cat" could be used to make high precision measurements less sensitive to external perturbations.

At Different Places at Once

According to the law of quantum physics, particles can be in different physical states at the same time. If, for example, a beam of neutrons is divided into two beams using a silicon crystal, it can be shown that the individual neutrons do not have to decide which of the two possible paths they choose. Instead, they can travel along both paths at the same time in a quantum superposition.

"This experimental technique is called neutron interferometry", says Professor Yuji Hasegawa from the Vienna University of Technology. "It was invented here at our institute in the 1970s, and it has turned out to be the perfect tool to investigate fundamental quantum mechanics."

To see if the same technique could separate the properties of a particle from the particle itself, Yuji Hasegawa brought together a team including Tobis Denkmayr, Hermann Geppert and Stephan Sponar, together with Alexandre Matzkin from CNRS in France, Professor Jeff Tollaksen from Chapman University in California and Hartmut Lemmel from the Institut Laue-Langevin to develop a brand new quantum experiment.

The experiment was done at the neutron source at the Institut Laue-Langevin (ILL) in Grenoble, where a unique kind of measuring station is operated by the Viennese team, supported by Hartmut Lemmel from ILL.

The crucial part of the experiment is: a crystal which splits the neutron beam into two parts and recombines them. Credit: TU Vienna

Where is the Cat …?

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.

… and Where is the Grin?

Things get tricky, when the system is used to measure where the spin is located: 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 exactly what can be seen in the measurement, if the magnetic field is applied at the lower beam – but that is the path which the neutrons considered in the experiment are actually never supposed to take. A magnetic field applied to the upper beam, on the other hand, does not have any effect.

"By preparing the neurons in a special initial state and then postselecting another state, we can achieve a situation in which both the possible paths in the interferometer are important for the experiment, but in very different ways", says Tobias Denkmayr. "Along one of the paths, the particles themselves couple to our measurement device, but only the other path is sensitive to magnetic spin coupling. The system behaves as if the particles were spatially separated from their properties."

High Hopes for High-Precision Measurements

This counter intuitive effect is very interesting for high precision measurements, which are very often based on the principle of quantum interference. "When the system has a property you want to measure and another property which makes the system prone to perturbations, the two can be separated using a Quantum Cheshire Cat, and possibly the perturbation can be minimized", says Stephan Sponar.

The idea of the Quantum Cheshire Cat was first discovered by Prof. Yakir Aharonov and first published by Aharonov's collaborator, Prof. Jeff Tollaksen (both now from Chapman University), in 2001. The measurements which have now been presented are the first experimental proof of this phenomenon. The experimental results have been published in the journal "Nature Communications".

Explore further: Superconductivity switched on by magnetic field

More information: Nature Communications DOI: 10.1038/ncomms5492

add to favorites email to friend print save as pdf

Related Stories

Superconductivity switched on by magnetic field

Dec 23, 2013

Superconductivity and magnetic fields are normally seen as rivals – very strong magnetic fields normally destroy the superconducting state. Physicists at the Paul Scherrer Institute have now demonstrated ...

Shielding for ambitious neutron experiment

Jul 24, 2008

In science fiction stories it is either the inexhaustible energy source of the future or a superweapon of galactic magnitude: antimaterial. In fact, antimaterial can neither be found on Earth nor in space, is extremely complex ...

Recommended for you

Cooling with molecules

8 hours ago

An international team of scientists have become the first ever researchers to successfully reach temperatures below minus 272.15 degrees Celsius – only just above absolute zero – using magnetic molecules. ...

Backpack physics: Smaller hikers carry heavier loads

Oct 21, 2014

Hikers are generally advised that the weight of the packs they carry should correspond to their own size, with smaller individuals carrying lighter loads. Although petite backpackers might appreciate the ...

User comments : 19

Adjust slider to filter visible comments by rank

Display comments: newest first

Doug_Huffman
not rated yet Jul 29, 2014
Thanks for an informative and interesting article and effective link URL the the real thing. Imagine, another Quantum Cat, first Schrödinger's cat and now the Quantum Cheshire.
Pexeso
1 / 5 (4) Jul 29, 2014
The Quantum Cheshire Cat: Can neutrons be located at a different place than their own spin
These experiments aren't conceptually new, the scientists already managed to separate for example the electrons to their three components: spinons, holons and orbitons. The key trick here follows from Couder&Fort experiments and dense aether model: the observable particle properties aren't properties of particle only, but its deBroglie wave, which is formed around particles during their motion trough vacuum foam in similar way, like the wake wave around boat floating at the water surface. And this wake wave can be separated from particle both spatially both temporally in limited extent.
Pexeso
1 / 5 (3) Jul 29, 2014
You can even observe the mechanical analogy of Cheshire cat experiment for neutrons at the water surface with so-called Falaco solitons. These solitons undulate and in certain moment they do manifest only with their spin component (i.e. the turbulence) at the water surface, i.e. in similar way, like the Cheshire cat "with smile only". The separation of particle properties from particles is otherwise quite common and all examples of so-called neutrino/meson/photon oscillations belong there too. Even the neutrons can occasionally disappear from our sight, despite they're quite massive particles. It's evident, it's just the deBroglie wave, what undulates here - not the particle itself.
antialias_physorg
4.3 / 5 (6) Jul 29, 2014
Well, that seems to be exactly the type of experiment that the other article (about 3 pigeons in 2 pigeonholes) was talking about: Dissociating properties from entities.

...that's all kinds of awesome (and freaky at the same time). I guess we'll have to let go of the notion that things have enumerable/firmly associated properties...and come to grips with that there is only a large likelyhood that the two coincide.

Interesting times, indeed.
Pexeso
1 / 5 (3) Jul 29, 2014
that seems to be exactly the type of experiment that the other article (about 3 pigeons in 2 pigeonholes) was talking about: Dissociating properties from entities
What it seems is just a dream. How the pigeon theorem is supposed to demonstrate the "dissociating properties from entities" according to you? Could you explain it, how did you mean it - or you're just twaddling here without sense? The quantum oscillations observed in Creshire cat experiment do apply to single particle - they don't require repetitive weak measurements to detect it. For example the separation of electron orbitals to spinons and orbitons is very classical effect, well known as a Zeeman effect and as such it's well explained within existing QM framework. it's just another fancy name for well known quantum phenomena.
Pexeso
1 / 5 (4) Jul 29, 2014
The Cheshire cat type experiments are done with photons routinely: they're separated and recombined again by their spin components (angular and orbital ones) in Mach-Zehnder interferometerroutinely and nobody calls it a surprising today. Such an experiments are therefore very common in current QM research and they don't involve the weak measurement. The only new here is the usage of neutrons for demonstration of such separation due to technical difficulties (the neutrons are routinely difficult to detect and manipulate with).
antialias_physorg
4.2 / 5 (5) Jul 29, 2014
This has nothing whatsoever to do with Mach Zehnder interferometry. The crucial apsect here is post-selection (as in the other article) which isn't done at all in what you post.
At least bother to read the articles before commenting, won't you?
Dr_toad
Jul 29, 2014
This comment has been removed by a moderator.
antialias_physorg
3.7 / 5 (6) Jul 29, 2014
@AP: Remember who you're talking to?

Yeah, but Zeph is slipping. He used to at least put some effort into his type of crazy...now he just seems to be posting random word jumbles without even bothering to read (much less try to have his posts relate to) the articles.

It's just sorta sad to watch.
MrVibrating
5 / 5 (1) Jul 29, 2014
Probably a weak analogy, but i'm reminded of a mass hanging from a length of string, say - the point of application of the weight is at the top of the string's length, where it's suspended from, which is remote from the location of the mass below possessing the weight.

Like i say, weak analogy and not intended to denigrate the research or any comments above... but strictly it IS a decoupling of properties from entities, if only in a classical mechanical way...

OK i'll shut up now..
ichisan
not rated yet Jul 29, 2014
Once you realize that, like time, space/distance is a perceptual illusion, then it all begins to make sense. The "spooky action at a distance" is no longer spooky and, suddenly, as if by magic, you know why particle decay is probabilistic. Then you grok motion, at last. But then you realize that nobody has the faintest idea what you're talking about. So back to the cave, it is.
del2
not rated yet Jul 29, 2014
Once you realize that, like time, space/distance is a perceptual illusion, then it all begins to make sense.

"Time is an illusion. Lunchtime doubly so." (Douglas Adams)
The Quantum Cheshire cat is radioactive; it has 18 half-lives.
antialias_physorg
4 / 5 (4) Jul 30, 2014
Once you realize that, like time, space/distance is a perceptual illusion, then it all begins to make sense.

Does it? On a numerical sense?

Kicking LSD is all fine and dandy - but there's quite a ways from going "Whoah, man...everything is one" to being able to predict (and use) the effects of spin, position separation.
George_Rajna
Jul 30, 2014
This comment has been removed by a moderator.
dedereu
not rated yet Jul 30, 2014
read freely the paper more clear.
"Due to the weakness of the coupling between the system and the measurement device, the information obtained by a single weak measurement is limited..the measurement has to be repeated several times..the wavefunction evolves through both paths of the interferometer. A weak interaction involving a coupling to the neutrons spatial wavefunction has on average no effect along path I , while a weak coupling involving the neutron spin wavefunction has observational consequences on average only when the coupling takes place along path I ..any probe system that interacts with the Cheshire Cat system weakly enough will on average be affected as if the neutron and its spin are spatially separated."
But in reality of the quantum mechanic they are absolutely not separated, the quantum equations are not changed, nothing fundamentally new. The title is misleading, only the interferences effects on average are different for spatial and spin !!
Pexeso
not rated yet Jul 30, 2014
the wavefunction evolves through both paths of the interferometer
vs.
this has nothing whatsoever to do with Mach Zehnder interferometry.
antialias_physorg
5 / 5 (2) Jul 30, 2014
But in reality of the quantum mechanic they are absolutely not separated,

You still don't get what 'post selection' means.
They are selecting only neutrons that go along one path. The others are discarded. However the experiment STILL shows interference with regards to spin even though all the neutrons of one path are eliminated.

This means the spin registered at the detector went along both paths while the neutrons themselves did not (as the ones going along one path were post-selected against).

While this is a direct result of QM (and neither this, nor the article about the pigeons claims otherwise) it is something that hasn't been observed before.
It means the probability density function of the spin is independent of the probability density function of the entity it's associated with.
swordsman
not rated yet Jul 30, 2014
They appear to have completely neglected the effect of the Coulomb forces. A separation from reality.
katastrofa
not rated yet Jul 31, 2014
One should note that if they measured sigma_x instead of sigma_z, they'd find the spin and the neutron on the same path.
katastrofa
not rated yet Jul 31, 2014
"They are selecting only neutrons that go along one path. The others are discarded. "

In what way are they discarded? The absorbers they used had high transmissivity.

Overall, I agree that the importance of this work is overblown. Sadly, this is often the case with "groundbreaking" papers about QM published in Nature or Science.
antialias_physorg
not rated yet Aug 03, 2014
In what way are they discarded?

It says right in the article
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.