Quantum Cheshire Cat effect may be explained by standard quantum mechanics

June 8, 2015 by Lisa Zyga feature
The vanishing Cheshire Cat leaves its grin behind. Image from Lewis Carroll's Alice in Wonderland.

(Phys.org)—"A grin without a cat" is how Lewis Carroll describes the Cheshire Cat's mysterious way of disappearing while leaving its grin behind in his 1865 classic, Alice in Wonderland. The fanciful character raises a question that has captured physicists' attention over the past few years: can an object be separated from its properties?

In 2013, Yakir Aharonov and his coauthors conceived of an experiment suggesting that a photon can be separated from its polarization (a property that tells the direction in which a wave oscillates). The following year, Tobias Denkmayr and coauthors carried out a similar experiment in which neutrons seemed to be separated from their spin (a property involving angular momentum). Aharonov's group called the effect a "quantum Cheshire Cat."

However, in a new paper published in the New Journal of Physics, Raul Corrêa, Pablo Saldanha, Marcelo Santos, and C. H. Monken from the Federal University of Minas Gerais in Belo Horizonte, Brazil, have questioned this interpretation of the results. Instead of a particle being separated from its properties, they suggest that the results can be explained by a standard quantum effect, quantum interference, in which an individual particle interferes with itself due to its wave-like properties.

"The possibility of separating a particle from one of its intrinsic properties, as suggested by Aharonov and coauthors, is rather intriguing and questions a very basic everyday notion, by which the properties of things are always with the things themselves," Corrêa told Phys.org. "Nobody sees colors going around without the objects that carry them, for instance. The experiment carried out by Denkmayr, et al., makes it still more intriguing, as this completely weird phenomenon is said to happen in the physical world. What we do is to take their results (which are completely correct) and propose an explanation in which no particle is separated from its properties and hence there is no paradox. Quantum interference is indeed a weird phenomenon, but no more than this usual weirdness is necessary to understand these experiments."

Unknown histories

As Corrêa and his coauthors explain, the basis of the controversy lies in the attempt to attribute physical reality to a situation that simply cannot be perceived as physical reality. In this case, the situation that cannot be considered physically real is the past history of particles traveling through an interferometer.

The interferometer set-up with two arms used by Aharonov, et al., in their 2013 study. Credit: Corrêa, et al.

An interferometer allows particles to travel down one of two arms, and was the device originally used to demonstrate the quantum Cheshire Cat. In these experiments, the physicists thought that they could tell which arm a photon or neutron had traveled through by making measurements of the particle after it exited the device. In the case of the photons, for example, a displacement of the photons by a certain amount seemed to indicate that the photons must have traveled down, say, the left arm, since a device in the left arm (such as a glass sheet) had been placed there specifically to displace the photons by that amount. At the same time, measurements of the photons' polarization seemed to indicate that the same photons must have traveled down the right arm for similar reasons.

The physicists in the earlier experiments concluded, quite logically, that the photons were in the left arm while their polarization was in the right arm. But now Corrêa and coauthors interpret the results differently, suggesting that the measurements of photon displacement made after the photons had exited the interferometer cannot reveal information about their past trajectories—that is, the measurements cannot tell which arm the photons traveled through.

The reason why such a seemingly simple assumption cannot be made, the physicists explain, is quantum interference. As the travel through the interferometer, their positions are measured by the photon propagation beam. A sufficiently large beam can make a "strong measurement," causing a large displacement of a photon's position and allowing researchers to determine which arm the photon traveled through. However, the scientists in the earlier studies used "weak measurements," which, as Corrêa and his coauthors explain, cause such a small displacement that they do not allow researchers to determine which arm the photon traveled through.

"In the case of the weak measurement considered by Aharonov, et al., the displacements are small compared to the beam diameter, and hence each part of the beam associated with each arm and polarization is overlapped with every other," Corrêa explained. "This characterizes the interference and prevents us from associating the detection position with the propagation of the photon through each arm or polarization. The paradox of having a photon somewhere and its polarization elsewhere doesn't exist if the problem is seen from this angle. The only mystery left is the usual weirdness, in which particles can be detected individually, while their propagation satisfies wave-like properties."

Different interpretations

The new paper highlights a fundamental feature of quantum mechanics, which is that interpretation plays a pivotal role in understanding the quantum world. While the paper suggests that the quantum Cheshire Cat effect may not be accurate, is also perplexing, although in a more familiar way. With so many ways to interpret the results, controversies and paradoxes inevitably arise. After a century of investigating the quantum realm, physicists know that common sense cannot be trusted, but they may never know for sure what can be.

"In no way this is a definitive answer," Corrêa said. "As usual in science, new explanations can always show up and are always welcome, and that's what characterizes its development. In fact, we can't even say that we proved the authors wrong in their interpretation—we simply provided a different interpretation of the results.

"Nonetheless, both Aharonov, et al., and Denkmayr, et al., proposed a useful application of this separation of particle and properties [high-precision measurements that separate out a particle's different properties], but in our final remarks we provide arguments as to why this application would fail, under the light of usual quantum mechanics. This could also offer a possible path to plan an experimental test which would say whether the particle and its intrinsic property are in fact in different places. Then we could think of the possibility of having an accurate physical answer to the interpretation."

Explore further: Physicists add 'quantum Cheshire Cats' to list of quantum paradoxes

More information: Raul Corrêa, et al. "'Quantum Cheshire Cat' as simple quantum interference." New Journal of Physics. DOI: 10.1088/1367-2630/17/5/053042

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Dhiresh Yadav
not rated yet Jun 08, 2015
Here is what I don't understand. Bosons don't follow Pauli's exclusion principle and so can acquire same position in space and time. So, how is it possible to detect a photon using photon beam? OR is it possible? Also, if this is true how can a photon interfere with itself even if it can duplicate and pass through both the slits in double slit interference phenomenon? Okay, even if it can it must be at the same point in both space and time, which it always is when it hasn't duplicated, and as a single photon. So, the only way it can interact with itself, and thus interfere, is by emitting some interacting (force carrying) particles/virtual bosons, as fermions do. Does it do that? If it does, how can a quantized particle like photon, which itself is a force carrier, emit/break/split itself apart into such virtual particles in mid space-time? and how and why does it do it?
arom
Jun 08, 2015
This comment has been removed by a moderator.
Noumenon
5 / 5 (2) Jun 08, 2015
@Dhiresh, when they say that the photon interferes with itself, they are referring to its wave-function description, which is not observable itself and is a mathematical construct representing superposition of all possible paths (which interfere).

In fact, there is really no such thing as an 'isolated photon',.... a photon is either an emission event or an absorption event, .....nothing else, .....since they travel on null geodesics with zero proper time, the events are 'connected' in some weird way irrespective of spatial separation.
Mimath224
not rated yet Jun 09, 2015
Ha!, not a good name for quantum theory. The Cheshire Cat might have become invisible, all but the grin, but that doesn't mean to say it wasn't there. Cloaking at the nano level having been achieved is more like the CC. Imho, as a layman, separating a particle (or anything else for that matter) from its properties is quite another thing. If the 'particles' of the std mod QM are classified by their properties (say spin 1 or ½ ) what then happens to the present classification. Think I'll have to think, and read, a bit more about this!
flag
3 / 5 (2) Jun 09, 2015
docile
Jun 09, 2015
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someone11235813
not rated yet Jun 09, 2015
Isn't this what George Berkeley was on about? He claimed that matter did not even exist, there was just properties. And it was all sustained by the mind of god. Pretty crazy stuff, but they were crazy times.
Noumenon
not rated yet Jun 09, 2015
Isn't this what George Berkeley was on about? He claimed that matter did not even exist, there was just properties. And it was all sustained by the mind of god. Pretty crazy stuff, but they were crazy times.


Yes, we naturally have an a-priori intuitive predisposition to presume a substratum in which qualities adhere,.... given how our minds operate to synthesize experience.

However, that is not what the above is saying;.... they seem to be saying that only on an interpretational basis are the attributes (quantum numbers) spatially separated in the experiment they referenced.

However, if they ARE presuming that the wavefunction itself, is a physical substratum, in which attributes (quantum numbers) adhere,.... it is wrong imo.
Noumenon
not rated yet Jun 09, 2015
.... "inhere" would have been a better term than "adhere".
swordsman
3 / 5 (2) Jun 09, 2015
Physicists still do not fully understand the characteristics of electromagnetic radiation. The radiation properties of dipole antenna were modeled accurately in the year 1936. The resulting equations are still used today to design antennas. When these equations are plotted in three dimensional space, they show that radiation is NOT spherical! Einstein's theory was based on spherical radiation, and is therefore highly flawed. Mistake of the centurey.
Returners
not rated yet Jun 14, 2015
Instead of a particle being separated from its properties, they suggest that the results can be explained by a standard quantum effect, quantum interference, in which an individual particle interferes with itself due to its wave-like properties.


If mass is bestowed by the Higgs Boson then it would seem to me that it is entirely possible to separate a particle's angular momentum from the particle itself, because momentum requires mass. If the mass is in the Higgs, then you could separate the Higgs, and if you take away the mass you have inherently taken away the angular momentum.

What if the particles are made of still smaller particles, and there is a particle for each property, including polarization? In such a model, one component particle might record orientation (polarization), another component particle records the frequency, and perhaps another component particle records the direction of propagation important for linear momentum upon impact.
Returners
not rated yet Jun 14, 2015
Physicists still do not fully understand the characteristics of electromagnetic radiation. The radiation properties of dipole antenna were modeled accurately in the year 1936. The resulting equations are still used today to design antennas. When these equations are plotted in three dimensional space, they show that radiation is NOT spherical! Einstein's theory was based on spherical radiation, and is therefore highly flawed. Mistake of the centurey.


Newton's gravity is based on spherical radiation too, as are newton's optics. This is where the inverse square law comes from and is the basis of pretty much all modern physics.

I'm not going to say that it's absolutely true, but it is so close to the truth locally that NASA can predict the velocity of orbiting objects at any point in their trajectory to within plus or minus a few centimeters per second among 15km/s to 40km/s velocities.

It is possible that on the galactic scale whatever flaw there is becomes noticeable.
Returners
not rated yet Jun 14, 2015
However, when I say "noticeable" I don't mean that scientists would realize it, I mean that the flaw might be mistaken for something "mysterious" and "invisible" force, such as Dark Matter or Dark Energy, that is, if there is a flaw.

Returning to the notion of the possibility of a particle becoming separated from one or more of its own properties, if you imagine the spherical propagation of a particle-wave then as the wave expands it might seem possible for some of the particles to not be able to track their past existence any more, in which case they would actually lose some of their properties, perhaps "forgetting" their polarization or their direction of propagation.

You might reply that such a situation would be noticed, but can you be sure? If Polarization were in fact stored in a separate particle, it's just as likely that the photon could acquire a new copy of the particle carrying polarization (call it a "polariton" I guess) and appear unchanged.
Returners
not rated yet Jun 14, 2015
Like in the virtual particle background, if the virtual particle were to interfere with an ordinary photon, then it would change the apparent polarization of the photon, and if this were happening all the time over cosmic distances, then the polarization of photons coming from distant objects would appear randomized, and if there is no bias in the background media, the distribution of the randomization would be even. If there is a bias in the background media, for example if the universe moves or expands faster in one direction than in another, the distribution would be ever so slightly biased in that direction. The scientists don't seem to track this bias via polarity though, they try to track it via frequency, which is how they track the velocity of Earth with respect to the background radiation, or more correctly the background media, since the velocity of the actual radiation is supposed to be the same for all observers, but the frequency is not the same for all observers.
theon
not rated yet Jun 15, 2015
I heard a talk by de Raedt and Michielsen who also completely do away with this Chesire nonsense. Sorry to see that in posts only the nonsense counts.

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