Physicists add 'quantum Cheshire Cats' to list of quantum paradoxes

November 25, 2013 by Lisa Zyga feature
In the proposed experimental set-up, the quantum Cheshire Cat paradox is demonstrated when photons travel through the left arm of the interferometer while photon polarizations travel through the right arm. Some measurements disturb the photons and cause them to travel through the right arm with their polarizations, making the paradox seem to disappear. However, weak measurements that do not cause this disturbance bring the paradox back to life. Credit: Aharonov, et al. ©2013 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft

( —Given all the weird things that can occur in quantum mechanics—from entanglement to superposition to teleportation—not much seems surprising in the quantum world. Nevertheless, a new finding that an object's physical properties can be disembodied from the object itself is not something we're used to seeing on an everyday basis. In a new paper, physicists have theoretically shown that this phenomenon, which they call a quantum Cheshire Cat, is an inherent feature of quantum mechanics and could prove useful for performing precise quantum measurements by removing unwanted properties.

The physicists, Yakir Aharonov at Tel Aviv University in Tel Aviv, Israel, and Chapman University in Orange, California, US, and his coauthors have published a paper on quantum Cheshire Cats in a recent issue of the New Journal of Physics.

The physicists begin their paper with an excerpt from Lewis Carroll's 1865 novel Alice in Wonderland:

'All right', said the Cat; and this time it vanished quite slowly, beginning with the end
of the tail, and ending with the grin, which remained some time after the rest of it had

'Well! I've often seen a cat without a grin', thought Alice, 'but a grin without a cat!
It's the most curious thing I ever saw in my life!'

Just as the grin is a property of a cat, polarization is a property of a photon. In their paper, the physicists explain how, "in the curious way of , photon polarization may exist where there is no photon at all."

Disturbing measurements

In their proposed experimental set-up, the physicists show that a photon will travel through the left arm of an interferometer with 100% certainty, yet its polarization can be detected in the right arm, where there is 0% probability of the photon traveling. That is, the photon is in one place while its polarization is in another.

However, there is a caveat with this experiment: it does not measure the location of the photon and its polarization simultaneously, but instead measures the location of some photons and the polarization of others at different times. This raises the question of whether it is possible that the polarization measurement disturbs the photons, causing them to change course and travel through the right arm.

To address this issue, the researchers proposed two additional variations of this experiment.

In the first variation, the physicists theoretically showed that simultaneously measuring the photon's location and polarization does, in fact, cause the photon to change course and travel through the right arm. That is, the act of measurement changes the outcome, and the paradox seems to disappear. This explanation of measurement-induced disturbance is actually the standard resolution of many paradoxes in quantum mechanics.

Reviving the paradox

Here, however, the physicists take things a step further and show that there really is a quantum Cheshire Cat by proposing another experiment that limits the measurement-induced disturbance. In this proposed experiment, the physicists take advantage of the tradeoff between disturbance and precision by performing a so-called "weak" measurement—one that is not very precise, but causes very little disturbance.

In this proposed set-up, the detectors used in the previous two experiments are replaced by a CCD camera and an optical element, both of which cause very little disturbance. Now, when the photon's location and polarization are measured simultaneously, the results are identical to those of the original experiment: the photon is in the left arm while the polarization is in the right arm.

This finding means that separating a property from its object truly is a feature of quantum mechanics, and not only for photons. The physicists predict that the effect also holds for an electron and its charge or spin, as well as for an atom and its internal energy. Yet while the proposed optical experiment can be implemented with current technology (and the researchers hope it soon will be), realizing the electron version is beyond the reach of current technology.

Wider implications

As the physicists explain, weak measurements can be used to revive other quantum paradoxes besides this one.

"We have many examples in which weak measurements 'bring back' paradoxes; it is in fact a general strategy and it can be applied to all paradoxes that were dismissed until now as simple illusions that would disappear when actual measurements are performed," coauthor Sandu Popescu at the University of Bristol told

The physicists have explained this topic in greater detail in a recent paper on Hardy's paradox. They also have applied the same strategy to what is probably the most famous quantum paradox, dating from the early days of quantum mechanics, which is the idea of negative kinetic energy. Kinetic energy is, by definition, a positive quantity, but appears to be negative for particles that have tunneled into a barrier.

"The tunneling example has in fact been used over and over again, in almost all classic quantum mechanics textbooks, to explain how quantum mechanics works, how the paradoxes are nothing more than illusions stemming from the wrong desire to apply classical thinking, and that a 'correct' understanding of quantum mechanics, namely the fact that measurements produce disturbance completely removes the paradox," Popescu said. "What we showed is that this standard way to dismiss paradoxes generates a wrong intuition and misses all that is truly interesting in quantum mechanics, and that the paradoxes are brought back to life if one knows how to look at the problem."

As for the current paradox, the existence of quantum Cheshire Cats opens up many intriguing questions. For example, how will an electron with disembodied charge and mass affect nearby electrons? In an atom with disembodied internal energy, what will the gravitational field look like? Can photons with disembodied polarization impart their to one object and their radiation pressure to another object? The hope that future work will address these questions.

Quantum Cheshire Cats could also have applications for performing precision measurements. For instance, in an experiment in which a particle's charge causes unwanted disturbances, perhaps the charge could be removed by confining it to separate region.

Explore further: Classical physics shown to be equal to quantum theory when it comes to unusual experiments with light beams

More information: — Yakir Aharonov, et al. "Quantum Cheshire Cats." New Journal of Physics. DOI: 10.1088/1367-2630/15/11/113015

— Yakir Aharonov, et al. "Revisiting Hardy's paradox: counterfactual statements, real measurements, entanglement and weak values." Physics Letters A 301 (2002) 130–138

Related Stories

Quantum reality more complex than previously thought

October 28, 2013

Imagine you order a delivery of several glass vases in different colors. Each vase is sent as a separate parcel. What would you think of the courier if the parcels arrive apparently undamaged, yet when you open them, it turns ...

Playing quantum tricks with measurements

February 15, 2013

A team of physicists at the University of Innsbruck, Austria, performed an experiment that seems to contradict the foundations of quantum theory—at first glance. The team led by Rainer Blatt reversed a quantum measurement ...

Scientists cast doubt on renowned uncertainty principle

September 7, 2012

Werner Heisenberg's uncertainty principle, formulated by the theoretical physicist in 1927, is one of the cornerstones of quantum mechanics. In its most familiar form, it says that it is impossible to measure anything without ...

Recommended for you

Probe for nanofibers has atom-scale sensitivity

January 20, 2017

Optical fibers are the backbone of modern communications, shuttling information from A to B through thin glass filaments as pulses of light. They are used extensively in telecommunications, allowing information to travel ...

Magnetic recording with light and no heat on garnet

January 19, 2017

A strong, short light pulse can record data on a magnetic layer of yttrium iron garnet doped with Co-ions. This was discovered by researchers from Radboud University in the Netherlands and Bialystok University in Poland. ...

Studying the quantum vacuum: Traffic jam in empty space

January 18, 2017

An important step towards a completely new experimental access to quantum physics has been made at University of Konstanz. The team of scientists headed by Professor Alfred Leitenstorfer has now shown how to manipulate the ...


Adjust slider to filter visible comments by rank

Display comments: newest first

1.4 / 5 (7) Nov 25, 2013
One way of looking at this paradox is that part of the photons' "form" - their formal cause - can be separated from the photon and sent on a different trajectory than the rest of the particles. The polarization still "belongs" to its photon in that they are entangled and will be reunited when the system is disrupted in a strong enough way.

I am trying to rather publicly feel my way towards different way of thinking about quantum systems here, but I think that Aristotelian metaphysics might end up being a useful way of understanding the strange quantum world, particularly the teleological "final cause."
Nov 25, 2013
This comment has been removed by a moderator.
Nov 25, 2013
This comment has been removed by a moderator.
1.9 / 5 (9) Nov 25, 2013
A weak interaction is still an interaction. If the so called paradox persists it could also be interpreted as any measurement at all is enough to change the results so I have doubts this will resolve anything
4.3 / 5 (3) Nov 26, 2013
Being able to separate an object of its properties say the least. That's the sort of behavior I would expect in a simulation.
QM certainly has a way of throwing up interesting stuff.

A weak interaction is still an interaction

The point is that with a weak interaction you can put an upper limit on the amount of disturbance (i.e. the amount of 'falsified' results you should get over a number of measurements). It's a bit like loading a die: you get more sixes - but if you don't load it too much you can be fairly certain you don't get all sixes.
1.9 / 5 (8) Nov 26, 2013
antialias_physorg, yes it sounds weird but in a way we do it a lot of the time. In Logic we speak of 'relations' and write in such a way that the relation is separate from the variable though when the whole is turned out it becomes unified. Could be said of math functions etc. too. On the intuitive side it seems to me like looking for a criminal when you know only certain properties but have no idea what the criminal looks like. But I admit it'ss a weak analogy.
From the article it is a 'proposed experiemnt' yet other wording ' this finding...' suggests an actual experiement. Has it been performed or are the 'findings' conjecture? If the experiement hasn't yet been performed it might be interesting to read a follow up.
Does the technology exist where the difference between a maximum disturbance and weak be measured to give a figure on just how much disembodied polarization there is? Hope they publish results.
Just a slight point...real cats can't grin!
1 / 5 (8) Nov 26, 2013
Base reality may be nothing more than a web of paradoxes, which form vertices, and structure which forms edges linking the infinitely nested paradoxes. Evidence of infinitely nested vortices is found in the energy signature of hurricane heat dissipation. Some emergent repeatable structures are identified as particles as their nesting is harmonically resonant

One can see structure is more fundamental than classical reality by observing the quantum vacuum, which displays paradox and structure even without energy supplied
not rated yet Nov 26, 2013
I see a lot of cautious thinking here. Take it to the next level. Let's say I take a beam of electrons and separate the charge from the particles. Now run the particles through a low-pressure gas of an isotope which decays by electron capture... Can you produce energy this way? Yes, but it is likely to only be of laboratory interest. Now try the same experiment with protons. Since the charge is separated from the proton, you should be able to transmute just about anything in the periodic table. Fusion energy, and without the need for magnetic confinement, etc.

Would the nuclear reaction have the effect of destroying the quantum part of the process? Depends on what is produced. Probably the best choice would be a target which did not immediately emit a gamma ray, but emitted an (energetic) particle after a short delay. Say F19 to Ne20*, which has two decay modes, one a gamma emission to Ne20 the other an alpha emission to O16.
Nov 26, 2013
This comment has been removed by a moderator.
not rated yet Nov 27, 2013
Can you produce energy this way?

Only if you omit the fact that it took you some energy to separate the particle form its property. No free lunch here, either, I'm afraid.

Whether transmutation would be easier? I don't know. Certainly the rate of transmutation would be so low that it would take forever to make any usable quantity of some material (and since you're not adding neutrons along with the protons the atoms will just fall apart instantly again once the property separation ceases)

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.