Quantum Cheshire cats: A physics trick that separates the feline from its grin

K. S. Jayaraman

doi:10.1038/nindia.2020.104 Published online 4 July 2020

In the physical world, an object can not be separated from its properties. For instance, a smile and the resultant dimples on the cheeks are unique signatures that cannot be segregated from a person's body.

But it is not always so in the quantum world where the physical properties of a particle may not belong to the particle itself.

Quantum physicists Arun Kumar Pati and Debamalya Das from the Harish-Chandra Research Institute (HRI) in Allahabad now prove that a quantum object can permanently discard a physical property and obtain a new one it did not initially have1. "This sounds strange, but can have profound implications for our understanding of the quantum world," Pati told Nature India.

In quantum physics parlance, separating an object from its own properties is interestingly called the Quantum Cheshire Cat (QCC) effect — drawing inspiration from Lewis Carroll’s famous book "Alice in Wonderland", in which Alice is wonder struck by a magical cat that appears and disappears at will leaving its weak grin behind.

'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 gone.

'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!' 

-- from Alice in Wonderland.

In 2013, Israeli physicist Yakir Aharonov and his colleagues showed the possibility of separating an object from its own properties using the concept of "quantum weak measurement" thereby supporting the QCC effect theory2.

In what they call a "thought experiment", Pati and Das now propose that two photons — tiny elementary particles — can swap their polarisations (or spins) even if they are not at the same site. They consider the photons as Cheshire cats, and their polarisations as the grins.

They revealed this separation of the body and the grin of the 'elementary Cheshire cats' by taking two sets of "weak measurements" of the particles — one to establish the location of the particle and the other to locate its spin (grin).

"Physical attributes are not real and may not belong to a system", Pati explains their observations. The thing at play here was 'quantum entanglement', meaning the inextricable linking of two particles in which whatever happens to one immediately affects the other. "Entanglement plays a crucial role in the realization of this exchange process," Pati says. This means that it is possible to swap the polarizations of two photons without their being close to each other. 

A schematic representation of exchange of grins in a double Quantum Cheshire Cat setup using 'weak measurement'.

© Pati, A. K. et al

QCC has opened up a new window for the understanding of quantum systems, quantum information as well as in technological applications, adds Pati. It pertains not only to photons and their polarizations but can, in principle, be observed with any quantum system and its property, such as neutron and its magnetic moment and electron and its charge.

Dipankar Home, a quantum physicist at Bose Institute in Kolkata, says implications of QCC effect are much debated and an unambiguous empirical demonstration of this effect still remains to be seen. "This work is a stimulating twist and its implications need to be further explored," he told Nature India.

Pati says the spin swapping by photons predicted by his group has now been experimentally observed and reported3 by researchers in China. Jingling Chen of Nankai University, one of the authors of that study, says their experiment, a first demonstration of two photons exchanging their spins without classically meeting each other, would help foster new research in the area of quantum information. The apparent separation of physical properties from quantum objects and the exchanges of these properties "lucidly exhibit the genuine quantum feature of the Cheshire cat".

Quantum physicist Eliahu Cohen from Israel's Bar-Ilan University says the HRI work has not only separated two photons from their polarizations, but also let them take each other's polarization. The conceptual implication could relate to the question of what inherently characterizes a particle. "....the practical implication would be to attempt to utilize the proposed effect in quantum communication and computation," he says.


1. Das, D. & Pati, A. K. Can two quantum Cheshire cats exchange grins? New J. Phys. 22, 063032 (2020) doi: 10.1088/1367-2630/ab8e5a

2. Aharonov, Y. et al. Quantum Cheshire cats. New J. Phys. 15, 113015 (2013) doi: 10.1088/1367-2630/15/11/113015

3. Liu, Z-H et al. Experimental exchange of grins between quantum Cheshire cats. Nat. Commun. 11, 3006 (2020) doi: 10.1038/s41467-020-16761-0