“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!”
Fig 1. The Cheshire cat.
In Alice in Wonderland, among all those intriguing characters created by Lewis Caroll, there is a cat: the Cheshire Cat. Sometimes the full cat would appear, sometimes you could only see its body, and sometimes only its smile was visible. Now, a smile is a property that you do not expect to find without its owner (in this case, the cat) and that’s why the Cheshire Cat is so interesting and magical. And you may think: “Ok, that may happen in Alice in Wonderland, but that is make believe! Such a thing is impossible in the real world!”. And yet, physics would prove you wrong: there is such a thing as a real world analogous of a Cheshire cat. Only, in physics, it’s not the smile that gets separated from its owner.
The phenomenon called “Quantum Cheshire Cat” corresponds do the apparent separation of the location of a quantum system from one of its physical properties. This idea was originally proposed by the physicist Aharonov and it involved the separation of a particle from its properties using an interferometer. In quantum mechanics particles can also be thought of as waves and the interferometer can separate these waves, making them follow two different paths.
Quantum states, like the one generated by the interferometer, are very sensible to measurements. If we make a measurement in one of the paths we will disturb the whole system. Therefore, we can only make measurements in either one of the paths, we can’t simultaneously measure both paths. This is true if the measuring device is classical, but if it is quantic interesting things may happen. In this case there is one type of measurement, called “weak measurement”, in which it is possible to simultaneously measure physical properties in both paths. Aharonov did several theoretical works using weak measurement and he realized that this setting could, in theory, lead to a situation in which a photon was detected in one path and its polarization in the other one. Hence the physical property of the photon (polarization) would be disembodied from the photon just like the smile from the Cheshire cat.
Fig2. Experimental realization.
Though this idea seems a bit lunatic, experimentalists have shown that it is correct. In 2014 a group of scientists did an experiment in which neutrons were separated from its magnetic moment. Now, a group of physicists from the University of Portland (USA) has done something similar, now using single photons. They designed an experimental setting using an interferometer and they showed that it was possible do measure the photons in one path and its polarization in another.
The scientists made several sequential measurements (in one path and then in the other) and simultaneous weak measurement. In all the cases the results were in agreement with what was predicted by Aharonov.
Even though the idea of having a real world Cheshire cat may seem a bit crazy, physics has shown that it can, in fact be found. It’s quite unlikely that you’ll start to see grins without cats in every tree branch. However, if quantum mechanics is involved, we can find systems similar to a Cheshire cat outside of Lewis Carroll’s books.
Kellen Manoela Siqueira
The full paper can be found in the July, 2016 edition of Physical Review A.